Posters for June 4, 2019
Quantum state tomography across the exceptional point in a single dissipative qubit
Recently, non-Hermitian systems with parity-time (PT) symmetry has been investigated in classical systems with optical, electrical and mechanical setups. These PT-symmetric Hamiltonians exhibit both real and imaginary spectrum separated by an exceptional point – where eigenenergies and eigenvectors are degenerate. These systems have potential application in non-reciprocal devices. However, due to the challenges associated with the realization of gain in quantum domain, non-Hermitian Hamiltonians have not been explored in quantum regime. Fortunately, dissipative systems can also exhibit similar behavior, which enable us to look into PT-symmetric quantum systems. Here we use circuit QED as a verstile platform to explore non-Hermitian physics in the form of a dissipative quantum system. Using a transmon circuit and by applying bath engineering techniques, we manipulate the dissipations to realize an effective non-Hermitian two-level quantum system. Here, I will be presenting about our observation of PT-symmetric transition and features associated with the existence of exceptional point in our engineered quantum system.
Non-saturating Thermopower and Signature of Quantized Thermoelectric Hall Effect in a Topological Semimetal at the Quantum Limit
Thermoelectric materials directly generate electricity from waste heat and are thus promising for energy harvesting applications. However, (sub-)room-temperature thermoelectrics have been a long-standing challenge due to vanishing electronic entropy at low temperatures. To combat this, recent theories have proposed that topological semimetals at the quantum limit can lead to a large, non-saturating thermopower and a quantized thermoelectric Hall conductivity approaching a universal value. We experimentally demonstrate the non-saturating thermopower and the signature of quantized thermoelectric Hall conductivity in the topological Weyl semimetal (WSM) tantalum phosphide (TaP). Our findings highlight the unique advantage of WSMs toward low-temperature energy harvesting applications.
Tuning electron correlation through metastability in low-dimensional vanadium oxides: Implications for next-generation computing materials and multivalent-ion cathode materials
Charge ordering and the localization of electrons in periodic wells is an intrinsic property of extended solids. Synthetic approaches that allow for precise control over this property are greatly desirable; however, systematically modulating periodic electron localization with some measure of tunability of the electron migration barriers represents a difficult challenge. The wider energy dispersion of bands when directly compared to discrete molecular orbitals in single molecules typically favors far greater delocalization of electrons and the multiplicity of sites implies that dimensional confinement of carriers can be established only for low-dimensional crystallographic motifs. One promising approach to modifying carrier density is by varying the electronic coupling across adjacent metal sites, but such attempts often result in phase transitions to entirely different crystal structures. Given the complexity of the problem, the chosen chemical system should exhibit electronic behavior spanning extremes between highly correlated and itinerant. Vanadium oxides represent such a system due to availability of multiple accessible redox states and an unequaled variety of structural motifs that can accommodate the intercalation of ions spanning the breadth of the periodic table to create a diverse set of ‘bronzes’ with the stoichiometry MxV2O5. Finally, vanadium has narrow V 3d-bands and the resulting oxides tend to crystallize in low dimensional motifs which surprisingly have the ability to avoid collapse upon topochemical transformations. Successfully achieving precise control over the strength of electron correlation in this system has significant implications for the design of materials in disparate fields. Two such examples include the design of multivalent ion cathode materials and the design of materials that exhibit controlled and reversible electronic instabilities.
As a first example, charge localization in battery cathode materials represents a significant obstacle. In fact, the coupling of a highly localized electron to a phonon mode hinders diffusion of the donated electrons through the vanadium oxide framework and must be addressed through chemical modification of the cathode material. Although moving beyond Li+ to Mg2+ and other divalent species represents the holy grail of sustainable battery chemistry, charge localization for the doubly-polarizing divalent ions becomes a significant problem, with only a small number of materials capable of this difficult feat. An alternative approach to mitigate the self-trapping of polarons is to utilize metastable phase space to design vanadium oxide frameworks that mitigate charge localization in α-V2O5. We have recently shown that metastable phases (ζ-V2O5 and γ’-V2O5) introduce frustrated coordination environments which facilitate cation diffusion and mitigate charge localization, enabling the reversible intercalation of Mg2+ and Ca2+, respectively.
As a second example, one solution to the breakdown in Dennard scaling between transistor size and power density is to replace the traditional metal-oxide-semiconductor field-effect transistors (MOSFET) with novel computing architectures that encode complexity through highly parallelized operations. Highly correlated materials which teeter at the precipice of an electronic transition are of interest because they can switch internal resistance values (often from metallic to insulating) rapidly upon external perturbation of the system; however, tailoring the temperature or gate-voltage threshold of these electronic transitions is critical. Modulation of this threshold and its magnitude requires precise control over charge localization and electron diffusion barrier to promote electronic transitions. We have recently reported examples of methods for tuning electron correlation in MxV2O5 bronzes with electronic instabilities through dimensional reduction, interlayer separation, and stoichiometry."
