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Mesoscale and Nanoscale Physics

New submissions

[ total of 42 entries: 1-42 ]

New submissions for Tue, 27 Jul 21

[1]  arXiv:2107.11407 [pdf, ps, other]
Title: Surface-induced reduction of the switching field in nanomagnets
Comments: 7 pages, 5 figures

Magnetization reversal in a many-spin nanomagnet subjected to an rf magnetic field, on top of a DC magnetic field, is studied by numerically solving the system of coupled (damped) Landau-Lifshitz equations. It is demonstrated that spin-misalignment induced by surface anisotropy favors switching with a DC magnetic field lower than the Stoner-Wohlfarth switching field, for optimal intensities and frequencies of the rf field.

[2]  arXiv:2107.11484 [pdf, ps, other]
Title: Dzyaloshinskii-Moriya Induced Topological Magnon-Phonon Hybridization in 2D Antiferromagnetic Insulators with Tunable Chern Numbers

We theoretically study magnon-phonon hybrid excitations (magnon-polarons) in two-dimensional antiferromagnets on a honeycomb lattice. With an in-plane Dzyaloshinskii-Moriya interaction (DMI) allowed from mirror symmetry breaking from phonons, we find non-trivial Berry curvature around the anti-crossing rings among magnon and both optical and acoustic phonon bands, which gives rise to finite Chern numbers. We show that the Chern numbers of the magnon-polaron bands can be manipulated by changing the magnetic field direction or strength. We evaluate the thermal Hall conductivity reflecting the non-trivial Berry curvatures of magnon-polarons and propose a valley Hall effect resulting from spin-induced chiral phonons as a possible experimental signature. Our study complements prior work on magnon-phonon hybridized systems without optical phonons and suggests possible applications in spin caloritronics with topological magnons and chiral phonons.

[3]  arXiv:2107.11564 [pdf, other]
Title: Projectively enriched symmetry and topology in acoustic crystals
Comments: 7 pages, 4 figures

Symmetry plays a key role in modern physics, as manifested in the revolutionary topological classification of matter in the past decade. So far, we seem to have a complete theory of topological phases from internal symmetries as well as crystallographic symmetry groups. However, an intrinsic element, i.e., the gauge symmetry in physical systems, has been overlooked in the current framework. Here, we show that the algebraic structure of crystal symmetries can be projectively enriched due to the gauge symmetry, which subsequently gives rise to new topological physics never witnessed under ordinary symmetries. We demonstrate the idea by theoretical analysis, numerical simulation, and experimental realization of a topological acoustic lattice with projective translation symmetries under a $Z_2$ gauge field, which exhibits unique features of rich topologies, including a single Dirac point, M\"{o}bius topological insulator and graphene-like semimetal phases on a rectangular lattice. Our work reveals the impact when gauge and crystal symmetries meet together with topology, and opens the door to a vast unexplored land of topological states by projective symmetries.

[4]  arXiv:2107.11578 [pdf, other]
Title: Collective magnetic excitations in graphene bilayers
Comments: 11 pages, 6 figures

We discuss novel transverse plasmon-polaritons that are hosted by AA- and AB-stacked bilayer graphene due to perfect nesting. They are composed of oscillating counterflow currents in between the layers, giving a clear interpretation for these collective modes as magnetic excitations carrying magnetic moment parallel to the planes. For AA-stacked bilayer graphene, these modes can reach zero frequency at the neutrality point and we thus predict a symmetry broken ground-state leading to in-plane orbital ferromagnetism. Even though it could be hard to detect them in real solid-state devices, these novel magnetic plasmons should be observable in artificial set-ups such as optical lattices. Also, it might be possible to enhance the instability by a superconducting cavity.

[5]  arXiv:2107.11595 [pdf, other]
Title: Scattering in Monolayer Molybdenum Disulfide Quantum Dot

We investigate the propagation of electrons in a circular quantum dot of monolayer Molybdenium disulfide MoS_2, subjected to an electric potential. Using the continuum model, we present analytical expressions for the eigenstates, scattering coefficients, scattering efficiency, and radial component of the reflected current and electron density. We identify two scattering regimes as a function of physical parameters such as incident electronic energy, potential barrier, and quantum dot radius. For the incident electron low energy, we show that there is an appearance of scattering resonances. Also, we note that the Far-field scattered current has distinct preferred scattering directions.

