We report drive-response experiments on individual superconducting vortices on a plane, a realization for a (1+1)-dimensional directed polymer in random media. For this we use magnetic force microscopy to image and manipulate individual vortices trapped on a twin boundary in ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ near optimal doping. We find that when we drag a vortex with the magnetic tip, it moves in a series of jumps. As theory suggests, the jump-size distribution does not depend on the applied force and is consistent with power-law behavior. The measured power is much larger than widely accepted theoretical calculations.

We study the interplay of interactions and disorder in a one-dimensional fermion lattice coupled adiabatically to infinite reservoirs. We employ both the functional renormalization group (FRG) as well as matrix product state techniques, which serve as an accurate benchmark for small systems. Using the FRG, we compute the length- and temperature-dependence of the conductance averaged over ${10}^4$ samples for lattices as large as ${10}^5$ sites. We identify regimes in which non-Ohmic power law behavior can be observed and demonstrate that the corresponding exponents can be understood by adapting earlier predictions obtained perturbatively for disordered Luttinger liquids. In the presence of both disorder and isolated impurities, the conductance has a universal single-parameter scaling form. This lays the groundwork for an application of the functional renormalization group to the realm of many-body localization.

We present a formalism for strongly correlated electron systems which consists in a local approximation of the dynamical three-leg interaction vertex. This vertex is self-consistently computed using a quantum impurity model with dynamical interactions in the charge and spin channels, similar to dynamical mean field theory approaches. The electronic self-energy and the polarization are both frequency and momentum dependent. The method interpolates between the spin-fluctuation or GW approximations at weak coupling and the atomic limit at strong coupling. We apply the formalism to the Hubbard model on a two-dimensional square lattice and show that as interactions are increased towards the Mott insulating state, the local vertex acquires a strong frequency dependence, driving the system to a Mott transition, while at low enough temperatures the momentum dependence of the self-energy is enhanced due to large spin fluctuations. Upon doping, we find a Fermi arc in the one-particle spectral function, which is one signature of the pseudogap state.

We study the spin dynamics of carriers due to the Rashba interaction in semiconductor quantum disks and wells after excitation with light with orbital angular momentum. We find that although twisted light transfers orbital angular momentum to the excited carriers and the Rashba interaction conserves their total angular momentum, the resulting electronic spin dynamics is essentially the same for excitation with light with orbital angular momentum $l=+\left|l\right|$ and $l=-\left|l\right|$. The differences between cases with different values of $\left|l\right|$ are due to the excitation of states with slightly different energies and not to the different angular momenta per se and vanish for samples with large radii where a $k$-space quasicontinuum limit can be established. These findings apply not only to the Rashba interaction but also to all other envelope-function-approximation spin-orbit Hamiltonians like the Dresselhaus coupling.

Symmetry-protected topological (SPT) phases of matter have been the focus of many recent theoretical investigations, but controlled mechanisms for engineering them have so far been elusive. In this work, we demonstrate that by driving interacting spin systems periodically in time and tuning the available parameters, one can realize lattice models for bosonic SPT phases in the limit where the driving frequency is large. We provide concrete examples of this construction in one and two dimensions, and discuss signatures of these phases in stroboscopic measurements of local observables.

A momentum balance equation is developed to investigate the magnetotransport properties in monolayer molybdenum disulphide when a strong perpendicular magnetic field and a weak in-plane electric field are applied simultaneously. At low temperature, in the presence of intravalley impurity scattering, Shubnikov–de Haas oscillation shows up accompanied by a beating pattern arising from large spin splitting and its period may halve due to the high-order oscillating term at large magnetic field for samples with ultrahigh mobility. In the case of intervalley disorders, there exists a magnetic-field range where the magnetoresistivity almost vanishes. For a low-mobility layer, a phase inversion of oscillating peaks is acquired in accordance with recent experiment. At high temperature when Shubnikov–de Haas oscillation is suppressed, the magnetophonon resonances induced by both optical phonons (mainly due to homopolar and Fröhlich modes) and acoustic phonons (mainly due to intravalley transverse and longitudinal acoustic modes) emerge for a suspended system with high mobility. For the single layer on a substrate, another resonance due to surface optical phonons may occur, resulting in a complex behavior of the total magnetoresistance. The beating pattern of magnetophonon resonance due to optical phonons can also be observed. However, for a nonsuspended layer with low mobility, the magnetoresistance oscillation almost disappears and the resistivity increases with field monotonically.

