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We analyze the effective Landé factor g * and its dependence on the orientation of the external magnetic field for a quantum point contact defined in the two-dimensional electron gas. The paper simulates the experimental procedure for evaluation of the effective Landé factors from the transconductance of a biased device in an external magnetic field. The contributions of the orbital effects of the magnetic field, the electron-electron interaction, and spin-orbit (SO) coupling are studied in low-temperature conditions (0.5 K). The anisotropy of the g*factors for the in-plane magnetic field orientation, which seems counterintuitive from the perspective of the effective SO magnetic field, is explained in an analytical model of the constriction as due to the SO-induced subband mixing. The asymmetry of the transconductance as a function of the gate voltage is obtained in agreement with the experimental data and the results are explained as due to depletion of the electron gas within the quantum point contact constriction and the related reduction of the screening as described within the DFT approach. The results for transconductance and the g * factors obtained are in a good agreement with the experimental data [Martin et al., Phys. Rev. B 81, 041303 (2010)]. [ABSTRACT FROM AUTHOR]

Shot noise of electrons that are transmitted with probability T through a quantum point contact (biased at a voltage V0) leads to a fluctuating current that in turn emits radiation in the microwave regime. By calculating the Fano factor F for the case where only a single channel contributes to the transport, it has been shown that the radiation produced at finite frequency 0 close to eV0/h and at low temperatures is nonclassical with sub-Poissonian statistics (F<1). The origin of this effect is the fermionic nature of the electrons producing the radiation, which reduces the probability of simultaneous emission of two or more photons. However, the Fano factor, being a time-averaged quantity, offers only limited information about the system. Here, we calculate the second-order coherence g(2)(t) for this source of radiation. We show that due to the interference of two contributions, two photon processes (leading to bunching) are completely absent at zero temperature for T=50%. At low temperatures, we find a competition of the contribution due to Gaussian current-current fluctuations (leading to bunching) with the one due to non-Gaussian fluctuations (leading to antibunching). At slightly elevated temperatures, the non-Gaussian contribution becomes suppressed, whereas the Gaussian contributions remain largely independent of temperature. We show that the competition of the two contributions leads to a nonmonotonic behavior of the second-order coherence as a function of time. As a result, g(2)(t) obtains a minimal value for times t*≃-10. Close to this time, the second-order coherence remains below 1 at temperatures where the Fano factor is already above 1. We identify realistic experimental parameters that can be used to test the sub-Poissonian nature of the radiation. [ABSTRACT FROM AUTHOR]

Quantum point contacts (QPCs) and quantum dots (QDs), two elementary building blocks of semiconducting nanodevices, both exhibit famously anomalous conductance features: the 0.7 anomaly in the former case, the Kondo effect in the latter. For both the 0.7 anomaly and the Kondo effect, the conductance shows a remarkably similar low-energy dependence on temperature T, source-drain voltage Vsd, and magnetic field B. In a recent publication [F. Bauer et al., Nature (London) 501, 73 (2013)], we argued that the reason for these similarities is that both a QPC and a Kondo QD (KQD) feature spin fluctuations that are induced by the sample geometry, confined in a small spatial regime, and enhanced by interactions. Here, we further explore this notion experimentally and theoretically by studying the geometric crossover between a QD and a QPC, focusing on the B-field dependence of the conductance. We introduce a one-dimensional model with local interactions that reproduces the essential features of the experiments, including a smooth transition between a KQD and a QPC with 0.7 anomaly. We find that in both cases the anomalously strong negative magneto conductance goes hand in hand with strongly enhanced local spin fluctuations. Our experimental observations include, in addition to the Kondo effect in a QD and the 0.7 anomaly in a QPC, Fano interference effects in a regime of coexistence between QD and QPC physics, and Fabry-Perot-type resonances on the conductance plateaus of a clean QPC. We argue that Fabry-Perot-type resonances occur generically if the electrostatic potential of the QPC generates a flatter-than-parabolic barrier top. [ABSTRACT FROM AUTHOR]

Bilayer graphene in a perpendicular electric field can host domain walls between regions of reversed field direction or interlayer stacking. The gapless modes propagating along these domain walls, while not strictly topological, nevertheless have interesting physical properties, including valley-momentum locking. A junction where two domain walls intersect forms the analog of a quantum point contact. We study theoretically the critical behavior of this junction near the pinch-off transition, which is controlled by two separate classes of nontrivial quantum critical points. For strong interactions, the junction can host phases of unique charge and valley conductances. For weaker interactions, the low-temperature charge conductance can undergo one of two possible quantum phase transitions, each characterized by a specific critical exponent and a collapse to a universal scaling function, which we compute. [ABSTRACT FROM AUTHOR]

