## 2018 Posters

Posters for June 14, 2018

Excited Phonons Govern the Nonequilibrium Electron Relaxation

With the ability to probe/control matter at the timescales intrinsic to many-body interactions, pump-probe spectroscopy has become a driver of recent advances in uncovering the physics of quantum materials. Despite the richness and complexity of phenomena out-of-equilibrium, the understanding mostly rely on phenomenological models such as the two-temperature model or the semi-classical Boltzmann equation. A comprehensive understanding of nonequilibrium phenomena is at the frontier of ultrafast physics. We study the dynamics of electrons weakly interacting with the lattice by treating them quantum mechanically using the nonequilibrium Keldysh formalism. Particularly, the role of excited phonons in the relaxation dynamics of electrons is of interest. When significant energy is deposited by the pump, or when the phonons are resonantly excited, the effect of excited phonons on the electrons plays an important role. To address this, we incorporate the changes induced by electrons on the phonons by solving a self-consistent Dyson equation for phonons. Our results reveal several novel aspects of the population dynamics that are otherwise absent. We show that the excited phonon populations can determine the population relaxation pathways, which are no longer governed by equilibrium physics. They suppress the decay rates significantly and induce strong time dependence. These dynamic changes concomitantly renormalize the lattice properties. Our work is an important step toward a comprehensive understanding of the phenomena out-of-equilibrium and the opportunities offered to tune the material properties for desired functionalities by ultrafast lattice modulations.

Tomoya Asaba

Rotational Symmetry Breaking in a Trigonal Superconductor Nb-doped Bi2Se3

Topological superconductors (TSC) have been attracting huge interest due to their potential applications to topological quantum computation. Particularly, theories predicted that superconducting doped Bi2Se3 is a TSC and shows a nematic order in the TSC state. Recently, Cu, Nb, and Sr-doped Bi2Se3 have shown the rotational symmetry breaking by means of NMR, heat capacity, torque magnetometry and transport Hc2. We probed the rotational symmetry of doped Bi2Se3 samples in both normal and superconducting states by torque magnetometry. The magnetic field was applied in-plane and the symmetry of magnetic anisotropic susceptibility as well as hysteresis loop was measured. We observed that the superconducting hysteresis loop, as well as the anisotropic magnetic susceptibility, was enhanced along one direction, indicating the rotational symmetry breaking.

Sayan Basak

Distinguishing XY from Ising electron nematics

"At low temperatures in ultraclean AlAs-AlGaAs heterojunctions, high fractional Landau levels break rotational symmetry. We find that a classical model of an electron nematic in the presence of a crystalline lattice is a good model for the temperature evolution of the observed transport properties. Could the universality class be determined experimentally?

We propose an experimental test based on hysteresis that can distinguish whether any electron nematic is in the XY or Ising universality class."

Gloria Bazargan

Characterization of Electron Transit Times in Molecular Conductors using Time-Dependent Quantum Mechanics

The movement of quantum particles between spatial regions is prevalent in nanoscale devices and conductive materials. Consequently, theoretical methods for characterizing the transit time of this movement may aid in identifying and refining systems with desirable transport properties for specific applications. One example of a quantum particle moving between spatial regions is electron transfer from an electron-donating group to an electron-accepting group within a donor-bridge-acceptor (DBA) assembly. These molecular assemblies are made up of an electron-rich donor group connected to an electron-acceptor group by a molecular bridge. Photo-excitation of the donor initiates electron transfer from the donor to the acceptor. DBA systems show promise for molecular electronics because of their small size and tunable electronic properties. Herein we describe a generalized probabilistic method for characterizing the transit times of quantum particles between distinct spatial regions, and present estimates of electron transit times in five DBA systems containing p-phenylenevinylene oligomer bridges. Published experimental results suggest a change in electron transfer mechanism from superexchange at short bridge lengths to hopping at longer bridge lengths in this set of systems. The predicted trend in electron transfer times with increasing bridge length supports this experimental inference. The method also affords new insight into the electron transfer mechanism.

Shermane Benjamin

Physical Properties of Low Dimensional Transition-Metal Sulfides

Transition metal sulfides (TMS) contain sulfur and transition-metal elements in binary, ternary and even more complicated forms. TMSs as opposed to transition-metal oxides (TMO), due to sulfur’s higher covalency, can exhibit exotic quantum properties. In an effort to explore quantum effects in TMSs we have been investigating their chemistry and synthesizing numerous compounds. This work includes the growth of both polycrystals and single crystals, and measurements of their physical properties. We have been investigating superconducting properties of some of these, the extremely large magnetoresistance in others, and plan to investigate their thermoelectric properties.

Daria Blach

Ultrafast Imaging of Interfacial Energy and Charge Transfer in Two-Dimensional Heterostructure Nanomaterials

Ultrafast photoluminescence and transient absorption spectroscopy and microscopy approaches are utilized to study energy and charge transfer in heterostructure nanomaterials, specifically at the CdSe/CdS interface. Two-dimensional nanostructures maintain quantum confinement in one direction, while charge and exciton transport can occur over other two dimensions, making them attractive for achieving directional transport. CdSe/CdS heterostructure has a type I band alignment, in which charge transfer can occur from CdS to CdSe. Steady-state spectra as well as dynamics, together with transient absorption spectroscopy and microscopy, at and near the interface are obtained and used to elucidate the mechanism of charge transfer. Preliminary data suggests the existence of a charge transfer process between CdSe and CdS layers, which is confirmed by an increase in the lifetime of CdSe at the interface compared to an isolated material.