Understanding coherent dynamics of excitons, spins, or hard-core bosons (HCBs) has tremendous scientific and technological implications for quantum computation. Here, we study decay of excited-state population and decoherence in two models for HCBs, namely, a two-site HCB model with site-dependent strong potentials and subject to non-Markovian dynamics and an infinite-range HCB model governed by Markovian dynamics. Both models are investigated in the regimes of antiadiabaticity and strong HCB-phonon coupling with each site providing a different local optical phonon environment; furthermore, the HCB systems in both models are taken to be initially uncorrelated with the environment in the polaronic frame of reference. In the case of the two-site HCB model, we show clearly that the degree of decoherence and decay of excited state are enhanced by the proximity of the site-energy difference to the eigenenergy of phonons and are most pronounced when the site-energy difference is at resonance with twice the polaronic energy; additionally, the decoherence and the decay effects are reduced when the strength of HCB-phonon coupling is increased. For the infinite-range model, when the site energies are the same, we derive an effective many-body Hamiltonian that commutes with the long-range system Hamiltonian and thus has the same set of eigenstates; consequently, a quantum-master-equation approach shows that the quantum states of the system do not decohere.

The half-filled topmost valence band of ${\mathrm{Ir}}^{4+}$ in several iridates such as ${\mathrm{Sr}}_2{\mathrm{IrO}}_4,{\mathrm{IrO}}_2$, and ${\mathrm{CaIrO}}_3$ has been proposed to originate mainly from the spin-orbit coupled $J_{\mathrm{eff}}=1/2$ states. In pyrochlore iridates $R_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ ($R$: rare earth), some exotic electronic states are theoretically proposed by assuming $J_{\mathrm{eff}}=1/2$ states. However, the octahedral coordination around Ir is trigonally distorted, which may affect the energy level scheme of Ir $5d$ states. Here, we report spectra of resonant elastic and inelastic x-ray scattering in ${\mathrm{Eu}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ at the Ir $L$ edges. A large suppression of the magnetic scattering signal at the Ir $L_{\mathrm{II}}$ edge supports the $J_{\mathrm{eff}}=1/2$ picture rather than the $S=1/2$ one. The inelastic scattering spectrum indicates that the magnitude of the trigonal field on the ${\mathrm{Ir}}^{4+}$ states is evaluated to be comparable to the spin-orbit interaction. The energy diagram of the $5d$ state is proposed based on the simple cluster model.

Thin film YIG (${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$) is a prototypical material for experiments on thermally generated pure spin currents and the spin Seebeck effect. The $3\omega$ method is an established technique to measure the cross-plane thermal conductance of thin films, but cannot be used in YIG/GGG (${\mathrm{Ga}}_3{\mathrm{Gd}}_5{\mathrm{O}}_{12}$) systems in its standard form. We use two-dimensional modeling of heat transport and introduce a technique based on Bayesian statistics to evaluate measurement data taken from the $3\omega$ method. Our analysis method allows us to study material systems that have not been accessible with the conventionally used $3\omega$ analysis. Temperature-dependent thermal conductance data of thin film YIG are of major importance for experiments in the field of spin caloritronics. Here we show data between room temperature and 10 K for films covering a wide thickness range as well as the magnetic field effect on the thermal conductance between 10 and 50 K.

The interband optical transitions in cube-on-cube-type epitaxial ferroelectric $\mathrm{NaNb}{\mathrm{O}}_3$ thin films and a reference antiferroelectric $\mathrm{NaNb}{\mathrm{O}}_3$ crystal are studied in the spectral range of 0.74–9 eV using variable-angle spectroscopic ellipsometry. The energies and the types of the transitions in the films are found to differ significantly from those in the crystal and to vary with epitaxial strain. The results are discussed in terms of epitaxial effects, including higher crystal symmetry, anisotropic lattice strain, and ferroelectric polarization in the $\mathrm{NaNb}{\mathrm{O}}_3$ films compared to the crystal.