The quantum transmission problem for a particle moving in a quantum point contact in the presence of a Rashba spin-orbit interaction and applied magnetic field is solved semiclassically. A strong Rashba interaction and parallel magnetic field form emergent Dirac states at the center of the constriction, leading to the appearance of resonances which carry spin current and become bound at high magnetic fields. These states can be controlled in situ by modulation of external electric and magnetic fields, and can be used to turn the channel into a spin pump which operates at zero bias. It is shown that this effect is currently experimentally accessible in p-type quantum point contacts. [ABSTRACT FROM AUTHOR]

We report an electronic magnetization measurement of a quantum point contact (QPC) based on nuclear magnetic resonance (NMR) spectroscopy. We find that NMR signals can be detected by measuring the QPC conductance under in-plane magnetic fields. This makes it possible to measure, from Knight shifts of the NMR spectra, the electronic magnetization of a QPC containing only a few electron spins. The magnetization changes smoothly with the QPC potential barrier height and peaks at the conductance plateau of 0.5 × 2e2/h. The observed features are well captured by a model calculation assuming a smooth potential barrier, supporting a no bound state origin of the 0.7 structure. [ABSTRACT FROM AUTHOR]

We evaluate the joint distributions of electron waiting times in coherent conductors described by scattering theory. Successive electron waiting times in a single-channel conductor are found to be correlated due to the fermionic statistics encoded in the many-body state. Our formalism allows us also to investigate the waiting times between charge transfer events in different outgoing channels. As an application we consider a quantum point contact in a chiral setup with one or both input channels biased by either a static or a time-dependent periodic voltage described by Floquet theory. The theoretical framework developed here can be applied to a variety of scattering problems and can in a straightforward manner be extended to joint distributions of several electron waiting times. [ABSTRACT FROM AUTHOR]

Small differential conductance oscillations as a function of source-drain bias are observed and systematically studied in an asymmetric quantum point contact. These oscillations become significantly suppressed in a small in-plane magnetic field (~0.7 T) or at higher temperatures (~800 mK). Qualitatively, their temperature evolution can be simulated numerically based on smearing of the Fermi distribution, whereas features near zero bias cannot. Single-particle scenarios are unsatisfactory in accounting for the oscillations, suggesting that they are likely caused by electron- and spin-correlation effects. [ABSTRACT FROM AUTHOR]

We address the quantum-critical behavior of two-dimensional itinerant ferromagnetic systems described by spin-fermion models in which fermions interact with close-to-critical bosonic modes. We consider Heisenberg ferromagnets, Ising ferromagnets, and the Ising nematic transition. Mean-field theory close to the quantum critical point predicts a superconducting gap with spin-triplet symmetry for the ferromagnetic systems and a singlet gap for the nematic scenario. Studying fluctuations in this ordered phase using a nonlinear sigma model, we find that these fluctuations are not suppressed by any small parameter. As a result, we find that a superconducting quasi-long-range order is still possible in the Ising-like models but long-range order is destroyed in Heisenberg ferromagnets. [ABSTRACT FROM AUTHOR]

We study how the conductance of a quantum point contact is affected by spin-orbit interactions, for systems at zero temperature both with and without electron-electron interactions. In the presence of spin-orbit coupling, tuning the strength and direction of an external magnetic field can change the dispersion relation and hence the local density of states in the point contact region. This modifies the effect of electron-electron interactions, implying striking changes in the shape of the 0.7-anomaly and introducing additional distinctive features in the first conductance step. [ABSTRACT FROM AUTHOR]

We analyze transport through a quantum point contact in fractional quantum Hall states with counter-propagating neutral edge modes. We show that both the noise (as expected and previously calculated by other authors) and (perhaps surprisingly) the average transmitted current are affected by downstream perturbations within the standard edge state model. We consider two scenarios for downstream perturbations. We argue that the change in transmitted current should be observable in experiments that have shown increased noise. [ABSTRACT FROM AUTHOR]