Kirsten Blagg

Thermoelectric effects in superconductor hybrid structures

Thermoelectric materials convert a temperature gradient to an electrical current and vis versa. These materials have excited a wealth of research due to their potential applications in energy as a way to convert waste heat into usable power and in technology as a solid state cooling device. Thermoelectric effects which are usually negligible in superconductors have been found to increase dramatically in superconductor hybrid structures when in the presence of a spin splitting exchange field. These structures offer high Seebeck values making them the only viable thermoelectric materials at low temperatures.

Natalie  Briggs

2-Dimensional Materials via Confinement Heteroepitaxy

The realization of epitaxial graphene has enabled new perspectives and interest in many research areas, serving not only as an atomically-smooth substrate for material growth or versatile contact material for device technology, but also as a means of artificially confining materials to low dimensions. Specifically, the epitaxial graphene/silicon carbide (EG/SiC) system has led to studies of intercalation of elements such as nitrogen, hydrogen, and fluorine through graphene layers to the EG/SiC interface. We show that intercalation of elemental gallium, indium, and tin readily occurs in systems of epitaxial graphene, where atomic layers of metal elements reside between silicon carbide and the first layer of graphene rather than between individual graphene layers. Such studies reveal that the EG/SiC interface plays a key role promoting elemental intercalation and in stabilizing atomically thin layers of materials.

We show that one to three atomic layers of gallium, indium, and tin can be achieved through a simple process in which metallic sources are heated beneath substrates of EG/SiC. To promote intercalation, graphene layers are first exposed to a plasma treatment, resulting in an increased defect density in the graphene. Following this treatment, graphene is placed face-down over metallic gallium or indium, and heated in an argon atmosphere in a tube furnace from 600-800°C. Scanning transmission electron microscopy of resulting samples shows predominant formation of trilayer gallium, bilayer indium, and monolayer tin located between SiC and the first EG layer. Auger electron spectroscopy indicates intercalation on the lateral scale of 10s of microns. Additionally, increased gallium and indium signals are observed in x-ray photoelectron spectroscopy following intercalation in plasma-treated graphene samples, supporting the hypothesis that intercalant atoms reach the EG/SiC interface through defects in the graphene layers.

To further investigate the nature of bonding in the SiC/intercalant/EG stack, preliminary density functional theory calculations are performed. Results indicate covalency between the first and second layers of intercalated gallium in SiC/Ga/EG structures and decreased relative covalency between the second and third gallium layers, further supporting the notion that the SiC surface plays key role in the realization of these two-dimensional metals. Additionally, preliminary low-temperature measurements indicate a superconducting transition in SiC/Ga/EG structures around 3-4K. Further work aims to elucidate the TC of trilayer gallium, and to realize additional two-dimensional metals with this EG/SiC system in an effort to understand requirements for intercalation, the role of the SiC substrate, and the unique properties of metallic elements confined to ultra-thin dimensions.

Rebecca Cebulka

Enabling the Study of Anisotropy-driven Quantum Dynamics of Single-Molecule Magnet Spins at 100mK

We will present the results of an ongoing experimental project to allow pulse EPR studies (spin echo) of condensed samples of single-molecule magnets and single-atom magnets (non-diluted crystals) at temperatures at or below 100mK. The aim is to eliminate dephasing due to dipolar fluctuations by freezing the spin state of all molecules in the crystal in the ground state without the need of applying strong magnetic fields. We expect that these conditions would allow us to study the quantum dynamics of the spins as governed by the intrinsic molecular magnetic anisotropy, which should give rise to non-well defined Rabi oscillations of the spin state, including metastable precessional spin states.

Eli Chertkov

Hunting for Hamiltonians: A Computational Approach to Learning Quantum Models

The machine learning community has been widely successful in developing computational methods for learning models, i.e., probability distributions, from data sets. We present a novel numerical method, similar in spirit to machine learning techniques, for learning quantum models, i.e., Hamiltonians, from wave functions. The method receives as input a target wave function and produces as output a space of Hamiltonians with the target wave function as an energy eigenstate. We demonstrate that our method is able to discover multi-dimensional spaces of Hamiltonians with ground states exactly identical to the ground states of known model Hamiltonians, such as the Kitaev chain, the XX chain, the Heisenberg chain, and the Majumdar-Ghosh model. Using this method, we also find a space of Hamiltonians with a new type of antiferromagnetic ground state, which has not been previously observed in other models. Our results indicate that our new computational approach can systematically discover new Hamiltonians, and thereby potentially new materials, with exactly specified ground state properties.

Avik Dutt

Topological photonics and gauge potentials with synthetic frequency dimensions

Photons are neutral particles which don't respond to electric and magnetic fields the way electrons do. However, realizing topological physics often requires the breaking of time-reversal symmetry, especially for photons, since there is no spin-momentum locking. We show that periodically driven photonic systems using electro-optic modulation can still realize effective electric and magnetic fields for photons, opening the door to a host of interesting condensed matter phenomena in optics. We also show how synthetic frequency dimensions can be realized in coupled photonic resonators undergoing dynamic modulation, enabling the exploration of higher dimensional physics on lower dimensional systems. By controlling the modulation phase, we implement photonic gauge potentials to realize coherent dynamics such as Bloch oscillations, parity-time symmetric lasing, and topological phenomena such as the quantum Hall effect.

