Posters for June 8, 2017
Ferromagnetism and spin-dependent transport at a complex oxide interface
Interfacial oxide systems have garnered a lot of attention after the discovery of a conducting interface between two insulating oxides LaAlO3 and SrTiO3. This interfacial system has been shown to have two possible ground states, superconducting or ferromagnetic. Here we show via magneto-transport measurement at low temperatures that the MBE-grown polar/non-polar NdTiO3/SrTiO3 (NTO/STO) interface hosts a ferromagnetic state. We observe a very large negative magneto-resistance ratio (MR) of up to -95 % at 150 mK. The amplitude of the negative MR decreases with increasing temperature and acquires a positive curvature above ~ 4 K. By modeling electron transport at the interface using spin-dependent hopping, we obtain excellent quantitative agreement with the data over more that an order of magnitude in temperature. We find that the MR depends exponentially on the temperature (T) and magnetization (M) of the sample, following MR∝Exp[-α(M/M_s)^2/k_BT], where α characterizes the spin-dependent energy cost of hopping. The system develops superparamagnetism below ~4 K. Above 1.5K, all the superparamagnetic moments are above their blocking temperature and hence thermally fluctuate. Below 1.5K the larger moments are blocked, leading to a hysteretic MR. Furthermore, we show that at low temperatures time-dependent MR measurements are necessary to distinguish between magnetic effects intrinsic to the sample and extrinsic effects such as heating, which can also produce a hysteretic signal. We conclude by discussing possible microscopic mechanisms that could lead to the formation of localized magnetic moments at the NTO/STO interface. Oxygen vacancies, dislocations or substrate impurities do not play an important role in determining the electronic properties of this system.
ARPES Investigation of Strong Point Defects on the Surface of a Topological Insulator
Topological insulators (insulators with non-trivial band topology in the bulk) acquire unique gapless edge states when placed in an ordinary insulator (e.g. air). Because these edge states are symmetry protected, they exhibit useful properties such as ballistic transport and massless dispersion, and are resilient to even extreme local defects. Here we investigate the effect of non-magnetic defects on the surface of a topological insulator, and show numerically that defects give rise to resonant surface states which cluster around the defects. We also report on experimental efforts to find these resonant states through the use of synchrotron-based X-ray spectroscopy on defect-rich samples of bismuth selenide.
Statistical Testing of an Adiabatic Quantum Computer's Qubit Chains
We chain physical superconducting qubits to create a singular logical qubit, exploring how chain length influences result validity. We incorporate thermalization and annealing timing parameter variations to build statistical models predicting the expected reliability of future annealing samples. We also show a skewing or bias in relatively large qubit chains that deviate from expected uniform distributions.
Toward scalable quantum photonic devices based on InAs Quantum Dots
Quantum photonic devices promise dramatic advantages in secure communications, computing, and metrology. InAs quantum dots (QDs) have long been of interest for such applications and many required functions, such as all-optical coherent control of single spins confined in QDs, have been demonstrated. However, there are two primary roadblocks to the scalable production of single photon quantum photonic devices based on InAs QDs: poor spatial control and poor spectral control. Poor spatial control hampers the production of devices incorporating multiple QDs coupled to photonic device components. Although site-templated QDs have been grown, their optical quality tends to be very poor and thus randomly-positioned self-assembled QDs are most commonly used. Poor spectral control arises from inhomogeneous distribution of optical transition energies inherent to the self-assembly process and inhibits deterministic strong coupling of multiple QDs to either photonic device elements or external optical fields. We are developing a material platform intended to overcome both roadblocks. We aim to achieve spatial control by using a pre-patterned substrate to nucleate “tracer” QDs. The “tracer” layers of such “tracer” QDs can transfer the pattern through strain propagation that provides preferential nucleation sites for QDs in subsequent layers. Optically active QDs grown on top of these “tracer” QDs, far above the patterned substrate, can have much better optical properties. We aim to address spectral control by making our optically active element a quantum dot molecule comprised of two QDs stacked along the growth direction and embedded in a p-i-n junction. This structure allows for the local application of an electric field that can tune the indirect optical transitions of the quantum dot molecule over a range much larger than that of single QDs. We anticipate that this material platform will allow the deterministic and scalable fabrication of quantum photonic devices incorporating InAs QDs.