Spin-Based Chemical Sensing using the Diamond Nitrogen Vacancy Center
Transport and ARPES study of proximity induced superconductivity in Bi2Se3
To study how the superconductivity works in topological insulators we have carried out two experiments. One experiment studied the resistive transition on Bi2Se3 films covered with niobium islands, while the other used ARPES to measure E(k) at low temperatures in Bi2Se3 films of different thicknesses formed on niobium substrates to determine the dependence of the density of states on temperature and depth. We extract the pair amplitude as a function of energy and depth and use the Usadel equation to describe how this varies in space and extract parameters describing the Berezinskii Kosterlitz Thouless (BKT) phase stiffening transition in the resistance-temperature dependence.
Helical Chains of Diatomic Molecules as a Model for Solid-State Optical Rotation
Optical rotation (OR) measurements are a common method for distinguishing chiral compounds, but it is not well understood how intra- and intermolecular interactions affect this electronic property. Theoretical comparisons with solution-phase measurements are hampered by the difficulty of modeling solvent effects and the isotropic averaging of the experimental observable. Solid-state OR experiments/calculations could alleviate these difficulties, but experimental measurements are challenging and computational efforts have been limited due to a lack of existing procedures. We report calculated OR tensor values for a series of helices of diatomic molecules that may serve as a benchmark in the development of general-purpose electronic structure methods to compute the optical rotation of solids, in particular, molecular crystals. We find that the OR tensors for small helix clusters show poor agreement with values converged with respect to the helix size, regardless of unit cell size. The dependence of the converged OR on the dihedral angle of homonuclear helices is well described by the Kirkwood polarizability model, indicating that nearest-neighbor interactions are very important, albeit not the only relevant interactions. Basis set comparisons suggest that the aug-cc-pVDZ basis is sufficient to obtain qualitatively accurate results.
Electronic transport of exfoliated α-RuCl3 devices
The layered Mott insulator alpha-ruthenium(III) chloride (α-RuCl3) exhibits behavior similar to that expected for a Kitaev quantum spin liquid (QSL). This QSL is an intriguing phase of matter expected to exhibit fractional excitations and Majorana fermions, and therefore is a potential platform for topological quantum computation. By mechanically exfoliating α-RuCl3, we are able to probe this material in the few- to single-layer limit. We observe magnetic and structural phase transitions in α-RuCl3 as a function of sample thickness and temperature via Raman spectroscopy and electronic transport measurements. Furthermore, we demonstrate that van der Waals heterostructures composed of α-RuCl3 and graphene mechanically stacked on one another have a conductivity that is enhanced over that of isolated graphene, which we attribute to conduction through the adjacent α-RuCl3. This is unexpected since α-RuCl3 is a 1 eV Mott insulator. Signatures of magnetic phase transitions are also seen in the low-temperature electronic transport. Finally, we discuss ongoing measurements of optical spectroscopy aimed at elucidating the nature of the interaction between graphene and flakes of this strongly correlated material in our devices.
A superlattice approach to analyze the x-ray diffraction of Ruddlesden-Popper films
Ruddlesden-Popper phases are an important class of superlattices composed of perovskite (ABO3) and rock salt (AO) with general formula (ABO3)nAO. Diverse properties including high-temperature superconductivity, colossal-magnetoresistance, and record-breaking tunable dielectrics have all been observed in Ruddlesden-Popper systems, but the quality of these films is limited by the precision of the calibration. To improve the layering perfection of Ruddlesden-Popper superlattices, we apply the x-ray diffraction (XRD) approach developed for superlattices of III-V semiconductors to quantitatively compute flux corrections to Ruddlesden-Popper films grown by molecular-beam epitaxy. We demonstrate the precision of this approach by synthesizing an n=10 Ruddlesden-Popper phase, (SrTiO3)10SrO, with unparalleled layering perfection and by iteratively improving (SrSnO3)2SrO films. We also modify standard methods that correct for XRD sample height error to make them applicable to superlattices and demonstrate the generality of our approach by analyzing another homologous series, the Aurivillius phases.
Vortex Lattice Structure and Topological Superconductivity in the Quantum Hall Regime
Chiral topological superconductors are expected to appear as intermediate states when a quantum anomalous Hall system is proximity coupled to an s-wave superconductor and the magnetization direction is reversed. In this paper we address the edge state properties of ordinary quantum Hall systems proximity coupled to s-wave superconductors, accounting explicitly for Landau quantization. We find that the appearance of topological superconducting phases with an odd number of Majorana edge modes is dependent on the structure of the system's vortex lattice. More precisely, vortex lattices containing odd number of superconducting flux quanta per unit cell, always support an even number of chiral edge channels and are therefore adiabatically connected to normal quantum Hall insulators. We discuss strategies to engineer chiral topological superconductivity in proximity-coupled quantum Hall systems by manipulating vortex lattice structure.