Based on a systematic first-principles study, the lowest-energy migration mechanisms and barriers for small vacancy-solute clusters $\left(V_n{X}_m\right)$ are determined in $\alpha$-Fe for carbon, nitrogen, and oxygen, which are the most frequent interstitial solutes in several transition metals. We show that the dominant clusters present at thermal equilibrium ($VX$ and $V{X}_2$) have very reduced mobility compared to isolated solutes, while clusters composed of a solute bound to a small vacancy cluster may be significantly more mobile. In particular, $V_{3}X$ is found to be the fastest cluster for all three solutes. This result relies on the large diffusivity of the most compact trivacancy in a bcc lattice. Therefore, it may also be expected for interstitial solutes in other bcc metals. In the case of iron, we find that $V_{3}X$ may be as fast as or even more mobile than an interstitial solute. At variance with common assumptions, the trapping of interstitial solutes by vacancies does not necessarily decrease the mobility of the solute. Additionally, cluster dynamics simulations are performed considering a simple iron system with supersaturation of vacancies, in order to investigate the impacts of small mobile vacancy-solute clusters on properties such as the transport of solute and the cluster size distributions.

Lattice stability and metastability, as well as melting, are important features of the physics and chemistry of dense hydrogen. Using ab initio molecular dynamics (AIMD), the classical superheating limit and melting line of metallic hydrogen are investigated up to 1.5 TPa. The computations show that the classical superheating degree is about 100 K, and the classical melting curve becomes flat at a level of 350 K when beyond 500 GPa. This information allows us to estimate the well depth and the potential barriers that must be overcome when the crystal melts. Inclusion of nuclear quantum effects (NQE) using path integral molecular dynamics (PIMD) predicts that both superheating limit and melting temperature are lowered to below room temperature, but the latter never reaches absolute zero. Detailed analysis indicates that the melting is thermally activated, rather than driven by pure zero-point motion (ZPM). This argument was further supported by extensive PIMD simulations, demonstrating the stability of Fddd structure against liquefaction at low temperatures.

The liquid regime equation of state of silicon dioxide ${\mathrm{SiO}}_2$ is calculated via quantum molecular dynamics in the density range of $5\text{–}15{\mathrm{g}/\mathrm{cm}}^3$ and with temperatures from 0.5 to 100 eV, including the $\alpha$-quartz and stishovite phase Hugoniot curves. Below 8 eV calculations are based on Kohn-Sham density functional theory (DFT), and above 8 eV a new orbital-free DFT formulation, presented here, based on matching Kohn-Sham DFT calculations is employed. Recent experimental shock data are found to be in very good agreement with the current results. Finally both experimental and simulation data are used in constructing a new liquid regime equation of state table for ${\mathrm{SiO}}_2$.

We extend White's minimally entangled typically thermal states approach (METTS) to allow Abelian and non-Ablian symmetries to be exploited when computing finite-temperature response functions in one-dimensional (1D) quantum systems. Our approach, called SYMETTS, starts from a METTS sample of states that are not symmetry eigenstates, and generates from each a symmetry eigenstate. These symmetry states are then used to calculate dynamic response functions. SYMETTS is ideally suited to determine the low-temperature spectra of 1D quantum systems with high resolution. We employ this method to study a generalized diamond chain model for the natural mineral azurite $\mathrm{Cu}{}_3\left(\mathrm{CO}{}_3{)}_2\right({\mathrm{OH})}_2$, which features a plateau at $\frac{1}{3}$ in the magnetization curve at low temperatures. Our calculations provide new insight into the effects of temperature on magnetization and excitation spectra in the plateau phase, which can be fully understood in terms of the microscopic model.

We analyze the stability of excitonic ground states in the two-band Hubbard model with additional electron-phonon and Hund's rule couplings using a combination of mean-field and variational cluster approaches. We show that both the interband Coulomb interaction and the electron-phonon interaction will cooperatively stabilize a charge density wave (CDW) state which typifies an “excitonic” CDW if predominantly triggered by the effective interorbital electron-hole attraction or a “phononic” CDW if mostly caused by the coupling to the lattice degrees of freedom. By contrast, the Hund's rule coupling promotes an excitonic spin density wave. We determine the transition between excitonic charge and spin density waves and comment on a fixation of the phase of the excitonic order parameter that would prevent the formation of a superfluid condensate of excitons. The implications for exciton condensation in several material classes with strongly correlated electrons are discussed.