[6]  arXiv:2107.11634 [pdf, other]
Title: Sub-Sharvin conductance and enhanced shot noise in doped graphene
Comments: RevTeX, 7 pages, 5 figures

Ideal Sharvin contact in a multimode regime shows the conductance $G\approx{}G_{\rm Sharvin}=g_0k_F{}W/\pi$ (with $g_0$ the conductance quantum, $k_F$ the Fermi momentum, and $W$ the contact width) accompanied by strongly suppressed shot-noise quantified by small Fano factor $F\approx{}0$. For ballistic graphene away from the charge-neutrality point the sub-Sharvin transport occurs, characterised by suppressed conductance $G\approx{}(\pi/4)\,G_{\rm Sharvin}$ and enhanced shot noise $F\approx{}1/8$. All these results can be derived from a basic model of quantum scattering, involving assumptions of infinite height and perfectly rectangular shape of the potential barrier in the sample. Here we have carried out the numerical analysis of the scattering on a family of smooth barriers of finite height interpolating between parabollic and rectangular shapes. We find that tuning the barrier shape one can modify the asymmetry between electron- and hole-doped systems. For electronic dopings, the system crosses from Sharvin to sub-Sharvin transport regime (indicated by both the conductance and the Fano factor) as the potential becomes closer to the rectangular shape. In contrast, for hole dopings, the conductivity is strongly suppressed when the barrier is parabolic and slowly converges to $G\approx{}(\pi/4)\,G_{\rm Sharvin}$ as the potential evolves towards rectangular shape. In such a case the Sharvin transport regime is inaccessible, shot noise is generically enhanced (with much slower convergence to $F\approx{}1/8$) comparing to the electron-doped case, and aperiodic oscillations of both $G$ and $F$ are prominent due to the formation of quasibound states.

[7]  arXiv:2107.11649 [pdf, other]
Title: Characterizing and Tuning Exceptional Points Using Newton Polygons
Comments: 5+6 pages, 3+1 figures

The study of non-Hermitian degeneracies -- called exceptional points -- has become an exciting frontier at the crossroads of optics, photonics, acoustics and quantum physics. Here, we introduce the Newton polygon method as a general algebraic framework for characterizing and tuning exceptional points, and develop its connection to Puiseux expansions. We propose and illustrate how the Newton polygon method can enable the prediction of higher-order exceptional points, using a recently experimentally realized optical system. As an application of our framework, we show the presence of tunable exceptional points of various orders in $PT$-symmetric one-dimensional models. We further extend our method to study exceptional points in higher number of variables and demonstrate that it can reveal rich anisotropic behaviour around such degeneracies. Our work provides an analytic recipe to understand and tune exceptional physics.

[8]  arXiv:2107.11691 [pdf, ps, other]
Title: Anomalous nematic-to-stripe phase transition driven by in-plane magnetic fields
Comments: 5 pages, 4 figures

Anomalous nematic states, recently discovered in ultraclean two-dimensional electron gas, emerge from quantum Hall stripe phases upon further cooling. These states are hallmarked by a local minimum (maximum) in the hard (easy) longitudinal resistance and by an incipient plateau in the Hall resistance in nearly half-filled Landau levels. Here, we demonstrate that a modest in-plane magnetic field, applied either along $\left < 110 \right >$ or $\left < 1\bar10 \right >$ crystal axis of GaAs, destroys anomalous nematic states and restores quantum Hall stripe phases aligned along their native $\left < 110 \right >$ direction. These findings confirm that anomalous nematic states are distinct from other ground states and will assist future theories to identify their origin.

[9]  arXiv:2107.11694 [pdf, other]
Title: Third-order terahertz optical response of graphene in the presence of Rabi Oscillations
Comments: 5 pages, 2 figures, to be published

Graphene has been shown to exhibit a nonlinear response due to its unique band structure. In this paper, we study the terahertz (THz) response metallic armchair graphene nanoribbons, specifically current density and Rabi oscillations beyond the semiclassical Boltzman model. We performed quantum mathematical modeling by first finding a solution to the unperturbed Hamiltonian for a single Fermion in the dipole gauge and then applying a polarized, THz electrical field. After writing the solution in terms of the four eigenstates of the Dirac system, we numerically calculated the $x$ and $y$ components of the induced current density resulting from applying the terahertz electrical field. Due to the inclusion of the Rabi Oscillation in our calculation of the optical response, we predict both odd and even harmonics, as well as continuum oscillations of the power density spectrum in the THz regime. Lastly, we show a rapid decay of the power harmonics.