Arash Fereidouni

Toward gate-controlled magnetism in few-layer MnPSe3

We have fabricated dual-gated few-layer MnPSe3 devices with a goal of investigating gate-controlled magnetism in this material. Thermally-assisted mechanical exfoliation is used to obtain bilayer and monolayers of MnPSe3. Bulk-like and few-layer flakes are transferred onto HfO2-insulated gold back gates and contacted in a Hall bar geometry. Different contacting strategies have been employed, including variations of geometry and contact metals. We have tried evaporating contacts on top of transferred flakes, as well as transferring flakes on top of pre-patterned contacts. In addition, both gold and palladium have been tried as contact metals. Dual gating is achieved using ionic liquid (DEME-TFSI) in addition to the local back gate. Transport measurements of a thick flake at room temperature showed high resistance of 5GΩ p-type conduction, while initial measurements on few-layer samples are indistinguishable from leakage current. In addition to the device fabrication, a cryogenic polar-MOKE microscope has been assembled to measure magnetization of few-layer MnPSe3 and its potential changes with carrier density and/or electric field. Future efforts are directed at controlling the carrier density of MnPSe3 flakes in search of a predicted antiferromagnetic-ferromagnetic phase transition, as well as gate controllable MOKE in antiferromagnetic bilayers.

Rita Garrido Menacho

Disorder effects on Superconductor-Graphene-Superconductor arrays

Graphene coupled to an array of superconducting islands has proven to be an excellent platform for the study of continuous quantum phase transitions (QPT). This system has been shown to transition from a superconducting state to a metallic and/or insulating state. The islands are Josephson coupled, with the magnitude of the coupling controlled by a gate voltage on the graphene. To study the effects of disorder on this phase transition and the related ground states, we performed transport measurements on graphene proximity-coupled to an array of Sn superconducting islands, where we added point disorder (random displacements) to each island site. We studied the Superconductor-to-Insulator transition (SIT) as a function of gate voltage and applied magnetic field. For low disorder, a clear critical crossing point was observed from magnetoresistance measurements, and the extracted critical exponents were consistent with a continuous 2D SIT. In contrast, high disorder devices showed a disrupted crossing point and disrupted scaling. These signatures suggest unusual behavior as a function of disorder near the Quantum Critical Point (QCP).

This work was supported by the DOE Basic Energy Sciences under DE-SC0012649.

Luwei Ge

Spin structures and dynamics of a quasi-2D triangular spin-1/2 antiferromagnet

The quantum (spin-1/2) triangular-lattice antiferromagnets is a rather simple yet important model hosting exotic ground states and excitations. Ba3CoSb2O9, a triple perovskite where Co2+ ions form triangular lattices, is a very good experimental realization despite small interlayer coupling and easy-plane anisotropy. Based on results from my collaborative work, as well as other literature, I will present the current understanding of its magnetic properties and demonstrate how it is motivating further studies.

Danielle Hamann

Using Controlled Nanoarchitecture to Study Structure and Transport Properties in Metastable Thin Film Heterostructures.

Recent research on monolayers and heterostructures containing them has led to the discovery of numerous emergent properties. These emergent properties are markedly different from those of the respective bulk material and in heterostructures the resulting properties are not always additive. This behavior makes the properties of heterostructures difficult to predict and control. Self-assembly of designed precursors with composition profiles that mimic that of the desired product allows for the synthesis of metastable heterostructures with controllable nanoarchitecture. Using this unique control, series of compounds are prepared where the dimension of a single constituent is changed, allowing the study of emergent properties as a function of material thickness or layering order. One family of heterostructure compounds with properties dependent on nanoarchitecture is [(SnSe)1+]m[TiSe2]n. The structure of these materials was characterized by various X-ray diffraction techniques and high angle annular dark field scanning transmission electron microscopy. The composition was measured by x-ray fluorescence and informs on the number of atoms per unit area in the sample. Temperature dependent Hall coefficients and resistivity data was collected under vacuum on a custom build system utilizing a closed cycle He cryostat. Varying the value of m and n in the heterostructure results in unique structural changes as well as interesting resistivity and Hall data. The cause of both the structural transformations and intriguing transport data is not entirely understood, and study of the layer interactions is necessary to elucidate the origin.  Advanced knowledge about the source of these emergent properties will facilitate the future design of heterostructures to target and optimize a certain property for a specific application.

David Hynek

Synthesis of large-area transition metal dichalcogenide films through use of metallic seeding layers

Two-dimensional materials have attracted much attention in the past decade due to their unique van der Waals (vdW) interlayer bonding that gives rise to interesting physical phenomena that can be accessed through monolayer isolation and synthesis of heterostructures. Recently, semi-metallic WTe2 has shown novel electronic properties, such as large magnetoresistance, superconductivity, quantum spin hall states, and the predicted type-II Weyl semimetallic state, which are attractive for applications ranging from chemical sensing to quantum computing. One of the biggest challenges to vdW bonded materials is the lack of industrially scalable synthesis methods to create large scale, stable structures on substrates that are easily integrable in modern microfabrication processes. In this work, transition metal dichalcogenide thin films are synthesized from transition metal seed layers through chemical vapor deposition, and their properties are investigated. We show that the metallic seed layers can be used to create large area films and various heterostructures.  In particular for WTe2, thermal conductivity values lower than that of bulk were observed for WTe2 polycrystalline thin films.

Mingde Jiang

EXAFS study of overdoped cuprate superconductor YSCO-Mo

YBa2Cu3O7-δ (YBCO) is a well-known high Tc superconductor (HTSC), with Tc=95K at optimal doping. The material studied here, Cu0.75Mo0.25Sr2YCu2O7.54 (YSCO-Mo), is isostructural with YBCO with two exceptions: 1) The BaO planes are replaced by SrO planes, and 2) 25% of the Cu in the Cu-O chain layer is substituted with Mo, which allows for much higher valence states, as well as different local coordination geometries. This overdoped cuprate compound is not expected to be superconducting, yet is found to have Tc=84K. Utilizing extended X-ray absorption fine structure (EXAFS), we aim to extract the local structure around the Mo dopants in order to determine the role of Mo in producing superconductivity in this material. This is a work in progress.