Particle partition entanglement of one dimensional spinless fermions
We investigate the scaling of the Rényi entanglement entropies for a particle bipartition of interacting spinless fermions in one spatial dimension. In the Tomonaga-Luttinger liquid regime, we calculate the second Rényi entanglement entropy and show that the leading order finite-size scaling is equal to a universal logarithm of the system size plus a non-universal constant. Higher-order corrections decay as power-laws in the system size with exponents that depend only on the Luttinger parameter. We confirm the universality of our results by investigating the one dimensional t − V model of interacting spinless fermions via exact-diagonalization techniques. The resulting sensitivity of the particle partition entanglement to boundary conditions and statistics points to its utility in future studies of quantum liquids.
Coherent Quantum Control of NV Center Using a Mechanical Resonator
With both their spins and orbitals(cryogenic temperature) strongly coupled to environmental electric, magnetic and strain field, plus long coherence time that preserves at room temperature, nitrogen vacancy(NV) centers in diamonds are promising candidates for quantum information processing and sensing. Coherent spin-phonon(strain) and orbital-strain interaction of NV centers are of great interests for their applications in force sensing at nanoscale, qubit cooling/control of optomechanical system, constructing mechanical quantum transducers in hybrid quantum system, etc. Engineeing bulk diamond as high Q mechanical resonator, and GHz frequency piezoelectric transducer as phonon source, we explore the interplay between phonon and NV center Qubit in the quantum regime.
Theory of Exciton Energy Transfer in Carbon Nanotube Composites
Carbon nanotubes (CNTs) are promising building blocks for organic photovoltaic devices, owing to their tunable band gap, mechanical and chemical stability. We study intertube excitonic energy transfer between pairs of CNTs with different orientations and band gaps. The optically bright and dark excitonic states in CNTs are calculated by solving the Bethe-Salpeter equation. We calculate the exciton transfer rates due to the direct Coulomb interactions, as well as the second-order phonon-assisted processes. We show the importance of phonons in calculating the transfer rates that match the measurements. In addition, we discuss the contribution of optically inactive excited states in the exciton transfer process, which is difficult to determine experimentally. Furthermore, we study the effects of sample inhomogeneity, impurities, and temperature on the exciton transfer rate. The inhomogeneity in the CNT sample dielectric function can increase the transfer rate by about a factor of two. We show that the exciton confinement by impurities has a detrimental effect on the transfer rate between pairs of similar CNTs. We show that the second-order phonon-assisted hopping process between bright excitonic states is as fast as the first-order one (∼ 1 ps). Moreover, second-order exciton transfer between dark and bright states, facilitated by phonons with large angular momentum, has rates comparable to bright-to-bright transfer, unlike the orders-of-magnitude disparity between the corresponding first-order rates. Therefore, dark excitonic states, which are difficult to probe with common measurement techniques, provide an efficient pathway for exciton transfer in CNT composites. This work demonstrates the importance of second-order processes for the understanding and predictive modeling of exciton transfer in CNT composites.
On-chip quantum and nonlinear optics in silicon nitride
Silicon nitride (Si3N4) is a versatile platform for quantum and nonlinear optics on chip due to its high linear and nonlinear index of refraction combined with low optical losses and CMOS compatibility. This has made Si3N4 a platform of choice for generating quantum states of light on chip, such as squeezed states, as well as for performing integrated quantum interference experiments, with promising applications in continuous variable quantum information processing and quantum enhanced sensing. We have reported the generation of bright squeezed light using Si3N4 microring optical parametric oscillators above threshold. We have shown mechanisms to tune this degree of squeezing using integrated microheaters and coupled rings. Our work has the potential to realize continuous variable EPR-type entanglement on a fully integrated silicon photonic chip, making it a scalable resource for future quantum technologies.
Quantum Quench of Lieb-Liniger Hamiltonian
Controlling Adiabatic Quantum Computing
Adiabatic quantum computing (AQC) is a computational model that solves problems through continuous-time quantum dynamics. Per the adiabatic theorem, the quantum state can be continuously transformed under slowly evolving changes of the system Hamiltonian. An important part in designing programs for the AQC model is specifying the controls that transform this Hamiltonian from its initial to final form. We investigate the effects of different control schedules on the quantum dynamics underlying these programs and we evaluate the impact of these changes on the performance of various quantum-accelerated applications. As an example, we discuss the impact of local adiabatic evolution to use a variable rate of evolution based on the estimated size of the system energy gap. While this approach has been shown previously to realize the quadratic speedup found in Grover’s gate-based algorithm, we discuss this problem in the context of more general Hamiltonians and systems. Because local adiabatic evolution can only be applied if the shape of the energy landscape is understood, we make use of modeling and simulation of the adiabatic quantum dynamics to understand and evaluate novel control paradigms.