[10]  arXiv:2107.11716 [pdf, ps, other]
Title: Optically induced topological spin-valley Hall effect for exciton polaritons
Comments: 4 pages excluding references + supplementary material
Journal-ref: Physical Review B 103, L201406 (2021)

We consider exciton-polaritons in a honeycomb lattice of micropillars subjected to circularly polarized (${\sigma_\pm}$) incoherent pumps, which are arranged to form two domains in the lattice. We predict that the nonlinear interaction between the polaritons and the reservoir excitons gives rise to the topological valley Hall effect where in each valley two counterpropagating helical edge modes appear. Under a resonant pump, ${\sigma_\pm}$ polaritons propagate in different directions without being reflected around bends. The polaritons propagating along the interface have extremely high effective lifetimes and show fair robustness against disorder. This paves the way for robust exciton-polariton spin separating and transporting channels in which polaritons attain and maintain high degrees of spin polarization, even in the presence of spin relaxation.

[11]  arXiv:2107.11760 [pdf, other]
Title: Frustrated magnetic interactions in a cyclacene crystal

We study the emergence of magnetism and its interplay with structural properties in a two dimensional molecular crystal of cyclacenes, using density functional theory (DFT). Isolated cyclacenes with an even number of fused benzenes host two unpaired electrons in two topological protected zero modes, at the top and bottom carbon rings that form the molecule. We show that, in the gas phase, electron repulsion promotes an open-shell singlet with strong intramolecular antiferromagnetic exchange. We consider a closed packing triangular lattice crystal phase and we find a strong dependence of the band structure and magnetic interactions on the rotation angle of the cyclacenes with respect to the crystal lattice vectors. The orientational ground state maximizes the intermolecular hybridization, yet local moments survive. Intermolecular exchange is computed to be antiferromagnetic, and DFT predicts a broken symmetry $120^\circ$ spin phase reflecting the frustration of the intermolecular spin coupling. Thus, the cyclacene crystal realizes a bilayer of two antiferromagnetically coupled S = 1/2 triangular lattices. Our results provide a bottom-up route towards carbon based strongly correlated platforms in two dimensions.

[12]  arXiv:2107.11763 [pdf, ps, other]
Title: Energy resonance transfer between quantum defects in metal halide perovskites

Quantum defects have shown to play an essential role for the non-radiative recombination in metal halide perovskites (MHPs). Nonetheless, the processes of charge transfer-assisted by defects are still ambiguous. Herein, we theoretically study the non-radiative multiphonon processes among different types of quantum defects in MHPs using Markvart model for the induced mechanisms of electron-electron and electron-phonon interactions, respectively. We find that charge carrier can transfer between the neighboring levels of the same type shallow defects by multiphonon processes, but it will be distinctly suppressed with the increasing of the defect depth. For the non-radiation multiphonon transitions between donor- and acceptor-like defects, the processes are very fast and independence of the defect depth, which provide a possible explanation for the blinking phenomena of photoluminescence spectra in recent experiment. We also discuss the temperature dependence of these multiphonon processes and find that their variational trends depend on the comparison of Huang-Rhys factor with the emitted phonon number. These theoretical results fill some gaps of defect-assisted non-radiative processes in the perovskites materials.

[13]  arXiv:2107.11815 [pdf, other]
Title: Robust topology in quench dynamics simulated on near-term quantum computers

Programmable quantum processors are suitable platforms for simulating quantum systems, of which topological phases are of particular interest. We simulate the quench dynamics of a one-dimensional system on IBM Q devices. The topological properties of the dynamics are described by the dynamical winding number and the Berry phase, which are simulated with the quantum circuit model. The results show that despite the noise present in the current quantum computers, the dynamical topological invariants are robust. Moreover, to investigate the influence of open quantum system, we analytically solve the master equation in Lindblad form and show that the dynamical winding number and the change in Berry phase are not affected by the dissipation. This study sheds light on the robustness of topological phases on the noisy intermediate-scale quantum computers.

[14]  arXiv:2107.11833 [pdf, other]
Title: Demonstrating Majorana nonabelian exchange using fast adiabatic charge-transfer
Comments: 18 pages, 8 figures

Demonstration of Majorana nonabelian exchange properties is a major challenge in the field which would show the existence of a new topological quasiparticle. In this work, we propose a minimal device and protocol for testing nonabelian exchange properties using charge-transfer operations between a quantum dot and two Majorana bound states. We use an adiabatic perturbation theory to find fast adiabatic paths to perform operations and to account for non-adiabatic errors. We find the ideal parameter sweep and a region in parameter space where MBS nonabelian properties can be shown. The ideal parameter sweep reduces the charge-transfer operation time in 1-2 orders of magnitude with respect to constant velocity driving. Using realistic parameters, we estimate that the lower bound for the time scale can be reduced to $\sim10$ ns. Deviations from the ideal parameters lead to the accumulation of an undesired dynamical phase, affecting the outcome of the proposed protocol. We reduce the influence from the dynamical phase using an angular echo.