Wencan Jin

Unidirectional Charge Order in (Sr1-xLax)3Ir2O7 Revealed by Polarized Raman Spectroscopy

Charge order is universally observed in underdoped copper-oxide based high-Tc superconductors (cuprates) and known to compete with superconductivity. Recently, strongly spin orbit coupled analogues to the cuprates, the layered perovskite iridium oxides (iridates), were predicted to also manifest unconventional superconductivity, and have since, tantalizingly, exhibited a number of electronic phenomena paralleling those found in cuprates including Mott insulating, Fermi arc, magnetic multipolar order, and even d-wave gap behaviors. Whether the unidirectional charge order instability known to be intertwined with and thought competing with superconductivity in the cuprates also manifests as a nearby instability of the Jeff = ½ Mott state of the iridates, however, remains an open question. Here we establish such a parallel by using polarized Raman spectroscopy that resolves the emergence of unidirectional charge order in the metallic state of the iridate (Sr1-xLax)3Ir2O7. We show that an amplitude mode of the emergent charge order below TCO ~ 200 K exhibits two-fold rotational symmetry while the observable phonon modes of the crystal lattice obey four-fold rotational symmetry at all temperatures. Different from conventional charge density wave where the frequency of the amplitude mode has an order parameter like temperature dependence, this amplitude mode frequency remains nearly constant over wide ranges of temperature and La doping concentrations. These features are highly reminiscent of the charge order observed in underdoped cuprates, and our measurements establish an endemic phase competition between unidirectional charge order and the Mott insulating states within the phases diagram of electron-doped Jeff = ½ Mott and hole-doped S = ½ Mott states.

Strong nonlinear optical response of ultranarrow graphene nanoribbons

Nonlinear optics have a broad range of applications in nanophotonics. As a result, finding materials with strong nonlinear optical properties has been an ongoing research.  Here, we calculate the optical nonlinearity of graphene nanoribbons (GNRs) and show that, in the long-wavelength limit, GNRs have a remarkably strong optical response, particularly the third-order Kerr optical response, at the infrared frequencies. The strong optical nonlinearity of GNRs in the long-wavelength region enables integrating and embedding them in the on-chip semiconductor waveguides. We base our analysis on perturbatively solving a Lindblad-type quantum-master equation. We show that electron scattering plays a nontrivial role in the nonlinear optical response of GNRs, which can be captured exclusively with a full quantum-mechanical treatment.

Abe Levitan

Visualizing Inhomogeneous Electronic Order with Resonant Coherent Diffractive Imaging

Zhengguang Lu

Splitting of dark and bright exciton states in TMDCs  monolayers

Transition metal dichalcogenides (TMDCs) monolayers, as direct-gap materials with strong light–matter interactions, host tightly bound excitons and spin-valley degrees of freedom. The lack of inversion symmetry together with the strong spin–orbit interaction lift the spin degeneracy in the conduction (CB) and valence bands (VB) at the K and K` points related by time reversal symmetry. Excitons in monolayer TMDCs, depending on the spin configuration of the CB and VB, can be either optically bright or dark. The nature of dark exciton states may have a critical impact on the optical properties, on scattering processes and photoluminescence efficiency in particular, thus it is of high interest for the fundamental science as well as eventual optoelectronic applications.

In this work, we apply a strong in-plane magnetic field to mix the spin of the CB, which allows us to brighten forbidden spin-flip transitions and probe directly the dark exciton states. We have performed low temperature magneto-photoluminescence (PL) and broadband reflection contrast measurements on MoSe2 and WSe2 monolayers. In both materials, magnetic brightening of dark excitons enables accurate spectroscopic determination of the energy separation between the dark and bright states. Moreover, in MoSe2, we explored the field dependent splitting between dark and bright excitons. The field evolution of both bright and dark exciton branches follow the simple two-band model.

Shu Zhang

Dynamical structure factor of spin liquid candidate NaCaNi2F7

The pyrochlore antiferromagnet is a highly frustrated system that might harbor exotic phases such as a three dimensional quantum spin liquid. NaCaNi2F7 is a spin-1 spin liquid candidate described by a Heisenberg Hamiltonian with small anisotropic corrections. Our previous work arXiv:1711.07509 showed good agreement between the static structure factor in neutron scattering experiment and the classical modeling. Here, we show evidence that the dynamical structure factor is also qualitatively captured by the nearly Heisenberg Hamiltonian. The simulation of the full classical dynamics and the analytical calculation from a stochastic model are compared with the experimental measurements. Quantum effects should be take into account to explain the large spectral weight in the broad dispersions at high energy transfer.

Posters for June 20, 2018

Quantization of Fractional Corner Charge in $C_n$-Symmetric Topological Crystalline Insulators

We find that $C_{n=4,6,3}$ symmetric crystalline insulators have fractional corner charges in multiples of $\frac{e}{n}$. We first classify two-dimensional crystalline insulators having time-reversal symmetry and $C_n$ symmetry and build rotation topological invariants to characterize these phases.  Then, by constructing sets of primitive generators that span these classifications, we are able to characterize the existence of corner fractional charge exhaustively and systematically. Our findings are compiled in a set of second-order topological indices that diagnose the presence of quantized fractional corner charge in $C_n$-symmetric crystalline topological insulators.

Ian Leahy

Thermal, Electrical, and Magnetic Properties of Quantum Materials

Using thermal, electrical, and magnetic probes, we investigate a variety of quantum materials from Weyl/Dirac semimetals to low dimensional quantum magnets. We present experimental details and results of thermal transport and magnetic measurements on the honeycomb magnets RuCl3 and CrCl3. We also present recent magnetic and electrical transport measurement results on materials with large, non-saturating magnetoresistance: NbP, TaP, NbSb2, and TaSb2.