Emergent Properties in [SnSe]m[TiSe2]n Heterostructures Prepared from Designed Precursors.
The discovery of emergent properties has spurred Interest in monolayers and heterostructures. Emergent properties are those which exist in a monolayer or a heterostructure, but are absent in the individual bulk constituents. Emergent properties are difficult to predict, hard to control, and present challenges in understanding their origin. Preparing metastable compounds using the modulated elemental reactants method enables control of the nanoarchitecture in heterostructure compounds. This facilitates the synthesis of series of compounds with designed changes in structure, enabling the study of emergent properties as a function of the thickness of a constituent layer or layering order within a heterostructure. Several series of heterostructures within the [(SnSe)1+]m[TiSe2]n family have been synthesized and display systematic nanoarchitecture depend properties. Various diffraction techniques and high angle annular dark field scanning transmission electron microscopy were used to determine structure. The SnSe constituent structure changes significantly with thickness. Transport properties (temperature dependent Hall and resistivity data) vary systematically with m and n, with the changes reflecting both the structural changes in the heterostructures and the interaction between the constituent layers. Additional knowledge of how the layers interact is necessary to fully understand the origin of these unique properties. This understanding is required to design heterostructures with targeted properties for specific applications.
Quantum Anomalous Hall Effect in Magnetic Topological Insulators
The discovery of quantum Hall effect (QHE) showed great potential in the development of low-power consumption and high speed electronic devices due to dissipationless edge states. However, the requirement of extremely high magnetic fields prevented the practical realization of such devices. Naturally, the question arose whether it is possible to achieve the quantum Hall state in the absence of an external magnetic field. Thus began the search for the quantum anomalous Hall effect (QAHE), which was predicted to exhibit quantized Hall conductance without the application of a magnetic field. My poster provides an overview of the first experimental realization of the QAHE in magnetic topological insulators (TI) and my current work related to the topic.
Development of Ferromagnetic Contacts to InSb Nanowires
The poster will be a very brief introduction of motivations, theory and experiments on ferromagnetic contacts to lnSb nanowires. Some simple preliminary results are presented.
Frequency Domain Photonic Quantum Information Processing
Frequency encoding of information has had a profound impact on classical telecommunication technologies, but remains relatively unexplored for quantum information processing (QIP). The revolutionary potential of the frequency degree of freedom of a photon lies in the fact that it enables dense encoding and processing of information in a single spatial mode. Here we present the first demonstration of the Hong-Ou-Mandel interference with optical photons in the frequency domain, and potential applications to problems such as BosonSampling. Frequency encoding is uniquely positioned to address multiple challenges facing photonic QIP due to the inherently high-dimensional nature of the spectral degree of freedom of a photon, and the highly mature fiber and integrated technologies optimized for classical applications. This demonstration of frequency domain HOM interference serves as a fundamental experimental advancement in this relatively unexplored domain, that can potentially enable massively parallel, robust, and scalable all-optical quantum information processing.
Understanding the Pauli Twirling Approximation
The success of fault-tolerant error correction is necessary for understanding the performance of potential quantum computers, but requires physical error models that can be simulated efficiently with classical computers. The Gottesmann-Knill theorem guarantees such class of such error models. Of these, the simplest is the Pauli twirling approximation (PTA), which is obtained by twirling a completely positive channel over the Pauli basis. We test PTA's accuracy at predicting a code's logical error rate by simulating the 5-qubit code circuit with decoherence and unitary gate errors. We find evidence for good agreement with exact simulation, with the PTA typically overestimating the logical error rate by a factor of 3. Also restricting ourselves to analysis of free dynamics of qubits we show explicitly how application of PTA is equivalent to ignoring most of the quantum back action of the system and give a general argument as to why this approximation leads to low logical error rates in fault tolerant stabilizer circuits as compared to other quantum channels. We provide numerical evidence that PTA's performance in modeling noise gets worse as number of qubits increase.