[15]  arXiv:2107.11935 [pdf, other]
Title: Statistical analysis of spin switching in coupled spin-crossover molecules
Comments: 9 pages, 5 figures

We study the switching behavior of two spin-crossover molecules residing in a nanojunction device consisting of two closely spaced gold electrodes. The spin states are monitored through a real-time measurement of the resistance of the junction. A statistical analysis of the resistance values, the occupation probabilities, and the lifetimes of the respective spin states shows that the two spin-crossover molecules are coupled to each other. We extract the parameters for a minimal model describing the two coupled spin-crossover molecules. Finally, we use the time dependence of factorial cumulants to demonstrate that the measured data indicates the presence of interactions between the two spin-crossover molecules.

[16]  arXiv:2107.11973 [pdf]
Title: Pseudo-magnetic field-induced ultra-slow carrier dynamics in periodically strained graphene
Comments: Main: 21 pages, 4 figures / SI: 13 pages, 5 figures

The creation of pseudo-magnetic fields in strained graphene has emerged as a promising route to allow observing intriguing physical phenomena that would be unattainable with laboratory superconducting magnets. Scanning tunneling spectroscopy experiments have successfully measured the pseudo-Landau levels and proved the existence of pseudo-magnetic fields in various strained graphene systems. These giant pseudo-magnetic fields observed in highly deformed graphene can substantially alter the optical properties of graphene beyond a level that can be feasible with an external magnetic field, but the experimental signatures of the influence of such pseudo-magnetic fields have yet to be unveiled. Here, using time-resolved infrared pump-probe spectroscopy, we provide unambiguous evidence for ultra-slow carrier dynamics enabled by pseudo-magnetic fields in periodically strained graphene. Strong pseudo-magnetic fields of ~100 T created by non-uniform strain in graphene nanopillars are found to significantly decelerate the relaxation processes of hot carriers by more than an order of magnitude. Our finding presents unforeseen opportunities for harnessing the new physics of graphene enabled by pseudo-magnetic fields for optoelectronics and condensed matter physics.

[17]  arXiv:2107.12019 [pdf, other]
Title: Optomechanical lasing and domain walls driven by exciton-phonon interactions
Comments: 8 pages, 12 figures

We study theoretically interaction of optically-pumped excitons with acoustic waves in planar semiconductor nanostructures in the strongly nonlinear regime. We start with the multimode optomechanical lasing regime for optical pump frequency {above} the exciton resonance and demonstrate broadband chaotic-like lasing spectra. We also predict formation of propagating optomechanical domain walls driven by optomechanical nonlinearity for the optical pump {below} the exciton resonance. Stability conditions for the domain walls are examined analytically and are in agreement with direct numerical simulations. Our results apply to nonlinear sound propagation in the arrays of quantum wells or in the plane of Bragg semiconductor microcavities hosting excitonic polaritons.

[18]  arXiv:2107.12149 [pdf]
Title: Exploring Two-Dimensional Empty Space
Authors: A. K. Geim
Journal-ref: Nano Lett. XXX (2021)

Research on graphene is proving to have more lives than a cat, repeatedly coming back in new incarnations including graphene's recent alter ego, two-dimensional emptiness.

[19]  arXiv:2107.12232 [pdf, other]
Title: Lock-in Detection based Dynamic Widefield Magnetometry using Quantum Defects in Diamond
Comments: 17 pages, 11 figures

Wide field-of-view magnetic field microscopy has been realized by probing shifts in optically detected magnetic resonance (ODMR) spectrum of nitrogen-vacancy (NV) defect centers in diamond. However, these widefield diamond NV magnetometers require a few to several minutes of acquisition to get a single magnetic field image, rendering the technique as temporally static in its current form. This limitation prevents application of diamond NV magnetometers to novel imaging of dynamically varying microscale magnetic field processes. Here, we show that the magnetic field imaging frame rate can be significantly enhanced by performing lock-in detection of NV photo-luminescence (PL), simultaneously over multiple pixels of a lock-in camera. A detailed protocol for synchronization of frequency-modulated PL of NV centers with fast camera frame demodulation, at few kilohertz frequencies, has been experimentally demonstrated. This modified diamond NV imaging allows up to ten thousand pixels to simultaneously track an applied magnetic field waveform of sub-second temporal variation at imaging frame rates of 50-200 Hz. Our work demonstrates that widefield per-pixel lock-in detection, in combination with frequency-modulated NV ODMR, enables millisecond scale dynamic magnetic field microscopy.