Boundary Conditions for a Continuum Model of Lateral Interfaces in Transition Metal Dichalcogenides
Tight-binding models of in-plane, lateral heterostructures of commensurate transition-metal dichalcogenides (TMD), such as MoS2-WS2 and MoSe2-WSe2 have demonstrated the appearance of laterally localized effective one-dimensional interfacial and edge states with unique features. These states lie within the band gap of the bulk structure and may provide a stable, tunable one-dimensional platform for possible use in exploring Majorana fermions, plasma excitations, and potential spintronics applications . Motivated by the possible versatility of these modes in a variety of 2D systems, we now explore their appearance in continuum model descriptions of effective massive Dirac systems at low energy. We use different k×p models to characterize TMD nanoribbons and analyze proposals for the appropriate boundary conditions at lateral interfaces with various terminations. In particular we examine an M-Matrix approach  and envelope function approximation to obtain suitable boundary conditions.
 O. Avalos-Ovando et al., J. Phys.: Cond. Matt. 31, 213001 (2019).
 C. G. Peterfalvi et al., Phys. Rev. B 92, 245443 (2015).
Second Harmonic Generation as a Probe of Broken Mirror Symmetry in 1T-TaS2
Using the material 1T -TaS2, we demonstrate that rotational-anisotropy second harmonic generation (RA-SHG) is an efective probe of broken mirror symmetry. We also fnd that RA-SHG can differentiate between mirror symmetry-broken structures with opposite planar chirality. We identify a binary indicator for broken mirror symmetry, as well as an indicator for the sense of the planar chirality. Additionally, we fnd evidence for bulk mirror symmetry-breaking in the incommensurate charge density wave phase of 1T -TaS2, which we speculate can be attributed to long-range variations in the c-axis stacking order. Our results imply that RA-SHG can be used to identify broken mirror symmetry in other nontrivial contexts, such as in the cuprate superconductors, where various candidate pseudogap phases have been proposed which break mirror symmetry.
Emergence of Stripe domains in the Metal-Insulator Transition in Ti doped Bilayer Calcium ruthenate
Transition metal oxides (TMOs) have been widely studied in the last few decades since they have a wide range of unique behaviors owing to the simultaneously active charge, spin, lattice, and orbital degrees of freedom. Ruthenates, with Ruddlesden-Popper-type layered structure, represent a good example here due to a wealth of collective correlated electron phenomena. The 4d electron orbitals of Ru are more extended than 3d orbitals, giving rise to a rich variety of exotic properties in ruthenates. Emergent electronic and magnetic state has been reported in the bilayer ruthenate Ca3Ru2O7 upon doping with a small concentration of Ti on the Ru sites. Phase separation is predicted due to the first-order nature of the Metal Insulator transition. We perform real-space mapping of local conductance on Ca3(Ru0.9Ti0.1)2O7 bulk crystal by Microwave Impedance Microscopy. We observe metallic stripes that align preferentially along certain crystalline axes, suggesting the anisotropic elastic strain as the key interaction in this system.
Liquid-phase exfoliated metal diboride nanosheets: Solvent dispersibility and applications in polymer nanocomposites
Metal diborides comprise a class of inorganic compounds with a range of exceptional properties, including ultra-high temperature melting points, relatively high temperature superconductivity and high hardness. We have previously shown that by subjecting bulk metal diboride powders mixed with suitable solvents or surfactant solutions to ultrasonication, we can produce stable liquid dispersions of metal diboride nanosheets. In this work, we further characterize these liquid-phase exfoliated metal diboride nanosheets by assessing their behavior in different solvents using Hansen Solubility Theory. We determine Hansen Solubility Parameters for two representative metal diboride compositions to be able to predict their processability in solvents and solvent mixtures. Additionally, we explore the potential applications of these materials in producing functional polymer nanocomposites by mixing dispersions of metal diboride nanosheets with liquid photocurable resins and subjecting the mixtures to stereolithography-based 3D printing. We demonstrate the successful and uniform incorporation of several different metal diboride nanosheet compositions into various complex-shaped 3D-printed structures.
Controlling Nanoarchitecture to Investigation Novel Metastable Thin Film Heterostructures.