Yunqiu (Kelly) Luo

Opto-Valleytronic Spin Injection in MoS2/Graphene Hybrid Spin Valves

Two-dimensional (2D) materials provide a unique platform for spintronics and valleytronics due to the ability to combine vastly different functionalities into one vertically stacked heterostructure, where the strengths of each of the constituent materials can compensate for the weaknesses of the others. Graphene has been demonstrated to be an exceptional material for spin transport at room temperature; however, it lacks a coupling of the spin and optical degrees of freedom. In contrast, spin/valley polarization can be efficiently generated in monolayer transition metal dichalcogenides (TMD) such as MoS2 via absorption of circularly polarized photons, but lateral spin or valley transport has not been realized at room temperature. In this Letter, we fabricate monolayer MoS2/few-layer graphene hybrid spin valves and demonstrate, for the first time, the opto-valleytronic spin injection across a TMD/graphene interface. We observe that the magnitude and direction of spin polarization is controlled by both helicity and photon energy. In addition, Hanle spin precession measurements confirm optical spin injection, spin transport, and electrical detection up to room temperature. Finally, analysis by a one-dimensional drift-diffusion model quantifies the optically injected spin current and the spin transport parameters. Our results demonstrate a 2D spintronic/valleytronic system that achieves optical spin injection and lateral spin transport at room temperature in a single device, which paves the way for multifunctional 2D spintronic devices for memory and logic applications.

Thomas Mittiga

Toward Nanoscale Magnetometry of Quantum Materials under Ambient and High Pressure Conditions

The Nitrogen-Vacancy (NV) defect center in diamond has cemented its role as a nanoscale sensor that will bolster the exploration of next-generation quantum materials. NVs are atom-like optically addressable electronic spin-1 systems with dipolar electric and magnetic coupling to its environment. By minimizing the separation between detector and sample, NV-based sensors strengthen coupling while boasting nanoscale spatial resolution. Consequently, NVs have been employed as sensors in a variety of configurations to study magnetic phenomena. Shallow-implanted single NVs can perform nuclear magnetic resonance spectroscopy with nanometer resolution. Our lab is working towards using shallow NV centers to probe quantum materials as well as NV centers in diamond anvil cells to study magnetism under high pressure.

Brenton Noesges

Role of Oxygen Vacancy Defects in Two-Dimensional Hole Gas at SrTiO3/LaAlO3 Complex Oxide Interfaces

SrTiO3/LaAlO3 heterostructures have been widely studied due to the highly-mobile two-dimensional electron gas (2DEG) present at the interface between these insulating materials.  The 2DEG interface has been commonly measured but the complementary 2D hole gas (2DHG) has been more difficult to observe due to defect formation. Using advanced growth techniques designed to control oxygen vacancies and provide atomically abrupt interfaces, we established the role of oxygen vacancy defects in the formation of a 2DHG in epitaxially-grown STO/LAO/STO heterostructures.  Oxygen vacancies must be minimized near the p-type interface as V_O can act as double donor sites and provide negative charge to compensate the 2DHG. We used a combination of depth-resolved cathodoluminescence spectroscopy (DRCLS), Monte Carlo simulations, and grounding techniques to identify oxygen vacancies and track the relative concentration of defects as a function of depth in STO/LAO/STO heterostructures grown using pulsed laser deposition (PLD).   DRCLS is capable of measuring defect states from the top STO surface, across interfaces, and into the bulk of the substrate with near-nanometer depth resolution which is necessary for these ultrathin structures with layers only 16 nm thick. Monte Carlo simulations of electrons in these heterostructures determines the relationship between probe depth and beam energy. With this level of depth resolution, we can measure how growth conditions and interface type affects defect features near the interfaces. Particularly, we observed and tracked a V_O defect state 2.9 eV above the valence band maximum (E_V). Due to the high quality of the film growth, the top STO film showed a much smaller sign of oxygen vacancies than the bulk STO substrate. The low concentration of V_O-related features in the top film showed no depth-dependency, while oxygen defects increased with depth in the bulk STO substrate. Such a low concentration of V_O in the upper STO confirmed growth efforts to minimize oxygen deficiency which is required to produce the previously unseen 2DHG at an STO/LAO interface. DRCLS is a powerful tool for identifying and tracking defect concentration in ultrathin oxide layers to better understand the role of defects in space charge regions.

Tara Pena

Spin Seebeck Effect in Two-Dimensional Antiferromagnetic Insulator MnPS3

We explore the thermal generation of pure spin current by the spin Seebeck effect in a two-dimensional (2D) antiferromagnetic (AFM) insulator MnPS3. We introduce a fabrication method using direct-write laser photolithography to pattern on-chip magnetothermal devices on 2D materials systems. Using this technique, we are free from relying on large area 2D samples, and have the flexibility to explore exfoliated flakes, which still provides the highest quality materials. The flexibility and sensitivity of this method allows for fast turn-around time, and high throughput when characterizing magnetothermal transport as we reach the 2D limit in our AFM system. A spin detecting layer, consisting of Pt, is used to detect the generated spin current via the inverse spin Hall effect. An electrical insulating layer of MgO is deposited onto the MnPS3 flake followed by electrically resistive Ti heater layer, contacted by a less resistive Ag contact layer. We have so far used this technique to observe spin current generation at low temperatures (5 – 40 K) and high magnetic fields up to ± 12 T. Use of a 2D Van der Waals magnet allows for easy mechanical exfoliation of the material and therefore allows for exploration into AFM order as we reach the monolayer limit.