Pulsed Ferromagnetic Resonance Driven Inverse Spin-Hall Effect in Organic and Inorganic Materials
The spin-orbit coupling (SOC) strength and the spin diffusion length are crucial parameters for the applicability of a material for spintronics and measuring them accurately is therefore a crucial prerequisite for progress within this field. We have recently made progress on such measurement techniques by demonstration of pulsed inverse spin-Hall effect (ISHE) experiments for which we employed a pulsed ferromagnetic resonance (FMR) spin-pumping scheme in order to inject a pure spin current from a ferromagnetic (FM) substrate into organic semiconductor (OSEC) layers. When the FM is in resonance with pulsed microwave excitation, a strong, pure (that means charge free) spin-current is formed in the OSEC, which circumvents the impedance mismatch between the FM layer and the organic film. Because of the weak SOC in most OSECs, the inverse spin Hall effect (ISHE) that results from the spin pumping scheme is very subtle; yet with pulsed, high microwave power excitation of the FMR, strong p-ISHE signals can be measured, while, by choice of low duty cycles, measurements artifacts due to heating and other electromagnetic effects are minimized at the same time. As the magnitude of the ISHE scales linearly with the Pointing flux that drives FMR, quantitative ISHE measurements require precise control of the FMR driving field amplitude B1. This is achieved by monitoring the Rabi nutation of paramagnetic spin-probes in proximity of the device on which the ISHE is measured.
Electro-Nuclear Clock Transitions In A Ho(Iii) Molecular Nanomagnet
An obstacle in the employment of spin qubits in quantum information processing is their short coherence time. At low temperatures, the magnetic dipolar interaction is the primary source of decoherence. The contribution from the dipolar coupling to the surrounding spins can be suppressed using atomic clock transition (CTs) . CTs are referred to the transitions that are robust against the variations in the magnetic field. Here, we report pulsed electron paramagnetic resonance (EPR) studies of a Holmium Polyoxometalate, where we exploit CTs to enhance coherence time . Pulsed EPR measurements were performed on crystals with two different concentrations of holmium (diluted in an isostructural diamagnetic matrix). Aside from a dramatic enhancement in the phase coherence time, we observed that at the CTs the Electron Spin Echo Envelope Modulation (ESEEM) vanishes. This further confirms that the Ho (III) spins become decoupled from the neighboring protons at CTs. We also observed electro-nuclear clock transitions in the concentrated sample that involve coupled dynamics of the electron and nuclear spins. These transitions are formally forbidden in EPR, however, the symmetry of this molecule generates admixtures of the ground doublet through second order perturbation. Furthermore, application of a transverse magnetic field mixes the mI and mI+1 states, allowing such transitions to occur in the vicinity of avoided level crossings. Our experimental results suggest an enhancement in the coherence time at these electro-nuclear clock transitions. This is significant for applications in hybrid magnetic qubits, where manipulation of the nuclear spin is controlled by EPR pulses.
 G. Wolfowicz, et al., Nature Nanotechnology 8, 561 (2013).
 M. Shiddiq, D. Komijani, et al., Nature 531, 348 (2016)."
Investigating the two-dimensional conductivity of graphene using surface acoustic wave devices
Surface acoustic waves (SAW) propagating on a piezoelectric substrate can be a sensitive probe of the dynamical conductivity of a nearby two-dimensional electron system (2DES). Enhanced absorption of acoustic energy can occur when the wavelength, or frequency, of the SAW become comparable to some other length, or time, scale within the 2DES. Utilizing a flip-chip SAW device, we implement SAW measurements of the frequency and wavevector dependent conductivity of both chemical vapor deposition grown and exfoliated graphene. Our flip-chip architecture allows us to measure graphene conductivity at low temperatures and high magnetic fields, vary the graphene carrier density in situ, and change the resonant SAW frequency by interchanging devices with different SAW transducer geometry.