We consider an open system near a quantum critical point that is suddenly moved towards the critical point. The bath-dominated diffusive nonequilibrium dynamics after the quench is shown to follow scaling behavior, governed by a critical exponent that emerges in addition to the known equilibrium critical exponents. We determine this exponent and show that it describes universal prethermalized coarsening dynamics of the order parameter in an intermediate time regime. Implications of this quantum critical prethermalization are: (i) a power law rise of order and correlations after an initial collapse of the equilibrium state and (ii) a crossover to thermalization that occurs arbitrarily late for sufficiently shallow quenches. [ABSTRACT FROM AUTHOR]

Here we calculate the pairing interaction and the k dependence of the gap function associated with the nematic charge fluctuations of a CuO2 model. We find that the nematic pairing interaction is attractive for small momentum transfer and that it gives rise to d-wave pairing. As the doping p approaches a quantum critical point, the strength of this pairing increases and higher d-wave harmonics contribute to the k dependence of the superconducting gap function, reflecting the longer range nature of the nematic fluctuations. [ABSTRACT FROM AUTHOR]

We consider scanning gate microscopy imaging of the double slit interference for a pair of quantum point contacts (QPCs) defined within the two-dimensional electron gas. The interference is clearly present in the scattered electron wave functions for each of the incident subbands. Nevertheless, we find that the interference is generally missing in the experimentally accessible conductance maps for many incident subbands. We explain this finding on the basis of the Landauer approach. A setup geometry allowing for observation of the double slit interference by scanning gate microscopy is proposed. [ABSTRACT FROM AUTHOR]

Here we calculate the electron transmission phase through a quantum point contact (QPC). The QPC is considered in the saddle-point approximation in the single-electron picture. We show that when the electron energy is close to the height of the potential barrier, the transmission phase depends linearly on the energy. The coefficient in the linear dependence is logarithmically enhanced by the ratio of the height over the curvature of the barrier potential. We compare the calculated transmission phase with the first experimental measurements of the phase. [ABSTRACT FROM AUTHOR]

We study the transport properties of a four-terminal comer junction made by etching a two-dimensional topological insulator toform a quantum point contact (QPC). The QPC geometry enables interboundary tunneling processes allowing for the coupling among states with different hehcity, while the tight confinement in the QPC region activates charging effects leading to the Coulomb-blockade physics. Peculiar signatures of these effects are theoretically investigated using a scattering field theory modified to take into account the electron-electron interaction within a self-consistent mean field approach. The current-voltage characteristics and the current fluctuations (noise) are derived beyond the linear response regime. Universal aspects of the thermal noise of the comer junction made of helical matter are also discussed. [ABSTRACT FROM AUTHOR]

The nature of the fractional quantum Hall state with filling factor v = 2/3 and its edge modes continues to remain an open problem in low-dimensional condensed matter physics. Here, we suggest an experimental setting to probe the v = 2/3 edge by tunnel-coupling it to a v = 1 integer quantum Hall edge in another layer of a two-dimensional electron gas (2DEG). In this double-layer geometry, the momentum of tunneling electrons may be boosted by an auxiliary magnetic field parallel to the two planes of 2DEGs. The threshold behavior of the current as a function of bias voltage and the boosting magnetic field yields information about the spectral function of the v = 2/3 edge, and in particular about the nature of the chiral edge modes. Our theoretical analysis accounts also for the effects of Coulomb interaction and disorder. [ABSTRACT FROM AUTHOR]

We study the charge dynamics of a quantum dot as measured by a nearby quantum point contact probing the dot via individual single-particle wave packets. We contrast the two limiting cases of weak and strong system-detector coupling exerting vanishing and strong backaction on the system and analyze the resulting differences in the charge-charge correlator. Extending the study to multiple projective measurements modeling a continuous strong measurement, we identify a transition from a charge dynamics dominated by the system's properties to a universal dynamics governed by the measurement. [ABSTRACT FROM AUTHOR]

We present transport experiments performed in high-quality quantum point contacts embedded in a GaAs two-dimensional hole gas. The strong spin-orbit interaction results in peculiar transport phenomena, including the previously observed anisotropic Zeeman splitting and level-dependent effective g factors. Here we find additional effects, namely, the crossing and the anticrossing of spin-split levels depending on subband index and magnetic field direction. Our experimental observations are reconciled in a heavy-hole effective spin-orbit Hamiltonian where cubic- and quadratic-in-momentum terms appear. The spin-orbit components, being of great importance for quantum computing applications, are characterized in terms of magnitude and spin structure. In light of our results, we explain the level-dependent effective g factor in an in-plane field. Through a tilted magnetic field analysis, we show that the quantum point contact out-of-plane g factor saturates around the predicted 7.2 bulk value. [ABSTRACT FROM AUTHOR]