Tomas Polakovic

Ion Beam Assisted Sputtering of NbN for Quantum Sensing Applications

Transition metal nitrides are a large family of materials with wide range of electronic and superconducting properties, making them a popular choice for applications in quantum optics or superconducting electronic devices. A member of this family, Niobium Nitride (NbN), is used predominantly in the field of single photon detectors because of relatively high superconducting Tc and good metallic behavior in normal state. The drawback of using this material is the presence of multiple non-superconducting crystal modifications. This requires NbN thin film deposition processes to be carried out at elevated temperatures and lattice-matching substrates, which makes integration into complex device structures impossible.

We report on results of systematic characterization of NbN films grown by magnetron sputtering with assistance of low-energy nitrogen ion beam and compare them to films made by conventional reactive magnetron sputtering. We show that this method allows for room temperature growth of textured, superconducting films with Tc as high as 14.5 K, on non-epitaxial substrates. We also study the behavior of the superconducting state as a function of film thickness, which shows behavior consistent with the increase of Ginzburg-Landau energy due to film surfaces as the film thickness approaches the superconducting coherence length.

Songyang Pu

Composite Fermions and Its Fermi Sea on a Torus

The fractional quantum Hall effect (FQHE) is one of the most wonderful collective states discovered in nature, serving as a quintessential prototype for emergent topological order. It has triggered a wealth of novel physics and concepts. A broad class of fractional quantum Hall (FQH) states and their excitations are well explained by explicit wave functions using the framework of the composite fermion (CF) theory, which have been constructed in disk and sphere geometry. Meanwhile, torus is a more natural geometry for Composite-Fermion Fermi sea (CFFS) with assigned Fermi wave vectors. In this work, We achieve an explicit construction of the lowest Landau level (LLL) projected wave functions for both FQH states and CFFS, show the influence of Landau level (LL) mixing on Berry phase and find the critical Zeeman energy.

Carola Purser

Spinwave sensing and magnetic resonance spectroscopy using Nitrogen-Vacancy (NV) centers in diamond

Modern technological challenges include developing smaller, faster, more energy efficient sensors and information devices. As traditional electronics has approached the fundamental limits of what can be achieved using the charge degree of freedom, researchers have begun exploring spin-based platforms for future applications. While ferromagnetic excitations may themselves be used to store and transmit information (i.e., magnonics). Spinwaves have also drawn attention for their influence on spin diffusion in proximate spin channels. Here, we leverage the high sensitivity of nitrogen-vacancy (NV) centers in diamond to detect the fluctuating magnetic fields from spinwaves resonant with NV transitions. We systematically investigate NV-spin relaxation by spinwaves in which: 1) the NV-FM separation is controlled by a wedge-shaped spacer; 2) the applied magnetic field alters the wavevectors of spinwaves that are resonant with the NV spins; and 3) a microwave drive field enhances the populations of spinwaves scattered from coherently driven ferromagnetic resonance modes. We are thus able to probe both thermally excited spinwaves as well as scattering processes in poorly understood high-drive, low-field regimes. The atomic-size of the NV center suggests that the spatial resolution of these techniques may extend to the nanoscale pertinent to miniaturizing device elements.

Jordan Russell

Many-Body Interactions in the Cyclotron Resonance of Encapsulated Graphene

We present observations of interband Landau level transitions in high-mobility encapsulated graphene by way of infrared magneto-spectroscopy. By varying the carrier density in a constant magnetic field we observe a non-monotonic dependence of the transition energies on the Landau level filling factor. These findings support the idea that electron-electron interactions contribute to the cyclotron resonance in graphene, beyond the single-particle picture. Additionally, a splitting of transitions involving the zeroth Landau level is interpreted as a Dirac mass arising from the coupling of the graphene and boron nitride lattices.

Sai Krishna Manoj Settipalli

Electron and Phonon Transport in Multilayered Materials

The poster presents studies on electron and phonon transport through multilayered materials that are intended for thermoelectric applications. Specifically, it focuses on electron and phonon transport through Si/Ge superlattices. The phonon transport studies are performed employing classical molecular dynamics, and the variation of phonon thermal conductivity with period is presented. The electron transport studies are performed using a  Kronig-Penney type model for Si/Ge superlattices and Boltzmann transport equation (BTE) with constant relaxation time approximation (RTA), and the variation of Seebeck coefficient (S) with period is presented. Finally, the relaxation time computed from electron-phonon coupling calculations performed using the density functional theory (DFT) is presented, which could be used for a more accurate electron transport computation. The aim is to understand how the thermal conductivity and S can be modified in order to achieve a desired thermoelectric efficiency (ZT).

Junyi Shan

Signatures of an order-disorder transition in LiOsO3 from second harmonic optical anisotropy

Metallic LiOsO3 undergoes an unusual ferroelectric-like structural phase transition below Tc = 140 K that is believed to realize the first example of the long-sought Anderson-Blount ferroelectric metal. However, the microscopic origin of this phase transition remains unsettled. I will present optical second harmonic generation rotation anisotropy measurements on LiOsO3 single crystals to probe the presence of both static and dynamic inversion symmetry breaking. Our results reveal a strong order-disorder character to the phase transition.