Prediction and Characterization of Multiple Extremal Paths in Continuously-Monitored Qubits
We examine optimal paths between initial and final states for diffusive quantum trajectories in continuously monitored qubits, obtained as extrema of a stochastic path integral. We demonstrate the possibility of “multipaths” in the dynamics of continuously-monitored qubit systems, wherein multiple optimal paths travel between the same pre- and post-selected states over the same time interval. Optimal paths are expressed as solutions to a Hamiltonian dynamical system. The onset of multipaths may be determined by analyzing the evolution of a Lagrangian manifold in this phase space, and is mathematically analogous to the formation of caustics in ray optics or semiclassical physics. We apply our methods in two systems: a qubit with two non-commuting observables measured simultaneously, and a Rabi-driven qubit monitored through its fluorescence signal [arXiv:1612.03189]. We demonstrate that both systems contain multipaths generated by paths with different “winding numbers” about the Bloch sphere, and multipaths generated by catastrophes in the Lagrange manifold.
All-Silicon Valley Hall Photonic Topological Insulator
Based on the same physical principles as the conventional condensed-matter topological insulators, Photonic Topological Insulators (PTIs) guide unidirectional electromagnetic wave, instead of electrons. The PTI-based unidirectional waveguides do not suffer from back-reflections and local impurities, and therefore can be applicable to a large integrated photonic structure. The all-silicon PTI is an analogue to the quantum valley Hall effect condensed-matter topological insulator. We demonstrate the existence of a topological edge state between two all-silicon PTIs with different valley indices, the in- and out-coupling between the PTI and the free space and a design of an optical delay line based on this PTI.
Posters for June 15, 2017
Charge carrier holes and Majorana fermions
Understanding Luttinger holes in low dimensions is crucial for numerous spin-dependent phenomena and nanotechnology. In particular, hole quantum wires that are proximity coupled to a superconductor is a promising system for the observation of Majorana fermions. Earlier treatments of confined Luttinger holes ignored a mutual transformation of heavy and light holes at the heteroboundaries. We derive the effective hole Hamiltonian in the ground state. The mutual transformation of holes is crucial for Zeeman and spin-orbit coupling, and results in several spin-orbit terms linear in momentum in hole quantum wires. We discuss the criterion for realizing Majorana modes in charge carrier hole systems. GaAs or InSb hole wires shall exhibit stronger topological superconducting pairing, and provide additional opportunities for its control compared to InSb electron systems.
Simulations of Quantum Error Correction and Fault-Tolerant Quantum Computing Systems
Quantum computing offers a fundamentally new approach to storing and processing information, but the physical encoded qubits are susceptible to noise from the surrounding environment and the externally applied gate fields. Quantum error correction offers an important method for recovering from noisy operations provided it can be implemented fault tolerantly. Fault-tolerant operations within a quantum computer must act on the encoded data while adhering to the constraints imposed by the hardware. The threshold theorem provides evidence that it is possible to achieve fault-tolerance provided the physical error rate is sufficiently low. However, these results do not account for specific physical layouts and realistic noise models. We study the influence of realistic noise and device design on the ability for different QEC codes and fault-tolerant protocols to reach threshold. We investigate various quantum error correction systems by using numerical simulation techniques based on stochastic Clifford channels and tensor networks. These mathematical representations provide efficient models for quantum simulations that can be used to quantify logical error rate. We present a sample of simulation results of Bell state maintenance and the Steane [7,1,3] encoding and discuss future directions for research.
Thermopower and Nernst measurements in a half-filled lowest Landau level
Recently Son presented a particle-hole symmetric (PHS) fermionic quasiparticle theory for half-filled lowest Landau level - massless Dirac composite fermions (DCF) ，which is different from the PHS broken HLR theory . Subsequently, thermoelectric transport experiments were proposed to differentiate the DCF and HLR. Motivated by this, we systematically study the electronic and thermoelectric properties of v = 1/2 and 3/2 in high-mobility GaAs/AlGaAs 2DEGs. In this poster，preliminary results will be presented.
 Dam Thanh Son, Phys. Rev. X 5, 031027 (2015).
 B. I. Halperin, P. A. Lee, and N. Read, Phys. Rev. B 47, 7312 (1993).
 Andrew C. Potter, Maksym Serbyn, and Ashvin Vishwanath, Phys. Rev. X 6, 031026 (2016).