We consider a sequence of quantized Lorentzian pulses of noninteracting electrons impinging on a quantum point contact and study the waiting time distribution (WTD), for any transmission and any number of pulses. As the degree of overlap between the electronic wave functions is tuned, the WTD reveals how the correlations between particles are modified. In the weak overlap regime, the WTD is made of several equidistant peaks, separated by the same period as the incoming pulses, contained in an almost exponentially decaying envelope. In the other limit, the WTD of a single quantum channel subjected to a constant voltage is recovered. In both cases, the WTD stresses the difference between the fluctuations induced by the scatterer and the ones encoded in the incoming quantum state. A clear crossover between these two situations is studied with numerical and analytical calculations based on scattering theory. [ABSTRACT FROM AUTHOR]

We report an observation, via sensitive shot noise measurements, of charge fractionalization of chiral edge electrons in the integer quantum Hall effect regime. Such fractionalization results solely from interchannel Coulomb interaction, leading electrons to decompose to excitations carrying fractional charges. The experiment was performed by guiding a partitioned current carrying edge channel in proximity to another unbiased edge channel, leading to shot noise in the unbiased edge channel without net current, which exhibited an unconventional dependence on the partitioning. The determination of the fractional excitations, as well as the relative velocities of the two original (prior to the interaction) channels, relied on a recent theory pertaining to this measurement. Our result exemplifies the correlated nature of multiple chiral edge channels in the integer quantum Hall effect regime. [ABSTRACT FROM AUTHOR]

We study a superconducting quantum point contact made of a narrow In0.75Ga0.25 As channel with Nb proximity electrodes. The narrow channel is formed in a gate-fitted constriction of InGaAs/InAlAs/InP heterostructure hosting a two-dimensional electron gas. When the channel opening is varied with the gate, the Josephson critical current exhibits a discretized variation that arises from the quantization of the transverse momentum in the channel. The quantization of Josephson critical current persists down to the single-channel regime, providing an unambiguous demonstration of a semiconductor-superconductor hybrid Josephson junction involving only a .single ballistic channel. [ABSTRACT FROM AUTHOR]

We study the quantum point contact between the topological superconductor and the helical Luttinger liquid. The effects of the electron-electron interactions in the helical Luttinger liquid on the low-energy physics of this system are analyzed by the renormalization group. Among the various couplings at the point contact which arises from the tunneling via the Majorana edge channel, the induced backscattering in the helical Luttinger liquid is the most relevant for repulsive interactions. Hence, at low temperatures, the helical Luttinger liquid is effectively cut into two separated half wires. As a result, the low-temperature physics is described by a fixed point consisting of two leads coupled to the topological superconductor, and the electrical transport properties through the point contact at low temperature and low bias are dominated by the tunneling via the Majorana edge channel. We compute the temperature dependence of the zero-bias tunneling conductance and study the full counting statistics for the tunneling current at zero temperature. [ABSTRACT FROM AUTHOR]

We investigate the effect of a scanning gate tip on the nonlinear quantum transport properties of nanostructures. Generally, we predict that the symmetry of the current-voltage characteristic in reflection-symmetric samples is broken by a tip-induced rectifying conductance correction. Moreover, in the case of a quantum point contact (QPC), the tip-induced rectification term becomes dominant as compared to the change of the linear conductance at large tip-QPC distances. Calculations for a weak tip probing a QPC modeled by an abrupt constriction show that these effects are experimentally observable. [ABSTRACT FROM AUTHOR]

We investigate integer and fractional quantum Hall states in quantum point contacts (QPCs) of different geometries, defined in AlGaAs/GaAs heterostructures employing different doping and screening techniques. We find that even in the highest mobility samples, interference and localization strongly influence the transport properties. We propose microscopic models for these effects, based on single- and many-electron physics. For integer quantum Hall states, transport is modulated due to the self-consistent formation of compressible regions of enhanced or reduced density in the incompressible region of the constriction. In the fractional quantum Hall regime, we observe the localization of fractionally charged quasiparticles in the constriction and an interplay of single- and many-electron physics. At low electron densities and in comparatively weak magnetic fields, single-electron interference dominates transport. Utilizing optimized growth and gating techniques, the v = 5/2 state can be observed in a QPC, conserving the bulk properties in an unprecedented quality. Our results might improve the understanding of the influence of localization on the transmission properties of QPCs, which is necessary for the interpretation of interference experiments employing QPCs, especially at v = 5/2. [ABSTRACT FROM AUTHOR]