[20]  arXiv:2107.12271 [pdf, other]
Title: Anomalous photoluminescence emission of monolayer MoS2-QD heterostructure on hBN
Authors: Pradeepa H L
Comments: 4 pages, 4 figures

Monolayer transition metal dichalcogenides(2D) and zero dimensional quantum dots(QD) are known to have unique optical properties in their individual limit such as high binding energy of excitons. The combination of these two systems is of particular interest in understanding various aspects of energy transfer, charge transfer, dynamics of excitons, etc. In this manuscript, we report the anomalous photoluminescence(PL) emission in one such heterostructure MoS2-CdSe QD. We observe multiple exciton emission peaks of the heterostructure on hBN substrate which are absent on SiO2. Our observation open up the questions, whether the local potential due to the lattice mismatch between MoS2 and hBN has any role in deciding the emission of these peaks or the strain eld of MoS2 and hBN is the reason for the emergence of multiple emission. In addition, the altered quantum potential of QD due the presence of hBN and MoS2 may also leads to such multiple emissions.

[21]  arXiv:2107.12278 [pdf, other]
Title: Optimizing Topological Switching in Confined 2D-Xene Nanoribbons via Finite-Size Effects
Comments: 30 pages, 4 figures

In a blueprint for topological quantum electronics, edge state transport in a topological insulator material can be controlled by employing a gate-induced topological quantum phase transition. While finite-size effects have been widely studied in 2D-Xenes, less attention has been devoted to finite-size effects on the gate-induced topological switching in spin-orbit coupled 2D-Xene nanoribbons. Here, by studying width dependence of electronic properties via a tight binding model, we demonstrate that finite-size effects can be used to optimize both the spin-orbit interaction induced barrier in the bulk and the gate-controlled quantized conductance on the edges of zigzag-Xene nanoribbons. The critical electric field required for switching between gapless and gapped edge states reduces as the width decreases, without any fundamental lower bound. This size dependence of the threshold voltage stems from a unique feature of zigzag-Xene nanoribbons: width and momentum dependent tunability of the gate-induced coupling between overlapping spin-filtered chiral states on the two edges. Furthermore, when the width of zigzag-Xene nanoribbons is smaller than a critical limit, topological switching between edge states can be attained without bulk band gap closing and reopening. This is primarily due to the quantum confinement effect on the bulk band spectrum which increases nontrivial bulk band gap with decrease in width. Such reduction in threshold voltage accompanied by enhancement in bulk band gap overturns the conventional wisdom of utilizing wide channel and narrow gap semiconductors for reducing threshold voltage in standard field effect transistor analysis and paves the way towards next-generation low-voltage topological quantum devices.

Cross-lists for Tue, 27 Jul 21

[22]  arXiv:2107.11562 (cross-list from cond-mat.supr-con) [pdf, ps, other]
Title: Controllable Majorana vortex states in iron-based superconducting nanowires
Comments: 13 pages and 7 figures

There has been experimental evidence for the Majorana zero modes (MZMs) in solid state systems, which are building blocks for potential topological quantum computing. It is important to design devices, in which MZMs are easy to manipulate and possess a broad topological non-trivial parameter space for fusion and braiding. Here, we propose that the Majorana vortex states in iron-based superconducting nanowires fulfill these desirable conditions. This system has a radius-induced topological phase transition, giving a lower limit to the radius of the nanowire. In the topological phase, there is only one pair of MZMs in the nanowire over a wide range of radius, chemical potential, and external magnetic field. The wavefunction of the MZM has a sizable distribution at the side edge of the nanowire. This property enables one to control the interaction of the MZMs in neighboring vortex nanowires, and paves the way for Majorana fusion and braiding.

[23]  arXiv:2107.11739 (cross-list from cond-mat.mtrl-sci) [pdf, other]
Title: Long-range anisotropic Heisenberg ferromagnets and electrically tunable ordering
Comments: 11 Pages, 5 figures
Journal-ref: Physical Review B 103, 214411 (2021)

Recent realizations of intrinsic magnetic order in truly two-dimensional materials have opened new avenues in the fundamental knowledge and spintronic applications. Here we develop an anisotropic Heisenberg model with relativistic exchange interactions that are obtained from the first-principles calculations. We demonstrate the crucial importance of magnetic interactions beyond the first-neighbour to qualitatively and quantitatively reproduce the experimental results. Once we ascertain the predictive capacity of the model for chromium trihalides and CrGeTe$_3$, we investigate the feasibility of tuning the magnetic ordering by electrical means in these materials. A remarkable five-fold increase in the ferromagnetic Curie temperature is predicted in monolayer CrI$_3$ within experimentally obtainable hole density. The elusive microscopic mechanism behind the doping-dependent Curie temperature is illustrated. Further, in the present context, the effects of biaxial strain and chemical doping are also addressed. The results should trigger further experimental attention to test the present conclusions.