Estiaque Haidar Shourov

Epitaxial Growth and Electronic Structure of Semiconducting Half-Heusler FeVSb

Although FeVSb is experimentally known as a high figure of merit thermoelectric material, challenges associated with fabricating high quality single crystalline samples have hampered a fundamental understanding of its electronic structure. For example, while recent first-principles calculations show that the DFT band gap is highly sensitive to the choice of exchange and correlation functional (LDA predicts 0.36 eV and HSE predicts 1.45 eV), its experimental bandgap is not known. Here, we demonstrate the epitaxial growth of FeVSb on MgO (001) by solid source molecular beam epitaxy. The single crystalline phase and epitaxial alignment were confirmed by reflection high-energy electron diffraction (RHEED) and X-ray diffraction. By tuning the growth temperature and relative Sb flux, we find that FeVSb can be grown in a self-limiting, Sb adsorption-controlled window. Further tuning of the Fe:V flux ratio (by QCM and RBS measurements) then allows us to grow stoichiometric FeVSb. Our angle-resolved photoemission spectroscopy (ARPES) reveals that the band gap of FeVSb is at least 0.6 eV (Fig. 2), much larger than the 0.36 eV band gap predicted by LDA calculations, and the measured valence band width is smaller than the LDA width by nearly a factor of two.

Qian Song

Spectroscopic study of the competition between charge order and superconductivity

Changyan Wang

Photocurrents in Na3Bi thin film

Because of the high symmetries of Dirac semimetal, the photocurrents cancels each other when shedding a light on Dirac semimetal, resulting in zero net current. By adding electric field along the z direction to Na3Bi thin film, which can be describe by a k*p model, the inversion symmetry of it is broken. Then by carefully tuning the incident direction and polarization of the light, we obtain nonzero net photocurrents.

Ying Wang

Formation of helical domain walls in the fractional quantum Hall regime as a step toward realization of high order non-Abelian excitations

We propose an experimentally-feasible platform to realize parafermions (high order non-Abelian excitations) based on spin transitions in the fractional quantum Hall effect regime. As a proof-of concept we demonstrate a local control of the spin transition at a filling factor 2/3 and formation of a conducting fractional helical domain wall (fhDW) along a gate boundary. Coupled to an s-wave superconductor these fhDWs are expected to support parafermionic excitations. We present exact diagonalization numerical studies of fhDWs and show that they indeed possess electronic and magnetic structures needed for the formation of parafermions. Reconfigurable network of fhDWs will allow manipulation and braiding of parafermionic excitations in multi-gate devices.

Yiping Wang

Searching for Fractionalized particles in a potential Kiteav Spin Liquid

Relativistic Mott insulators on the honeycomb lattice are predicted to realize a novel topologically protected state known as the Kitaev Quantum Spin Liquid(QSL). Recently attention has focused on α-RuCl3 that appears to be close to this limit with preliminary evidence from Neutrons and Raman for Majorana fermions. Here I will discuss our recent high resolution Raman spectroscopy of both Stokes and anti-Stokes spectra. We find new evidence for the non-trivial nature of these excitations.

Yishu Wang

Antiferromagnetic quantum phase transitions: continuous tuning and direct probes of competing states

Antiferromagnets are choice systems to study quantum critical behavior. Unlike ferromagnets, they can experience continuous quantum phase transitions when tuned by pressure. However, the lack of a net magnetization renders experimental approaches difficult and often indirect. Here I demonstrate that both non-resonant and resonant x-ray magnetic diffraction under pressure provide the highly-desired direct probe for microscopic insights into the disappearance of the magnetic order, as well as the evolution of the charge and structural degrees of freedom. In MnP, a spiral magnetic order with tightened pitch was revealed in the high-pressure phase near a superconducting state at ~7 GPa. As the spiral pitch changes, fluctuations move from antiferromagnetic to ferromagnetic at long and short wavelengths respectively, thereby promoting spin-fluctuation-mediated superconductivity of of different symmetries. In all-in-all-out (AIAO) pyrochlore antiferromagnet Cd2Os2O7, we discovered a quantum critical point at 35.8 GPa using new techniques for resonant x-ray magnetic diffraction under pressure.  As the AIAO antiferromagnetic order is continuously suppressed to zero temperature, three phases of different time reversal and spatial inversion symmetries converge at a single critical point.  While phase lines of opposite curvature indicate a striking departure from a mean-field form at high pressure, the intertwined spin, charge, and phonon fluctuation modes in the quantum critical region demonstrate a strong-coupled scenario of quantum criticality.

References:

[1] Yishu Wang, Yejun Feng, J.-G. Cheng, W. Wu, J. L. Luo, T. F. Rosenbaum, Spiral Magnetic Order and Pressure-Induced Superconductivity in Transition Metal Compounds, Nature Communications 7, 13037 (2016).

[2] Yishu Wang, T. F. Rosenbaum, A. Palmer, Y. Ren, J.-W. Kim, D. Mandrus, Yejun Feng, Strongly-coupled quantum critical point in an all-in-all-out antiferromagnet, under review."

Pheona Williams

Temperature-dependent Raman Spectroscopy of Doped and Undoped Topological Insulators

Topological insulators, materials whose bulk interior is insulating but have an unusual two-dimensional electronic state at the surface, represent a new class of quantum materials now being intensively investigated. The objective of this work was to study how the introduction of magnetic dopants affect the Raman response of topological insulator thin films in terms of spectral peak position and linewidth. We performed temperature dependent Raman spectroscopy on 8nm thick doped and undoped samples of capped Bi2Te3 thin films grown by molecular beam epitaxy. The dopants used were Cr and V at 2% and 4% replacement of Bi. We describe the effect of this doping on the position, and full width at half maximum of the observed thin film spectra. From the analysis of the temperature dependent Raman response, we suggest that this behavior can be understood in terms of doping-induced strain. This study revealed information about the lattice dynamical properties of Bi2Te3; in particular, how these properties evolve with the manipulation of metallic and transition metallic atoms in the topological insulator lattice.