We consider an electronic analog of the Hong-Ou-Mandel (HOM) interferometer, where two single electrons travel along opposite chiral edge states and collide at a quantum point contact. Studying the current noise, we show that because of interactions between copropagating edge states, the degree of indistinguishability between the two electron wave packets is dramatically reduced, leading to reduced contrast for the HOM signal. This decoherence phenomenon strongly depends on the energy resolution of the packets. Insofar as interactions cause charge fractionalization, we show that charge and neutral modes interfere with each other, leading to satellite dips or peaks in the current noise. Our calculations explain recent experimental results [ E. Bocquillon Science 339 1054 (2013)] where an electronic HOM signal with reduced contrast was observed. [ABSTRACT FROM AUTHOR]

Electronic localization in narrow graphene constrictions is theoretically studied, and it is found that long-lived (∼1 ns) quasibound states (QBSs) can exist in a class of ultrashort graphene quantum point contacts (QPCs). These QBSs are shown to originate from the dispersionless edge states that are characteristic of the electronic structure of generically terminated graphene, in which pseudo-time-reversal symmetry is broken. The QBSs can be regarded as interface states confined between two graphene samples, and their properties can be modified by changing the sizes of the QPC and the interface geometry. In the presence of bearded sites, these QBSs can be converted into bound states. Experimental consequences and potential applications are discussed. [ABSTRACT FROM AUTHOR]

We study a quantum point contact in the fractional quantum Hall regime at Landau level filling factors v = 1/3 and 5/2. By using exact diagonalizations in the cylinder geometry, we identify the edge modes in the presence of a parabolic confining potential. By changing the sign of the potential, we can access both the tunneling through the bulk of the fluid and the tunneling between spatially separated droplets. This geometry is realized in the quantum point contact geometry for two-dimensional electron gases. In the case of the model Moore-Read Pfaffian state at filling factor v = 5/2, we identify the conformal towers of many-body eigenstates including the non-Abelian sector. By a Monte-Carlo technique, we compute the various scaling exponents that characterize the edge modes. In the case of hard-core interactions whose ground states are exact model wave functions, we find equality of neutral and charged velocities, both bosonic and fermionic, for the Pfaffian state [ABSTRACT FROM AUTHOR]

In classically chaotic systems, small differences in initial conditions are exponentially magnified over time. However, it was observed experimentally that the (necessarily quantum) "branched flow" pattern of electron flux from a quantum point contact (QPC) traveling over a random background potential in two-dimensional electron gases remains substantially invariant to large changes in initial conditions. Since such a potential is classically chaotic and unstable to changes in initial conditions, it was conjectured that the origin of the observed stability is purely quantum mechanical, with no classical analog. In this Letter, we show that the observed stability is a result of the physics of the quantum point contact and the nature of the experiment. We show that the same stability can indeed be reproduced classically, or quantum mechanically. In addition, we explore the stability of the branched flow with regards to changes in the eigenmodes of the quantum point contact. [ABSTRACT FROM AUTHOR]

We consider the absorption of microwaves by a quantum Hall droplet. We show that the number and velocities of charged edge modes can be directly measured from a droplet of known shape. In contrast to standard transport measurements, different edge equilibration regimes can be accessed in the same device. If there is a quantum point contact in the droplet, then quasiparticle properties, including braiding statistics, can be observed. Their effects are manifested as modulations of the microwave absorption spectrum that are, notably, first-order in the tunneling amplitude at the point contact. [ABSTRACT FROM AUTHOR]

We investigate the effect of interactions on shot noise in v=2 quantum Hall edges, where a repulsive coupling between copropagating edge modes is expected to give rise to charge fractionalization. Using the method of nonequilibrium bosonization, we find that even asymptotically the edge distribution function depends in a sensitive way on the interaction strength between the edge modes. We compute shot noise and the Fano factor from the asymptotic distribution function, and from comparison with a reference model of fractionalized excitations, we find that the Fano factor can be close to the value of the fractionalized charge. [ABSTRACT FROM AUTHOR]