Two dimensional (2D-) materials are fundamentally different from their bulk equivalents. When the dimensionality of bulk materials is reduced, the dominant interactions change from long-range and local to surface and local, respectively. Well-known examples of this behavior are MoX2 compounds where X is the dichalcogenide S or Se. Modifying the environment surrounding layers of 2D-materials is another avenue to alter its properties. These novel properties absent in the bulk structure or ‘emergent behavior’, make this class of materials a research priority. One way to prepare 2D materials is via designed precursors. In this method, finite amounts of atoms are deposited in a certain sequence that mimics the structure of the targeted product. These precursors are gently annealed to facilitate crystallization into the desired product. This method has been used to prepare new heterostructures with specific atoms in certain coordination environments to tune properties as well as study the influence of nanoarchitecture on transport and thermoelectric properties. It is the nuances of these materials at a finer level have the biggest influence on the resultant properties. In this poster, I will present of the synthesis of a new (PbMn0.5Se1.5)¬1.14 VSe2 compound to probe the magnetic properties of a single layer of MnSe and tune it by modifying the adjacent layer/s interfacing with the magnetic layer. I will also discuss the systematic study of layering sequence on the properties of a [(SnSe)1+delta]m[TiSe2]n family of compounds. Both of these projects will focus on how the precise synthesis and annealing conditions are required to prepare the desired compounds with unique properties.
Interfacing spin qubits in quantum dots and donors—hot, dense, and coherent
Semiconductor spins are one of the few qubit realizations that remain a serious candidate for the implementation of large-scale quantum circuits. Excellent scalability is often argued for spin qubits defined by lithography and controlled via electrical signals, based on the success of conventional semiconductor integrated circuits. However, the wiring and interconnect requirements for quantum circuits are completely different from those for classical circuits, as individual direct current, pulsed and in some cases microwave control signals need to be routed from external sources to every qubit. This is further complicated by the requirement that these spin qubits currently operate at temperatures below 100 mK. Here, we review several strategies that are considered to address this crucial challenge in scaling quantum circuits based on electron spin qubits. Key assets of spin qubits include the potential to operate at 1 to 4 K, the high density of quantum dots or donors combined with possibilities to space them apart as needed, the extremely long-spin coherence times, and the rich options for integration with classical electronics based on the same technology.
Lock-step evolution of electron-phonon coupling, pseudogap and superconductivity in high-Tc cuprates
Lattice vibrations are known to have strong presence in many cuprate high-temperature superconducting systems. I will first demonstrate a low-energy phonon hybridization that may act as a precursory instability to the formation of charge order in cuprates. Then I will demonstrate how a particular lattice vibrational mode can be activated via broken local symmetry or reduced doping in high-Tc cuprate Bi2Sr2CaCu2O8, subsequently helping double the superconducting Tc amid strong electronic correlation effects.
Defects in Antiferromagnetic Topological Insulator MnBi2Te4
The coupling of topological electronic states and magnetism can lead to various exotic phenomena such as quantum anomalous Hall effect, axion insulator, etc. Recently, single crystals and thin films of intrinsic antiferromagnetic topological insulator MnBi2Te4 has been successfully synthesized, so it becomes the focus of recent investigation. It is crucial to understand and control defects in this material because they affect its electronic and magnetic properties. In this talk, I will summarize the recent progress on this material and present our study of defects in single crystals of MnBi2Te4 using scanning tunneling microscopy (STM). We identify the dominant defects, Mn_Bi antisites in the second layer of Bi, which may impact the magnetic properties of MnBi2Te4. In addition, we observe another interesting defects with pronounced defect states near the conduction band edge. This work is supported by NSF DMR-1506618.
Effect of Conformation on Electron Localization and Delocalization in Infinite Helical Chains [X(CH3)2]∞ (X = Si, Ge, Sn, and Pb)
Ubiquitous σ bonds dominate bonding in all molecules and define the framework of a molecule. It has been long known that electrons in σ bonds can delocalize throughout a molecule. However, the nature of σ-electron delocalization is not fully understood due to its complexity. One of the important aspects of σ-electron delocalization is its conformational dependence, first noted in oligosilanes where properties of a molecule change drastically when it changes conformation. We examine characteristics of σ-electron delocalization in linear infinite polysilane, polygermane, polystannane, and polyplumbane, in which effects of conformation on the delocalization decouple from other interferences, such as delocalization through substituents. Effective hole mass is used to indicate an extent of the delocalization, while a position of the Fermi level in reciprocal space is used to determine the dominant mechanism of delocalization. The simple model used to explain the origin of the conformational dependence of electron delocalization in saturated compounds is verified by comparison with density functional theory calculations.
Posters for June 11, 2019
Symmetry Protected Qubit: Fluxonium with tunable interaction between states
I will discuss symmetry-protected qubits whose quantum states are encoded in the parity of fluxons in a superconducting loop. The qubit is composed of a Cooper pair box and a superinductor arranged in a superconducting loop. I will demonstrate that the interaction between logical states can be controlled by the charge on the CPB island thru the symmetry of qubit wavefunctions. By applying fast gate pulses, the fluxon-pairing qubit can by adiabatically switched between the protected and unprotected states for quantum state initialization and readout. Some preliminary time-domain experimental results will be presented.