[24]  arXiv:2107.11748 (cross-list from quant-ph) [pdf, other]
Title: Ancilla assisted Discrete Time Crystals in Non-interacting Spin Systems
Comments: 6 pages, 3 figures

We show here through experiments and exact analytical models the emergence of discrete time translation symmetry breaking in non-interacting systems. These time-periodic structures become stable against perturbations only in the presence of their interaction with the ancillary quantum system and display subharmonic response over a range of rotation angle errors. We demonstrate this effect for central spin and spin-mechanical systems, where the ancillary induced interaction among the spins stabilizes the spin dynamics against finite errors. Further, we extend these studies and show the possibility to even achieve non-local (remote) synchronization of such Floquet crystals.

[25]  arXiv:2107.11923 (cross-list from cond-mat.mtrl-sci) [pdf]
Title: Anomalous valley Hall effect in antiferromagnetic monolayers

Anomalous valley Hall (AVH) effect is a fundamental transport phenomenon in the field of condensed-matter physics. Usually, the research on AVH effect is mainly focused on 2D lattices with ferromagnetic order. Here, by means of model analysis, we present a general design principle for realizing AVH effect in antiferromagnetic monolayers, which involves the introduction of nonequilibrium potentials to break of PT symmetry. Using first-principles calculations, we further demonstrate this design principle by stacking antiferromagnetic monolayer MnPSe3 on ferroelectric monolayer Sc2CO2 and achieve the AVH effect. The AVH effect can be well controlled by modulating the stacking pattern. In addition, by reversing the ferroelectric polarization of Sc2CO2 via electric field, the AVH effect in monolayer MnPSe3 can be readily switched on or off. The underlying physics are revealed in detail. Our findings open up a new direction of research on exploring AVH effect.

[26]  arXiv:2107.11959 (cross-list from physics.optics) [pdf]
Title: Optically-Induced Symmetry Breaking via Nonlinear Phononics
Comments: 7 pages, 5 figures

Optical nonlinearities in solids reveal information about both the in-plane rotational and out-of-plane inversion symmetries of a crystal. In the van der Waals material hexagonal boron nitride (hBN) both these symmetries and the linear vibrational properties have led to the rich physics of mid-infrared phonon-polaritons. However, the role of strong electron-phonon nonlinearities requires further study. In this work, we investigate both theoretically and experimentally the rich interplay of phonon anharmonicity and symmetry in phonon-polariton mediated nonlinear optics. We show that large enhancements (>30x) of third-harmonic generation occur for incident femtosecond pulses that are resonant with the hBN transverse optical phonons. In addition, we predict and observe large transient sub-picosecond duration second-harmonic signals during resonant excitation, which in equilibrium is forbidden by symmetry. This surprising result indicates that instantaneous crystal inversion symmetry breaking can be optically induced and controlled via phonon interactions by both the power and polarization of the pump laser.

[27]  arXiv:2107.11980 (cross-list from cond-mat.mtrl-sci) [pdf, other]
Title: Influence of sample momentum space features on scanning tunnelling microscope measurements
Comments: 10 pages, 12 figures
Subjects: Materials Science (cond-mat.mtrl-sci); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Applied Physics (physics.app-ph); Computational Physics (physics.comp-ph); Quantum Physics (quant-ph)

Theoretical understanding of scanning tunnelling microscope (STM) measurements involve electronic structure details of the STM tip and the sample being measured. Conventionally, the focus has been on the accuracy of the electronic state simulations of the sample, whereas the STM tip electronic state is typically approximated as a simple spherically symmetric $ s $ orbital. This widely used $ s $ orbital approximation has failed in recent STM studies where the measured STM images of subsurface impurity wave functions in silicon required a detailed description of the STM tip electronic state. In this work, we show that the failure of the $ s $ orbital approximation is due to the indirect band-gap of the sample material silicon (Si), which gives rise to complex valley interferences in the momentum space of impurity wave functions. Based on direct comparison of STM images computed from multi-million-atom electronic structure calculations of impurity wave functions in direct (GaAs) and indirect (Si) band-gap materials, our results establish that whilst the selection of STM tip orbital only plays a minor qualitative role for the direct band gap GaAs material, the STM measurements are dramatically modified by the momentum space features of the indirect band gap Si material, thereby requiring a quantitative representation of the STM tip orbital configuration. Our work provides new insights to understand future STM studies of semiconductor materials based on their momentum space features, which will be important for the design and implementation of emerging technologies in the areas of quantum computing, photonics, spintronics and valleytronics.