Dynamics of 1D Josephson chains in the regime of E_C << E_J ~ kT
Engineering Hole Spins in InAs/GaAs Quantum Dot and Quantum Dot Molecule using 2-D Electric Field
Self-assembled InAs/GaAs quantum dot (QDs) and quantum dot molecules (QDMs) are promising solid-state material platforms for quantum information applications. People have demonstrated that the optical and spin properties of individual QD and QDM can be fine-tuned by external electric field, either in the growth direction or in-plane direction. In order to further understand the properties of hole spin in single QD and QDM, we use tight-binding atomistic simulation to map a single hole spin state under various electric field and magnetic field conditions. We show that under a 1T Voigt geometry magnetic field, in-plane electric field can induce out-of-plane hole spin component by pushing the wave function of hole state to the edge of the QD. We also show that a 2-D electric field that has a gradient can induce mixed hole spin state in a symmetric shaped QDM. We present our design and fabrication process of a 4-electrode device, in order to apply both growth direction and in-plane direction electric field to a single QD or QDM at the same time. We also discuss some of the preliminary results of a single QD in 2-D electric field using low-temperature photoluminescence experiment.
Designing Silicon Qubits with Advanced Simulation Methods
Electron and nuclear spins are strong candidates for qubits in a quantum processor due to their long coherence/relaxation times and compatibility with the microelectronics industry . There are three crucial elements of a spin based quantum computer - (i) a well-defined spin qubit that can be readout and controlled in a nanostructure (ii) precise control of the exchange coupling between two spin qubits for two-qubit operations and (iii) a robust method to transport the spin qubit to different parts of the quantum computer [1, 2]. Here, we model and design several silicon nanodevices with a range of classical electrostatic and quantum simulation techniques , to demonstrate the above building blocks. We first present a non-invasive spatial metrology procedure that locates donor spin qubits to a precision several thousand times smaller than current statistical techniques, for high fidelity spin readout and control [4, 5]. We then investigate a method that offers massive tunability (5 orders of magnitude) of the exchange coupling between donor electron spins (20 nm apart) . For spin transport, we model donor chains  under realistic experimental conditions, and highlight that transport (100 nm) fidelities greater than 99.99 % are achievable across them . Finally, we propose a novel technique to couple donor qubits (separated by several hundreds of nanometers) via their dipolar interaction, as well as through the photonic mode of a resonant cavity . With the above, our modeling aids to estimate the required device topologies, qubit positions and electric fields – for high fidelity readout, control, exchange and transport of quantum spin information. Our results thereby provide a range of design considerations and feedback to experimentalists, who are in a global race to develop a fully scalable quantum computer.
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 L. C. L. Hollenberg et al. Physical Review B 74, 045311 (2006).
 T. S. Humble et al. Nanotechnology 27, 42, 424002 (2016).
 A. Laucht et al. Science Advances, 1, 3 (2015).
 F. A. Mohiyaddin et al. Nano Letters, 13, 1903 (2013).
 F. A. Mohiyaddin, Ph.D. thesis, University of New South Wales (2014).
 M. Friesen et al. Phys. Rev. Lett. 98, 230503 (2007).
 F. A. Mohiyaddin et al. Physical Review B, 94, 045414 (2016).
 G. Tosi et al. arxiV:1509.08538 (2017).
Stabilization of a manifold of coherent states for quantum error correction.
Stabilization of quantum manifolds is at the heart of error-protected quantum information storage and manipulation. Stabilizing a manifold of four coherent states of a harmonic oscillator against energy relaxation and dephasing facilitates realization of an error corrected logical qubit. Such stabilization requires a four-photon drive and dissipation on the harmonic oscillator. In this poster, we explain a theoretical proposal to engineer such a four-photon driven-dissipative process by cascading experimentally demonstrated two-photon exchange processes.
Probing and manipulating multi-vortex states in superconducting structures
New ways to investigate and manipulate superconducting vortices are of great practical and fundamental interest. Multi-vortex states could be employed to study vortex interactions and interference effects, to braid Majorana bound states by winding vortices, and to create novel superconducting devices. We demonstrate a new mode of magnetic force microscopy ($\Phi_0$-MFM), that enables us to induce, probe and control multi-vortex states in superconducting structures. By using a MFM tip as a source of inhomogeneous magnetic field, we can efficiently explore the configuration space of vortex states supported by the structure. $\Phi_0$-MFM enables us to map the transitions between tip-induced vortex states during a scan: at the positions of the tip, where the two lowest energy vortex configurations become degenerate, small oscillations of the tip drive transitions between these states, which causes a significant shift in the resonant frequency and dissipation of the cantilever. We show that measured patterns of vortex transitions allows us to identify these states, manipulate them, investigate their energetics and dynamics.