Simulation Tool for Coupled Quantum Transport and Electrodynamics
Accurate simulation of light-matter interaction at the nanoscale requires a computational approach that combines quantum transport and electrodynamics, self-consistently and at every time step. Here we present the development of a new tool for this purpose — Quantum Transport and Electrodynamics Simulation Tool (QuTEST) — which self-consistently couples a full-wave finite-difference time-domain (FDTD) electrodynamics solver for the potentials with a quantum transport solver. The current density and charge density are calculated from the quantum solver and inputted back into the FDTD; FDTD produces the vector potential and scalar potential, both of which are needed in the Hamiltonian that dictates quantum transport. The non-traditional 3D FDTD formulation for the potentials is implemented by discretizing Maxwell’s equations in the Lorentz gauge.
Multi-Disciplinary Approach to Rotoelectric Materials
Extraordinary advances in nanoscience and molecular rotors have paved the way to an investigation of an as-yet unknown class of ferroelectric materials, referred to as roto-electric materials. These materials are defined by ordered systems of free-rotating permanent dipole moments which can rotate about an axis, or “roto-electric” systems. As such, a 2-dimensional film of such rotating dipoles holds great potential for the observation of ferroelectric and antiferroelectric phases.This is interesting as i) this is fundamentally interesting and not naturally occurring, except in surface adsorbed gases and ii) it has applications in signal processing and electronics.
To create a roto-electric lattice, one approach is to use hexagonal tris(o-phenylene)cyclotriphosphazene (TPP), which has hexagonal channels into which a specially-designed dipolar molecule can fit. Thin films of these host-guest compounds can be and have been grown, as successful guest rotors will evaporate with hexagonal TPP directly into the channels. By evaporating a thin film onto the surface of a capacitor, collective behavior of included rotor molecules can be probed by measuring the capacitance and the loss tangent of the capacitance.
Another approach is using Langmuir-Blodgett films, which are more convenient for controlling the distance between dipoles, allowing careful synthesis to produce a tailor-made one-dimensional structure. High Curie temperatures and collective behavior should also be easier to achieve. However, LB films come with their own challenges, such as transferring these long, tipsy rods from their native aqueous subphase to a sturdy substrate with a capacitor.
An overview of both these approaches, their successes and challenges, will be discussed.
Tunable Strong Coupling in Carbon-Based Hybrid Terahertz Metasurface
Here, we consider an array of metallic resonators on ultraflexible thin polyimide films and show that these metasurfaces exhibit intrinsically strong interactions in the terahertz (THz) frequency regime. Instead of coupling to external cavities, bright and quasi-dark eigenmodes of four gap symmetric split ring resonators (SRRs) couple mutually and internally, which results in the opening of a transparency window within the absorption band of the SRRs. This new approach could have important implications for the development of future active and nonlinear metadevices.
In-situ Fermi-level tuning of epitaxial films in STM
Graphene-based platform for thin film growth and STM with back-gating capabilities
Doping a material provides access to quantum phases such as superconductivity or the pseudo-gap phase. For a detailed atomically-resolved in-depth study of a material with a complex phase diagram, it is beneficial to implement an in-situ Fermi level tuning which can be done by back-gating. Gating is a relatively reliable technique in transport measurements, but combining it with epitaxial film growth and scanning tunneling microscopy (STM) presents many technical challenges. Here, we report the design of a robust back-gating device for a versatile thin film growth and subsequent in-situ Fermi level tuning in STM. Graphene or graphene-like material serves as the platform for the epitaxial growth, while a range of materials can be used as an insulating gate depending on the particular requirements of the experiment, including but not limited to SiO2, hBN, Al2O3, SrTiO3, etc. We demonstrate successful Fermi level tuning in Bi2Te3 on graphene and SnTe on graphene in STM. The same platform also allows studying exfoliated samples as well as polymer-transferred. We demonstrate successful implementation of the same platform for studying twisted-bilayer graphene devices based on CVD-grown graphene.
Spin-Phonon Coupling, Spin Waves, and Other Phenomena in Layered Magnetic Materials via Raman Spectroscopy
Raman spectroscopy, imaging, and mapping are powerful non-contact, non-destructive optical methods to probe the fundamental physics of graphene and related two-dimensional (2D) layered materials. An amazing amount of information is quantified from the spectra such as layer thickness, disorder, edge and grain boundaries, strain, etc. More interestingly for 2D materials is that Raman efficiently probes the evolution of the electron-phonon and spin-phonon interactions as a function of temperature, laser energy, polarization, and magnetic field. Using our unique magneto-Raman capabilities, we have studied the magnetic properties of the metal phosphorus trisulfide family (XPS3, where X = Fe, Mn, and Ni) which are layered antiferromagnetic semiconductors. While the three materials have the same crystal structure, their varying spin structures result in distinct behavior as a function of temperature and magnetic field, which is presented here. In FePS3, we investigate spin-phonon coupling, as well as the splitting and shifting of non Γ–point phonon modes below the Neel temperature that is not present in MnPS3, as well as the emergence of a spin-wave with anomalous symmetry behavior. In addition, we have studied the apparent two-magnon mode in NiPS3 under various conditions.