[28]  arXiv:2107.11981 (cross-list from quant-ph) [pdf, other]
Title: An exchange-based surface-code quantum computer architecture in silicon
Comments: 7 pages, 5 figures

Phosphorus donor spins in silicon offer a number of promising characteristics for the implementation of robust qubits. Amongst various concepts for scale-up, the shared-control concept takes advantage of 3D scanning tunnelling microscope (STM) fabrication techniques to minimise the number of control lines, allowing the donors to be placed at the pitch limit of $\geq$30 nm, enabling dipole interactions. A fundamental challenge is to exploit the faster exchange interaction, however, the donor spacings required are typically 15 nm or less, and the exchange interaction is notoriously sensitive to lattice site variations in donor placement. This work presents a proposal for a fast exchange-based surface-code quantum computer architecture which explicitly addresses both donor placement imprecision commensurate with the atomic-precision fabrication techniques and the stringent qubit pitch requirements. The effective pitch is extended by incorporation of an intermediate donor acting as an exchange-interaction switch. We consider both global control schemes and a scheduled series of operations by designing GRAPE pulses for individual CNOTs based on coupling scenarios predicted by atomistic tight-binding simulations. The architecture is compatible with the existing fabrication capabilities and may serve as a blueprint for the experimental implementation of a full-scale fault-tolerant quantum computer based on donor impurities in silicon.

[29]  arXiv:2107.12097 (cross-list from physics.optics) [pdf]
Title: Helicity-selective Raman scattering from in-plane anisotropic α-MoO$_3$
Comments: The following article has been submitted to Applied Physics Letters

Hyperbolic crystals like {\alpha}-MoO$_3$ can support large wavevectors and photon density as compared to the commonly used dielectric crystals, which makes them a highly desirable platform for compact photonic devices. The extreme anisotropy of the dielectric constant in these crystals is intricately linked with the anisotropic character of the phonons, which along with photon confinement leads to the rich Physics of phonon polaritons. However, the chiral nature of phonons in these hyperbolic crystals have not been studied in detail. In this study, we report our observations of helicity selective Raman scattering from flakes of {\alpha}-MoO$_3$. Both helicity-preserving and helicity-reversing Raman scattering are observed. We observe that helical selectivity is largely governed by the underlying crystal symmetry. This study shed light on the chiral character of the high symmetry phonons in these hyperbolic crystals. It paves the way for exploiting proposed schemes of coupling chiral phonon modes into propagating surface plasmon polaritons and for compact photonic circuits based on helical polarized light.

[30]  arXiv:2107.12104 (cross-list from cond-mat.mtrl-sci) [pdf]
Title: Addressing Raman features of individual layers in isotopically labeled Bernal stacked bilayer graphene
Journal-ref: 2D Materials, 3(2), 025022 (2016)

The most important bands for the evaluation of strain in graphene (the 2D and 2D prime modes) are investigated. It is shown that for Bernal-stacked bilayers, the two-phonon Raman features have three different components that can be assigned to processes originating solely from the top graphene layer, bottom graphene layer, and from a combination of processes originating both from the top and bottom layers. The individual components of the 2D and 2D prime modes are disentangled. The reported results enable addressing the properties of individual graphene layers in isotopically labelled turbostratic and Bernalstacked graphene systems.

[31]  arXiv:2107.12107 (cross-list from cond-mat.mtrl-sci) [pdf]
Title: Enhanced Raman Scattering on Functionalized Graphene Substrates
Journal-ref: 2D Materials, 4, 025087 (2017)

The graphene-enhanced Raman scattering of Rhodamine 6G molecules on pristine, fluorinated and 4-nitrophenyl functionalized graphene substrates was studied. The uniformity of the Raman signal enhancement was studied by making large Raman maps. The relative enhancement of the Raman signal is demonstrated to be dependent on the functional groups, which was rationalized by the different doping levels of pristine, fluorinated and 4-nitrophenyl functionalized graphene substrates. The impact of the Fermi energy of graphene and the phonon energy of the molecules was considered together for the first time in order to explain the enhancement. Such approach enables to understand the enhancement without assuming anything about the uniformity of the molecules on the graphene surface. The agreement between the theory and our measured data was further demonstrated by varying excitation energy.