Quantum Walks and Spatial Searches on Free Groups and Networks
Quantum walks have been utilized by many quantum algorithms which provide improved performance over their classical counterparts. Quantum search algorithms, the quantum analogues of spatial search algorithms, have been studied on a wide variety of structures. We study quantum walks and searches on the Cayley graphs of finitely-generated free groups. Return properties are analyzed via Green’s functions, and quantum searches are examined. Additionally, the stopping times and success rates of quantum searches on random networks are experimentally estimated.
Thermalization in Driven Quantum Systems
Periodic driving is of considerable interest due to its promising potential for engineering and control of quantum systems. In these works, we study thermalization in both open and closed driven (Floquet) systems in the presence of interactions. In the case of open systems, we analyze the steady states and show how one may control them with dissipation. In the case of closed systems, we study finite size systems and analyze the scaling behavior of heating.
Ballistic graphene Josephson junctions from the short to the long regime
We explore the critical current (I_C) temperature scaling of ballistic Josephson junctions. Using encapsulated graphene/boron-nitride heterostructure devices, we vary device length from the short to the long junction regime. We extract the carrier-density-independent energy δE by calculating the ballistic cavity level spacing through the Fabry-Perot oscillations of the normal resistance. In the long and intermediate junction regimes, we find I_C scales as exp(-kBT/ δE) at higher temperatures. For short junctions, we find strong agreement with theoretically predicted I_C behavior. In the zero temperature limit, I_C of a long (short) junction saturates at a magnitude determined only by the product of δE (Δ) and the number of transversal modes in the junction.
Anisotropic Responses in Magnetic Materials
Studies of materials’ anisotropic responses offer a convenient window into characterizing the behavior of magnetic structures. We investigated the angular dependence of electron transport properties in Mn0.9Fe0.1Si within the A-phase. We found the fully formed Skyrmion planes rotate freely with the applied field, decoupled from Fe impurities and the underlying crystalline lattice. The presence of Fe impurities only plays a minor role in the field and angle dependence, highlighting the robustness of the electrical signals resulting from these spin textures. We also studied the strain dependence of sample resistance in Cr1/3NbS2, and BaIrO3. We find highly strain dependent negative resistance changes in these ferromagnetic materials. BaIrO3 shows highly anisotropic and hysteretic strain dependence, demonstrating that its bonds and bond angles coupled with the ferromagnetic phases play a vital role in the material’s electronic properties.
Three-wave mixing element for parametric amplifiers in circuit QED
Parametric conversion and amplification based on three-wave mixing are powerful primitives for efficient quantum operations. For superconducting qubits, such operations can be realized with a quadrupole Josephson junction element, the Josephson Ring Modulator (JRM), which behaves as a loss-less three-wave mixer. Since combining multiple quadrupole elements is a difficult task, it would be advantageous to have a pure three-wave dipole element that could be tessellated for increased power handling and/or information throughput. Here, we present a novel dipole circuit element with third-order nonlinearity, which implements three-wave mixing while minimizing harmful Kerr terms present in the JRM.
Optical Signatures of Spin-Orbit Exciton in Bandwidth Controlled Sr2IrO4 Epitaxial Films via High-Concentration Ca and Ba Doping
We have investigated the electronic and optical properties of (Sr1-xCax)2IrO4 and (Sr1-yBay)2IrO4 (x and y = 0 - 0.375) epitaxial thin-films, in which the bandwidth is systematically tuned via chemical substitutions of Sr ions by Ca and Ba. Transport measurements indicate that the thin-film series exhibits insulating behavior, similar to the Jeff = 1/2 spin-orbit Mott insulator Sr2IrO4. The optical conductivity spectra of doped Sr2IrO4 shows that the increased U/W from (Sr1-xCax)2IrO4 to (Sr1-yBay)2IrO4 causes a red shift in spectral weight, which cannot be explained by the simple picture of well-separated Jeff = 1/2 and Jeff = 3/2 bands. We suggest that the two-peak-like optical conductivity spectra of the layered iridates originates from the overlap between the optically-forbidden spin-orbit exciton and the inter-site optical transitions within the Jeff = 1/2 band. Our experimental results are consistent with this interpretation as implemented by a multi-orbital Hubbard model calculation.