John Woods

Suppression of magnetoresistance in thin WTe2 flakes by Surface Oxidation

Transition metal dichalcogenides (TMDCs) have received increased attention due to their exotic electronic properties, especially in the few- or mono-layer limit.  Fabricating nano-devices based on monolayer TMDCs necessitates that the desired electronic phases are preserved as the layer number decreases.  The extremely large non-saturating magnetoresistance of WTe2 and quantum spin Hall state in the mono-layer limit make WTe2 an attractive system for both technological applications and fundamental science.  Here, we relate layer-dependent transport measurements of WTe2 flakes with X-ray photoelectron spectroscopy and transmission electron microscopy characterization to show that the transport properties of thin flakes are degraded by surface oxidation and Fermi level pinning.  We find that a roughly 2nm oxide layer forms at the WTe2 surface.  The amorphous oxide layer degrades the overall mobility due to scattering from the increased disorder that is observed with decreasing flake thickness when fitting the transport behavior to a two-band electrical conduction model.   This oxide layer also induces Fermi level pinning as observed by a ~300meV work function shift at the flake surface, but due to the limited spatial extent of the work function shift, it is not the primary cause of the magnetoresistance degradation.  Our observation of the extrinsic effects on the electrical properties of WTe2 underscores the need to understand and mitigate detrimental surface conditions in order to preserve the interesting electronic phases in the few- and mono-layer limit for TMDCs and other 2D layered materials.

Lin Xiong

Pump-probe nano-spectroscopy in α-RuCl3

The Kitaev quantum spin liquid (KQSL) is an exotic state of matter predicted to exhibit Majorana fermions and gauge flux excitations. This novel state may be realized in α-RuCl3. Here we report optical pump-probe nano-spectroscopy study in an α-RuCl3 thin film. We observed a fast increase in mid-infrared (MIR) near-field reflectance for 1ps. A spectral feature is created at 0ps delay time at 900cm-1. It moves to higher frequency as delay time increases and stops at 1250cm-1. The feature becomes sharper with higher pump fluence but the highest frequency doesn’t have a clear fluence dependence. This is the first reported pump-probe spectroscopy study in α-RuCl3 and it provides a hint for photo-induced carrier and possible route to transient high-temperature superconductor.

Tsung-Han Yang

Investigating the Conduction Mechanisms in In2O3 by Nuclear Magnetic Resonance

We discuss the conduction mechanisms in In2O3 under stoichiometric (S-) and oxygen deficient (OD-) conditions by Nuclear Magnetic Resonance (NMR) measurements. The domination of oxygen hopping mechanism which has a 45 meV barrier contributes to the semiconductor-like behavior of S-In2O3 in the higher temperature region (T>220K). On the contrast, the OD-In2O3 cannot be explained by the same model, but the modified Korringa relation for electron delocalized and nondegenerate states. This new state which has 7 meV below the conduction band minimum has been located by our NMR measurements. In this model, electrons are thermally excited into delocalized states which explains the metallic behavior for OD-In2O3.

Yuan-Chi Yang

Theory of strong driving of silicon quantum dot hybrid qubits

Performing qubit gate operations as quickly as possible can suppress the effects of decoherence. For resonant gates, this requires applying a strong ac drive, resulting in strong driving effects such as fast oscillations and Bloch-Siegert shift. Here we analyze resonant X rotations of a silicon quantum double dot hybrid qubit and show that gate fidelities above 99.99% are possible, in the absence of decoherence. When coupled to 1/f charge noise typical to our device, we further show that, by applying strong driving, gate fidelities can be above 99.9%. This suggests a plausible path for achieving higher gate fidelities in silicon quantum dot qubits.

Jie Zhang

Microwave Photocurrent in Strain Layer InAs/InGaSb Edge State

We measure microwave photocurrent generated in strain layer InAs/GaSb double quantum wells (DQWs). To distinguished photoresponse between bulk and edge state, we prepare Hall bar and Corbino disk from the same wafer with same fabrication procedure. A back gate is used to tune Fermi level in the bulk gap. Results show that the Corbino disk has a negligible photocurrent demonstrating that the non-trivial photoresponse from the Hall bar sample is purely from the edge. We also study the frequency, temperature and magnetic field dependence of signal.

Songtian Sonia Zhang

Vector field STM study of iron-based superconductor

Magnetic field studies of iron-based superconductors have been largely limited to c-axis applied fields. Using a combination of a vector magnetic field and high resolution scanning tunneling microscopy/spectroscopy, we study the three dimensional field based phase diagram of vortex electronic matter in a correlated iron-based superconductor. We find that the lattice structure of the vortices is strongly correlated with the magnitude and direction of the magnetic field in an anisotropic manner. Probing the quasiparticle excitations and interferences reveals an unusual magnetic pair-breaking effect with distinct scattering vectors. These results will help us understand the interplay between Cooper pairing and emergent three dimensional vortex matter in iron-based superconductors.

Wenhan Zhang

Topological Phase Transition with Nanoscale Inhomogeneity in (Bi1−xInx)2Se3

Topological insulators are a class of band insulators with nontrivial topology, a result of band inversion due to the strong spin−orbit coupling. The transition between topological and normal insulator can be realized by tuning the spin−orbit coupling strength and has been observed experimentally. However, the impact of chemical disorders on the topological phase transition was not addressed in previous studies. Herein, we report a systematic scanning tunneling microscopy/spectroscopy and first-principles study of the topological phase transition in single crystals of In-doped Bi2Se3. Surprisingly, no band gap closure was observed across the transition. Furthermore, our spectroscopic-imaging results reveal that In defects are extremely effective “suppressors” of the band inversion, which leads to microscopic phase separation of topological-insulator-like and normal-insulator-like nano regions across the “transition”. The observed topological electronic inhomogeneity demonstrates the significant impact of chemical disorders in topological materials, shedding new light on the fundamental understanding of topological phase transition.