[32]  arXiv:2107.12129 (cross-list from cond-mat.soft) [pdf, other]
Title: How water wets and self-hydrophilizes nanopatterns of physisorbed hydrocarbons
Comments: 23 pages, 5 figures, Supplementary information including 2 movies as ancillary file, in press
Journal-ref: Journal of Colloids and Interface Science (2021)

Weakly bound, physisorbed hydrocarbons could in principle provide a similar water-repellency as obtained by chemisorption of strongly bound hydrophobic molecules at surfaces. Here we present experiments and computer simulations on the wetting behavior of water on molecularly thin, self-assembled alkane carpets of dotriacontane (n-C32H66 or C32) physisorbed on the hydrophilic native oxide layer of silicon surfaces during dip-coating from a binary alkane solution. By changing the dip-coating velocity we control the initial C32 surface coverage and achieve distinct film morphologies, encompassing homogeneous coatings with self-organised nanopatterns that range from dendritic nano-islands to stripes. These patterns exhibit a good water wettability even though the carpets are initially prepared with a high coverage of hydrophobic alkane molecules. Using in-liquid atomic force microscopy, along with molecular dynamics simulations, we trace this to a rearrangement of the alkane layers upon contact with water. This restructuring is correlated to the morphology of the C32 coatings, i.e. their fractal dimension. Water molecules displace to a large extent the first adsorbed alkane monolayer and thereby reduce the hydrophobic C32 surface coverage. Thus, our experiments evidence that water molecules can very effectively hydrophilize initially hydrophobic surfaces that consist of weakly bound hydrocarbon carpets.

[33]  arXiv:2107.12302 (cross-list from quant-ph) [pdf, other]
Title: Quantum heat engines with complex working media, complete Otto cycles and heuristics
Comments: RevTex 4-2, 26 pages, 7 figures; Comments are welcome

Quantum thermal machines make use of non-classical thermodynamic resources, one of which is interactions between elements of the quantum working medium. In this paper, we examine the performance of a quasi-static quantum Otto engine based on two spins of arbitrary magnitudes subject to an external magnetic field and coupled via an isotropic Heisenberg exchange interaction. It has been earlier shown that the said interaction provides an enhancement of cycle efficiency for two spin-1/2 particles, with an upper bound which is tighter than the Carnot efficiency. However, the necessary conditions governing engine performance and the relevant upper bound for efficiency are unknown for the general case of arbitrary spin magnitudes. Analyzing extreme-case scenarios, we formulate heuristics to infer the necessary conditions for an engine with uncoupled as well as coupled spins model. These conditions lead us to a connection between performance of quantum heat engines and the notion of majorization. Further, the study of complete Otto cycles inherent in the average cycle also yields interesting insights into the average performance.

Replacements for Tue, 27 Jul 21

[34]  arXiv:2104.14211 (replaced) [pdf, ps, other]
Title: Quantum and thermal fluctuations in the dynamics of a resistively and capacitively shunted Josephson junction
Comments: 8 pages, 6 figures; published version
Journal-ref: Phys. Rev. B 104, 014519 (2021)
[35]  arXiv:2105.02411 (replaced) [pdf, other]
Title: Quantum Oscillation of Thermally Activated Conductivity in a Monolayer WTe$_2$-like Excitonic Insulator
Comments: 6 pages, 3 figures. 9 pages of supplemental material. Comments are welcome
Journal-ref: Phys. Rev. B 104, L041110 (2021)
[36]  arXiv:2105.08056 (replaced) [pdf, other]
Title: A graphene edge-mediated quantum gate
Comments: 6 pages, 8 figures, supplemental material attached
[37]  arXiv:2106.03908 (replaced) [pdf, other]
Title: Scanning probe-induced thermoelectrics in a quantum point contact
Comments: 5+ pages. Includes supplementary material (another 4 pages). Accepted version
Journal-ref: Appl. Phys. Lett. 119, 043101 (2021)
[38]  arXiv:2106.08216 (replaced) [pdf, other]
Title: A mesoscopic model of an adsorbate clusters self-assembly by diffusion on a graphene Moiré
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Materials Science (cond-mat.mtrl-sci); Pattern Formation and Solitons (nlin.PS); Atomic and Molecular Clusters (physics.atm-clus)
[39]  arXiv:2106.10510 (replaced) [pdf, other]
Title: The effect of multiple energy-scales on mesospin ordering
Comments: 9 pages, 7 figures
[40]  arXiv:2107.09823 (replaced) [pdf, other]
Title: In-flight confinement of an electron surfing on a sound wave
Comments: Main paper: 10 pages, 3 figures, Supplementary material: 5 pages, 4 figures
[41]  arXiv:2107.10192 (replaced) [pdf, other]
Title: Non-monotonic spin-phase gathering in curved spintronic circuits
Comments: 8 pages, 5 figures, 1 appendix
[42]  arXiv:2107.00698 (replaced) [pdf, other]
Title: Evidence for Higher order topology in Bi and Bi$_{0.92}$Sb$_{0.08}$
Journal-ref: Nature Communications,2021
[ total of 42 entries: 1-42 ]

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