Atomic manipulation of defects in the layered semiconductor 2H-MoTe2
Here we present a migration of a native defect in the bulk transition metal dichalcogenide, MoTe2 by voltage pulsing a scanning tunneling microscopy (STM) tip. Bulk MoTe2 was cleaved at room temperature in ultrahigh vacuum and imaged with a cut PtIr tip at 9K. Native defects in the MoTe2 are present throughout the sample. In topographic imaging, the long-range protrusion of a bright defect indicates the species is charged and we image the defects at different depths below the surface. After pulsing the tip at 6V, we notice the subsurface defects change in appearance to resemble the brighter and more pronounced ones that exist near the surface. We attribute this to migration of these defects between layers of the MoTe2. Additionally, the bright protrusions present with an ionization feature in tunneling spectroscopic mapping which indicates that the charge state of this defect can be manipulated by the band bending caused by the tip. The migrated defects exhibit a similar spectroscopic signature. We also present DFT results that we use to clarify the identification of these native defects and energy barriers for migration between layers of 2H-MoTe2.
Ultrafast Spin and Charge Dynamics in Monolayer WSe2-Graphene Heterostructures
Monolayer transition metal dichalcogenides (TMDs) have attracted interest due to their long spin/valley lifetimes and the ability couple spin/valley polarization to helicity of light. In addition, TMDs can strongly complement other materials, such as graphene, by acting as a means of optical spin injection or proximity coupling. Recently, multiple groups have reported proximity mediated charge transfer and optical spin injection in TMD/graphene heterostructures. However, the spin transfer dynamics across TMD/graphene interfaces remain largely unexplored.
Here we employ time-resolved Kerr rotation (TRKR) microscopy to image spin/valley dynamics in monolayer WSe2/graphene heterostructure devices. Spatial maps demonstrate long-lived spin/valley lifetimes on bare WSe2 but reveal a suppression of spin-valley signal at the WSe2/graphene overlap interfaces. Time delay scans show these interface lifetimes to be quenched up to 3 orders of magnitude in comparison to those in bare WSe2. Furthermore, photoluminescence maps exhibit quenched emission at the interfaces while photoconductivity is enhanced in these regions, demonstrating efficient charge transfer from WSe2 to graphene. Consequently, we attribute the ultrafast spin/valley quenching to the transfer of spin information by conducted charge carriers.
Symmetry Analysis of the Resonant X-Ray Scattering Cross Section for Long-Range Electronic Order
Resonant X-ray scattering (RXS) has developed into the choice technique to study the long-range electronic order and low-energy excitation spectra in condensed matter systems. In this poster, I will describe the unique capabilities of RXS to probe the symmetry of the underlying ground state and excitations, disentangling the magnetic, orbital, and charge nature. When the incident photon energy is tuned to an electronic resonance of a constituent atom, the atomic scattering factor becomes a tensor and is sensitive to the charge anisotropy of the resonant atom. Such anisotropy is subject to the point or local site-symmetry group, which can experimentally inferred for the corresponding symmetry-broken phase. I will show how this symmetry-based approach can be effectively employed to gain insight into the nature of the electronic order and how this approach can be extended to the spectra of elementary excitations. I will put particular emphasis on our recent results on the antiferromagnetic metal RuO2 and the novel electronic phases in Sr2CrO4, where electronic order emerges due to the coupling between the spin and orbital degrees of freedom.
Spin valve-like magnetoresistance of a topological insulator in proximity to a perpendicular magnet
Topological insulators (TIs) have a variety of unique transport properties, one of which is symmetry-protected surface states. Efforts have been made to control the surface states by symmetry breaking via proximity magnetization, which is typically achieved by placing a ferromagnetic or antiferromagnetic insulator in direct contact with the TI. We report a spin valve-like magnetoresistance switching phenomena when the TI Bi2Se3 is in contact with a Co/Pt multilayer, which has a perpendicular magnetic anisotropy. The magnetoresistance is mostly isotropic, as evidenced by the resistance switching when either in-plane or out-of-plane magnetic fields are applied. This effect occurs at low temperatures, and the switching fields are incompatible with the behavior of Co/Pt. It appears the phenomenon is related to a magnetized TI surface and/or bulk states.