A quantum point contact was used to observe single-electron fluctuations of a quantum dot in a GaAs heterostructure. The resulting random telegraph signals (RTS) contain statistical information about the electron spin state if the tunneling dynamics are spin dependent. We develop a statistical method to extract information about spin-dependent dynamics from RTS and use it to demonstrate that these dynamics can be studied in the thermal energy regime. The tunneling rates of each spin state are independently measured in a finite external magnetic field. We confirm previous findings of a decrease in overall tunneling rates for the spin excited state compared to the ground state as an external magnetic field is increased. [ABSTRACT FROM AUTHOR]

Two recent experiments [ Radu et al. Science 320 899 (2008); Lin et al. Phys. Rev. B 85 165321 (2012)] measured the temperature and voltage dependence of the quasiparticle tunneling through a quantum point contact in the ν=5/2 quantum Hall liquid. The results led to conflicting conclusions about the nature of the quantum Hall state. In this paper, we show that the conflict can be resolved by recognizing different geometries of the devices in the experiments. We argue that in some of those geometries there is significant unscreened electrostatic interaction between the segments of the quantum Hall edge on the opposite sides of the point contact. Coulomb interaction affects the tunneling current. We compare experimental results with theoretical predictions for the Pfaffian, SU(2)2, 331, and K=8 states and their particle-hole conjugates. After Coulomb corrections are taken into account, measurements in all geometries agree with the spin-polarized and spin-unpolarized Halperin 331 states. [ABSTRACT FROM AUTHOR]

Linear conductance below 2e2/h shows resonance peaks in highly asymmetric quantum point contacts (QPCs). As the channel length increases, the number of peaks also increases. At the same time, differential conductance exhibits zero-bias anomalies (ZBAs) in correspondence with every other peak in the linear conductance. This even-odd effect, observable in the longer channels, is consistent with the formation of correlation-induced quasilocalized states within the QPC. In rare cases, triple peaks are observed, indicating the formation of a spin-1 Kondo effect when the electron filling number is even. Changing the gate voltage tunes this spin triplet to a singlet which exhibits no ZBA. The triple peak provides evidence suggestive of a spin singlet-triplet transition in a QPC, and the presence of a ferromagnetic spin interaction between electrons. [ABSTRACT FROM AUTHOR]

We investigate near-equilibrium ballistic transport through a quantum point contact (QPC) along a GaAs/AlGaAs heterojunction with a transfer matrix technique as a function of temperature and the shape of the potential barrier in the QPC. Our analysis is based on a three-dimensional (3D) quantum-mechanical variational model within the Hartree-Fock approximation that takes into account the vertical depletion potential from ionized acceptors in GaAs and the gate-induced transverse confinement potential that reduce to an effective slowly varying one-dimensional (1D) potential along the narrow constriction. The calculated zero-temperature transmission exhibits a shoulder ranging from 0.3 to 0.6 depending on the length of the QPC and the profile of the barrier potential. The effect is a consequence of the compressibility peak in the 1D electron gas and is enhanced for antiferromagnetic interaction among electrons in the QPC, but is smeared out once temperature is increased by a few tenths of a Kelvin. [ABSTRACT FROM AUTHOR]

A systematic theory of the conductance measurements of noninvasive (weak probe) scanning gate microscopy is presented that provides an interpretation of what precisely is being measured. A scattering approach is used to derive explicit expressions for the first- and second-order conductance changes due to the perturbation by the tip potential in terms of the scattering states of the unperturbed structure. In the case of a quantum point contact, the first-order correction dominates at the conductance steps and vanishes on the plateaux where the second-order term dominates. Both corrections are nonlocal for a generic structure. Only in special cases, such as that of a centrally symmetric quantum point contact in the conductance quantization regime, can the second-order correction be unambiguously related with the local current density. In the case of an abrupt quantum point contact, we are able to obtain analytic expressions for the scattering eigenfunctions and thus evaluate the resulting conductance corrections. [ABSTRACT FROM AUTHOR]