Optical Parametric Amplification Effects in Two-Color Free-Electron Lasers
Newly available two-color X-ray free-electron lasers (XFELs) have raised questions with immediate experimental relevance: Are there optical parametric amplification effects in these systems? Can sum and difference-frequency generation (SFG and DFG) be achieved? These phenomena are explored using the broadband XFEL simulations Aurora and PUFFIN with the implication that SFG and DFG may be achievable with current hardware. SFG could prove to be a promising technique for boosting the XFEL frequency; very preliminary results are presented from an optimization study to this effect. In the particular case of SLAC National Accelerator Laboratory, this technique could may promise for achieving one of the lab’s overarching goals—single-shot protein structure determination—by allowing the LCLS XFEL to access frequency resonances for multi-wavelength anomalous diffraction studies.
High-Performance Electron Acceptor with Thienyl Side Chains for Organic Photovoltaics
The poster presents the findings of Y. Lin et al. in the Journal of American Chemical Society (J. Am. Chem. Soc. 2016, 138, 4955−4961).
We develop an efficient fused-ring electron acceptor (ITIC-Th) based on indacenodithieno[3,2-b]- thiophene core and thienyl side-chains for organic solar cells (OSCs). Relative to its counterpart with phenyl side-chains (ITIC), ITIC-Th shows lower energy levels (ITIC-Th: HOMO = −5.66 eV, LUMO = −3.93 eV; ITIC: HOMO = −5.48 eV, LUMO = −3.83 eV) due to the σ-inductive effect of thienyl side-chains, which can match with high-performance narrow-band-gap polymer donors and wide-band-gap polymer donors. ITIC-Th has higher electron mobility (6.1 × 10−4 cm2 V−1 s−1) than ITIC (2.6 × 10−4 cm2 V−1 s−1) due to enhanced intermolecular interaction induced by sulfur−sulfur interaction. We fabricate OSCs by blending ITIC-Th acceptor with two different low-band-gap and wide-band-gap polymer donors. In one case, a power conversion efficiency of 9.6% was observed, which rivals some of the highest efficiencies for single junction OSCs based on fullerene acceptors.
Induced superconductivity in 2D system
Induced superconductivity in semiconductors has been an area of active research for the last 20 years, and it gained a renewed attention recently with the search for states with non-Abelian statistics. Especially attractive is a combination of superconductivity and the quantum Hall effect as well as superconductivity in 1D semiconductor with strong spin orbit coupling. In the poster, I will present our progress on inducing superconductivity in GaAs 2D electron system as well as recent progress in InAs 2D electron system.
Probing the origin of extremely large magnetoresistance in topological semimetals
The recent discovery of extremely large magnetoresistance (XMR) in the exotic topological Dirac and Weyl semimetals has triggered extensive research to uncover its origin. Both topologically protected surface states and bulk Dirac/Weyl fermions have been proposed as the cause of the observed XMR. XMR can also be associated with other mechanisms such as magnetic-field induced metal-insulator transition and electron-hole compensation. Here, we investigate novel transport phenomena in candidate topological semimetals with the focus on unravelling the origin of XMR and to establish methods to separate surface bulk transport effects, a major challenge even in the quintessential topological insulator Bi2Se3.
Multi-Photon Transitions in Coupled Plasmon-Cyclotron Resonance Measured by Millimeter-Wave Reflection
We construct a low-temperature microwave waveguide interferometer for measuring high-frequency properties of two-dimensional electron gases (2DEGs). Coupled plasmon-cyclotron resonance (PCR) spectra are used to extract effective mass, bulk plasmon frequency, and carrier relaxation times. In contrast to traditional transmission spectroscopy, this method does not require sample preparation and is nondestructive. PCR signals can be resolved with a microwave power source as low as 10 nW. We observe PCR in the multi-photon transition regime, which has been proposed to be relevant to the microwave-induced resistance oscillations.
Josephson effect occurs at the surface of topological insulators
S-TI-S (superconductor-topological insulator-superconductor) Josephson junction is theoretically proved to be a candidate to support Majorana bound states. Then such a system (S-TI-S) is very important for basic study of Majorana fermions. My poster will introduce some S-TI-S systems and some results already achieved experimentally.