On-Chip Strain Enhanced Superconductivity in 1T’-MoTe2
Many two-dimensional (2D) materials, such as transition metal ditellurides (TMDs), have been found to have unique phase transitions with respect to strain. Demonstrating on-chip control over these exotic phase transitions could lead to a revolution of new devices structures, the sensitivity of these phase transitions to strain could even lead to strain-based electronics that may potentially supplant conventional field-effect transistors (FETs). Recent work from our group has combined the use of dynamic and static strain, from ferroelectrics and thin film stress, to switch between semimetallic and semiconducting states in MoTe2 . Additionally, semimetallic 1T’-MoTe2 has been known to exhibit increased superconductivity transitions temperatures from the nominal value of 100 mK up to to 7 K under applied pressure. In this work, we explore enhancing the superconductivity transition temperature in 2D 1T’-MoTe2 via on-chip static straining techniques for quantum nanoelectronics applications. We observe this enhancement electrically from pseudo 4 point device structures patterned on exfoliated 1T’-MoTe2. The device structures utilize the combination of differential thermal contraction from the electrodes and thin film compressive stress from a deposited encapsulation layer as the sources of strain. We find that variations in device geometry drastically affect the onset transition temperature of the MoTe2 channel. We have seen onset transition temperatures in our device structures as high as 3.5 K and the disappearance of these transitions upon magnetic field application.
Fabrication and characterization of Van der Waals Josephson junction with ferromagnetic weak link
Two-dimensional van der Waals materials can be assembled into atomically thin heterostructures that gives rise to interesting hybrid electronic systems. These heterostructures are highly tunable with gate and number of layers. Superconducting proximity effects in superconductor-normal metal(S-N) heterostructures have garnered interest in recent time. Here we try to combine 2D TMD superconductor(S) with 2D ferromagnetic material(F) to study the interplay of superconductivity and ferromagnetism in S-F-S Josephson junction. We will perform low temperature characterization of these Josephson Junctions to search for junctions and spin triplet superconductivity.
Higher-Order Floquet Topological Phases
In this poster, we discuss the theoretical discovery and characterization of higher-order Floquet topological phases dynamically generated in a periodically driven system with mirror symmetries. We demonstrate that these phases support lower-dimensional Floquet bound states, such as corner Floquet bound states at the intersection of edges of a two-dimensional system, protected by the nonequilibrium higher-order topology induced by the periodic drive. We characterize higher-order Floquet topologies of the bulk Floquet Hamiltonian using mirror-graded Floquet topological invariants. Moreover, we show that bulk vortex structures can be dynamically generated by a drive that is spatially inhomogeneous. We show these bulk vortices can host multiple Floquet bound states. This "stirring drive protocol" leverages a connection between higher-order topologies and previously studied fractionally charged bulk topological defects.
Topological phase transition in WTe2
Multifunctional quantum materials are important for realizing critical phenomena, manifesting novel switching behavior and producing robust interface physics. The multifunctionality of 2D materials is exemplified by WTe2 which is a type-II Weyl semimetal with non-saturating magnetoresistance in its bulk, transforming into a superconductor under hydrostatic pressure, or transforming into a quantum spin hall insulator (QSHI) in its monolayer limit, which can be tuned to a superconductor with electrostatic gating. In many of these transformations, the tuning of the carrier density has been shown to play an important role. Here we demonstrate a non-trivial non-monotonic change in electronic structure of WTe2 upon in-situ alkali metal dosing, realizing an interface field effect as a pathway for tuning behavior of 2D materials
Area-Selective Atomic Layer Deposition and Atomic Layer Etching
Due to the introduction of 7nm-and-beyond technology nodes in the semiconductor industry and the rising need for bottom-up device nanopatternings, there is a growing interest for area-selective deposition with angstrom-scale resolution, high selectivity and uniformity. Thermal area-selective deposition, which integrates atomic layer deposition (ALD) and atomic layer etching (ALE), can be one of the methods to address these requirements.
Herein, I will be giving a brief overview of atomic layer deposition and atomic layer etching methods, and show how combining these two can be used for selective deposition in future devices by taking advantage of inherent material properties. For this I will present experimental data of TiO2 thin film deposition on SiO2 vs hydrogen-terminated Si (Si-H), performed in a homebuilt reaction chamber. By combining super-cycles of TiO2 ALD with intermittent TiO2 ALE, we achieved significant selectivity on the substrates compared to ALD-only process.
Overall, our results provide a fundamental understanding of the area-selective deposition process, show, how the selectivity can be further improved, and indicate, that this method is viable in producing future thin-film devices with atomic-scale precision.