We present a first-principles approach for inelastic quantum transport calculations based on maximally localized Wannier functions. Electronic-structure properties are obtained from density-functional theory in a plane-wave basis, and electron-vibration coupling strengths and vibrational properties are determined with density-functional perturbation theory. Vibration-induced inelastic transport properties are calculated with nonequilibrium Green's function techniques; since these are based on a localized orbital representation we use maximally localized Wannier functions. Our formalism is applied first to investigate inelastic transport in a benzene molecular junction connected to monoatomic carbon chains. In this benchmark system the electron-vibration self-energy is calculated either in the self-consistent Born approximation or by lowest-order perturbation theory. It is observed that upward and downward conductance steps occur, which can be understood using multieigenchannel scattering theory and symmetry conditions. In a second example, where the monoatomic carbon chain electrode is replaced with a (3,3) carbon nanotube, we focus on the nonequilibrium vibration populations driven by the conducting electrons using a semiclassical rate equation and highlight and discuss in detail the appearance of vibrational cooling as a function of bias and the importance of matching the vibrational density of states of the conductor and the leads to minimize joule heating and breakdown. [ABSTRACT FROM AUTHOR]

Low-temperature transport measurements on a p-GaAs quantum point contact are presented which reveal the presence of a conductance anomaly that is markedly different from the conventional "0.7 anomaly." A lateral shift by asymmetric gating of the conducting channel is utilized to identify and separate different conductance anomalies of local and generic origins experimentally. While the more generic 0.7 anomaly is not directly affected by changing the gate configuration, a model is proposed which attributes the additional conductance features to a gate-dependent coupling of the propagating states to localized states emerging due to a nearby potential imperfection. Finite bias conductivity measurements reveal the interplay between the two anomalies consistently with a two-impurity Kondo model. [ABSTRACT FROM AUTHOR]

We present transport measurements in superconductor-nanowire devices with a gated constriction forming a quantum point contact. Zero-bias features in tunneling spectroscopy appear at finite magnetic fields and oscillate in amplitude and split away from zero bias as a function of magnetic field and gate voltage. A crossover in magnetoconductance is observed: Magnetic fields above ∼0.5 T enhance conductance in the low-conductance (tunneling) regime but suppress conductance in the high-conductance (multichannel) regime. We consider these results in the context of Majorana zero modes as well as alternatives, including the Kondo effect and analogs of 0.7 structure in a disordered nanowire. [ABSTRACT FROM AUTHOR]

We investigate the current noise correlations at a quantum point contact in a quantum spin Hall structure, focusing on the effect of a weak magnetic field in the presence of disorder. For the case of two equally biased terminals we discover a robust peak: the noise correlations vanish at B = 0 and are negative for B ≠ 0. We find that the character of this peak is intimately related to the interplay between time reversal symmetry and the helical nature of the edge states and call it the Z2 peak. [ABSTRACT FROM AUTHOR]

For electronic transport at the nanoscale, coherent scattering at defects plays an important role. Therefore, the capability of visualizing the influence of defects on the conductivity of single atomic junctions may benefit the development of future nano-electronics. Here, we report imaging the coherent scattering from a defect with well-controlled geometry by quantum point contact microscopy recently developed by us. An ~10% modulation in transport conductance of a single atomic junction is observed, with a phase shift of nearly π compared to the tunneling conductance. With the well-defined scattering geometry, we performed a theoretical calculation of the conductance and found the result consistent with the experiment. [ABSTRACT FROM AUTHOR]

We investigate the dynamics following sudden quenches across quantum critical points belonging to different universality classes. Specifically, we use matrix product state methods to study the quantum Ising chain in the presence of two additional terms which break integrability. We find that in all models the rate function for the return probability to the initial state becomes a nonanalytic function of time in the thermodynamic limit. This so-called "dynamical phase transition" was first observed in a recent work by Heyl, Polkovnikov, and Kehrein [Phys. Rev. Lett. 110, 135704 (2013)] for the exactly-solvable quantum Ising chain, which can be mapped to free fermions. Our results for "interacting theories" indicate that nonanalytic dynamics is a generic feature of sudden quenches across quantum critical points. We discuss potential connections to the dynamics of the order parameter. [ABSTRACT FROM AUTHOR]

Occurrence of the Berezinskii-Kosterlitz-Thouless (BKT) transition is investigated by superfluid density measurements for two-dimensional (2D) disordered NbN films with disorder level very close to a superconductor-insulator transition (SIT). Our data show a robust BKT transition even near this 2D disorder-tuned quantum critical point. This observation is in direct contrast with previous data on deeply underdoped quasi-2D cuprates near the SIT. As our NbN films approach the quantum critical point, the vortex core energy, an important energy scale in the BKT transition, scales with the superconducting gap, not with the superfluid density, as expected within the standard 2D-.XT model description of BKT physics. [ABSTRACT FROM AUTHOR]