Single molecule conductance of TCNQ and F4TCNQ
By investigating the conductance of single molecules we gain insight into the fundamental physics of electron transport. Such an understanding can lead to molecular-based electronic components and sensors. We study the transport properties of tetracyanoquinodimethane (TCNQ) and tetrafluoro-TCNQ (F4TCNQ) at the level of density functional theory within the non-equilibrium Green’s function formalism (NEGF-DFT). Experiments have measured at least three distinct conductance values for a single molecule of TCNQ and F4TCNQ. We construct a model system which consists of two gold electrodes with a single molecule arranged in different orientations. Four distinct orientations are defined, bidentate-bidentate (b-b), monodentate-bidentate (m-b), monodentate-monodentate (m-m), and flat. We find that the conductance depends on the molecule’s orientation between the electrodes which ranges over an order of magnitude. In both molecules, the b-b and m-b orientations produce lower conductance (0.02 – 0.1 G0) while the flat orientation gives a slightly higher conductance of 0.2 G0. Surprisingly, the m-m shows the largest conductance at 0.4 and 0.6 G0 for F4TCNQ and TCNQ respectively. The results are in qualitative agreement with scanning tunneling microscopy break junction (STM-BJ) experiments, which show two low and one high conductance value. These results open up the possibility of single molecules being used as a switch if the orientation of the molecule can be controlled.
Jesus del Carmen Valdiviezo Mora
The Effect of Backbone on the Interplay of Coherence and Incoherence in Nucleic Acid Conductance
DNA duplexes are promising structures to build functional nanodevices, such as biosensors and chip technologies, given their ability to efficiently transport current over long distances. In these systems charge transfer proceeds mainly by coherent tunneling over short distances and by incoherent charge hopping over longer distances. Recent works reported the existence of an intermediate coherent-incoherent regime in DNA sequences with stacked guanine–cytosine (GC) base pairs supported by charge delocalization along several base pairs. In this work we study the single molecule conductance of peptide nucleic acid (PNA) duplexes with stacked GC base pairs and compare the finding with previous studies on DNA . Our results show that PNA has higher conductance than DNA and incoherent transport is the dominant mechanism, despite the stronger charge delocalization present in PNA. Theoretical simulations suggest that the peptide backbone enhances the electrode-molecule coupling, and in consequence the coherent contribution to the total conductance is reduced.
Coherence properties of shallow donor qubits in ZnO
Defects in crystals are leading candidates for photon-based quantum technologies, but progress in developing practical devices critically depends on improving defect optical and spin properties. Motivated by this need, we study a new defect qubit candidate, the shallow donor in ZnO. We demonstrate all-optical control of the electron spin state of the donor qubits and measure the spin coherence properties. We find a longitudinal relaxation time T1 exceeding 100 ms, an inhomogeneous dephasing time T2* of ~17 ns, and a Hahn spin-echo time T2 of ~50 us. The magnitude of T2* is consistent with the inhomogeneity of the nuclear hyperfine field in natural ZnO. Possible mechanisms limiting T2 include instantaneous diffusion and nuclear spin diffusion (spectral diffusion). These dephasing mechanisms suggest that with isotope and chemical purification qubit coherence times can be extended. This work motivates further research on high-purity material growth, quantum device fabrication, and high-fidelity control of the donor:ZnO system for quantum technologies.
Uniaxial strain effect on superconductivity in LaAlO3/SrTiO3 nanostructures
We investigate the effects of uniaxial strain on superconductivity in nanowires created at the LaAlO3/SrTiO3 interface using conductive atomic force microscope (c-AFM) lithography . C-AFM-written areas are associated with Z-oriented ferroelastic domains, surrounded by in-plane insulating regions . Application of external uniaxial stress is expected to displace the ferroelastic domain boundaries, either inward or outward, depending on the sign. Our initial experiments indicate that tensile and compressive strains profoundly affect the superconducting state at milli-kelvin temperatures. Uniaxial stretching of the nanowire in the parallel direction is found to completely suppress the superconducting state, while reversal of the applied strain restores superconductivity. We discuss implications for understanding possible role of ferroelastic domain walls in electron-pairing mechanisms.
 C. Cen, et al., Nature Materials 7, 298 (2008).
 Y.-Y. Pai et al., Phys Rev Lett 120, 147001 (2018).
A multi-scale model of mechanical relaxation in twisted trilayers
Two-dimensional van der Waals layered materials (e.g., twisted bilayer graphene) provide a platform to study correlated many-body physics and have potential device applications. However, these layered systems are computationally challenging to model by conventional methods due to their large supercells. Here, we present a multi-scale model to efficiently calculate the in-plane mechanical relaxation pattern in incommensurate van der Waals heterostructures at arbitrary twist angles and lattice mismatch. We adopt a continuum model to describe lattice relaxation and a generalized stacking fault energy, computed from the density functional theory, to account for interlayer couplings. We obtain the optimized structure by minimizing the total energy. Our model extends the computationally accessible regime to layered systems with relatively small twist angles and large moiré patterns. This model can be applied to a wide range of materials, including those with no empirical interlayer coupling potential available such as transition metal dichalcogenides. In this presentation, we use twisted bilayer graphene and twisted trilayer graphene as our model system.