We report measurements of the electrical and thermal transport on the hexagonal heavy-fermion metal YbAgGe for temperatures T=40⩾mK and in magnetic fields H ∥ ab up to 14 T. This distorted kagome-lattice system displays a series of magnetic states and a quantum critical point at Hc=4.5 T. The Lorenz ratio L(T)/L0 displays a marked reduction only close to Hc. A T-linear contribution below 120 mK, present at all different fields, allows us to extrapolate the Lorenz ratio towards T=0. At the critical field this yields L/L0=0.92±0.03, suggesting a violation of the Wiedemann-Franz law due to strong inelastic scattering. [ABSTRACT FROM AUTHOR]

We demonstrate that the combination of an external magnetic field and the intrinsic spin-orbit interaction results in nonadiabatic precession of the electron spin after transmission through a quantum point contact (QPC). We suggest that this precession may be observed in a device consisting of two QPCs situated in series. The pattern of resonant peaks in the transmission is strongly influenced by the non-Abelian phase resulting from this precession. Moreover, an unusual type of resonance which is associated with suppressed, rather than enhanced, transmission (i.e., antiresonance) emerges in the strongly nonadiabatic regime. The shift in the resonant transmission peaks is dependent on the spin-orbit interaction and therefore offers a way to directly measure these interactions in a ballistic one-dimensional system. [ABSTRACT FROM AUTHOR]

We report a study of the magnetoresistivity of high purity Sr3Ru2O7, in the vicinity of its electronic nematic phase. By employing a triple-axis (9/1/1 T) vector magnet, we were able to precisely tune both the magnitude and direction of the in-plane component of the magnetic field (H∥). We report the dependence of the resistively determined anisotropy on H∥ in the phase, as well as across the wider temperature-field region. Our measurements reveal a high-temperature anisotropy which mimics the behavior of fluctuations from the underlying quantum critical point, and suggest the existence of a more complicated phase diagram. [ABSTRACT FROM AUTHOR]

The presence of edges locally breaks the inversion symmetry of heterostructures and gives rise to lateral (edge) spin-orbit coupling (SOC), which, under some conditions, can lead to the formation of helical edge states. If the edge SOC is strong enough, the helical edge states can penetrate the band gap and be energetically isolated from the bulk-like states. As a result, backward scattering is suppressed, dissipationless helical edge channels protected against time-inversion symmetric perturbations emerge, and the system behaves as a two-dimensional topological insulator (TI). However, unlike in previous work on TIs, the mechanism proposed here for the creation of protected helical edge states relies on the strong edge SOC rather than on band inversion. [ABSTRACT FROM AUTHOR]

We investigate adiabatic and nonadiabatic emission of single particles into an edge state using an analytically solvable dynamical scattering matrix model of an on-demand source. We compare adiabatic and nonadiabatic emissions by considering two geometries: a collider geometry where two emitters are coupled to two different edge states and a series geometry where two emitters are coupled to the same edge state. Most effects observed for adiabatic emitters also occur for nonadiabatic emitters. In particular this applies to effects arising due to the overlap of wave packets colliding at a quantum point contact. Specifically we compare the Pauli peak (the fermionic analog of the bosonic Hong-Ou-Mandel dip) for the adiabatic and nonadiabatic collider and find them to be similar. In contrast we find a striking difference between the two operating conditions in the series geometry in which particles are emitted into the same edge state. Whereas the squared average charge current can be nullified for both operating conditions, the heat current can be made to vanish only with adiabatic emitters. [ABSTRACT FROM AUTHOR]

Integrating the Kondo correlation and spin-orbit interactions, each of which have individually offered unprecedented means to manipulate electron spins, in a controllable way can open up new possibilities for spintronics. We demonstrate electrical control of the Kondo correlation by coupling the bound spin to leads with tunable Rashba spin-orbit interactions, realized in semiconductor quantum point contacts. We observe a transition from single to double peak zero-bias anomalies in nonequilibrium transport--the manifestation of the Kondo effect--indicating a controlled Kondo spin reversal using only spin-orbit interactions. Universal scaling of the Kondo conductance is demonstrated, implying that the spin-orbit interactions could enhance the Kondo temperature. A theoretical model based on quantum master equations is also developed to calculate the nonequilibrium quantum transport. [ABSTRACT FROM AUTHOR]