The Lieb-Liniger model describes one-dimensional bosons interacting through a repulsive contact potential. In this work, we introduce an extended version of this model by replacing the contact potential with a decaying exponential. Using the recently developed continuous matrix product states techniques, we explore the ground-state phase diagram of this model by examining the superfluid and density correlation functions. At weak coupling superfluidity governs the ground state, in a similar way as in the Lieb-Liniger model. However, at strong coupling quasicrystal and super–Tonks-Girardeau regimes are also found, which are not present in the original Lieb-Liniger case. Therefore the presence of the exponentially decaying potential leads to a superfluid/super–Tonks-Girardeau/quasicrystal crossover, when tuning the coupling strength from weak to strong interactions. This corresponds to a Luttinger liquid parameter in the range $K\in (0,\infty )$, in contrast with the Lieb-Liniger model, where $K\in [1,\infty )$, and the screened long-range potential, where $K\in (0,1]$.

The spin-$1/2$ single-modulation (SM) and double-modulation (DM) photoluminescence (PL) detected magnetic resonance (PLDMR) in poly(2-methoxy-5-(2′-ethyl)–hexoxy-1,4- phenylene vinylene) (MEH-PPV) films and poly(3-hexylthiophene) (P3HT) films is described, analyzed, and discussed. In particular, the models based on spin-dependent recombination of charge pairs (SDR) and triplet-polaron quenching (TPQ) are evaluated. By analyzing the dependence of the resonance amplitude on the microwave chopping (modulation) frequency using rate equations, it is demonstrated that the TPQ model can well explain the observed resonance behavior, while SDR model cannot reproduce the results of the observed DM-PLDMR. Thus the observed spin-$1/2$ PLDMR is assigned to TPQ rather than SDR, even though the latter may also be present.

We study finite temperature transport in the Luttinger-Abrikosov-Beneslavskii phase—an interacting, scale invariant, non-Fermi-liquid phase found in quadratic semimetals. We develop a kinetic equation formalism to describe the dc transport properties, which are dominated by collisions, and compute the shear viscosity $\eta$. The ratio of shear viscosity to entropy density $\eta /s$ is a measure of the strength of interaction between the excitations of a quantum fluid. As a consequence of the quantum critical nature of the system, $\eta /s$ is a universal number and we find it to be consistent with a bound proposed from gauge-gravity duality.

We report on low-temperature spatially resolved photoluminescence (SRPL) experiments to study the diffusion of free excitons in a $1.5\text{−}\mu \mathrm{m}$-thick layer of high-purity epitaxial GaAs. Extending previous SRPL experiments, we analyze the stationary diffusion profiles detected on the second LO-phonon replica of the free exciton. This allows us to circumvent the inherent interpretation ambiguities of the free-exciton zero-phonon line. Moreover, a spatially resolved line shape analysis of the $\left(FX\right)-2\hslash {\mathrm{\Omega }}_{\text{LO}}$ replica provides direct experimental access to the pump-induced exciton temperature profile. We demonstrate that only resonant optical excitation prevents the buildup of a temperature gradient in the carrier system, which otherwise severely distorts the stationary and time-resolved free-exciton diffusion profiles.

The nonzero thickness of single-layer (SL) ${\mathrm{MoS}}_2$ manifests in electron states forming classes of states that are even and odd with respect to reflections through the central plane. These states are energetically well separated: In particular, we show that pristine SL ${\mathrm{MoS}}_2$ exhibits two band gaps, $E_{g\parallel }=1.9$ eV and $E_{g\perp }=3.2$ eV, for the optical in-plane and out-of-plane susceptibilities ${\chi }_{\parallel }$ and ${\chi }_{\perp }$, respectively. Because of this, odd states are often neglected, which effectively reduces SL ${\mathrm{MoS}}_2$ to a perfect two-dimensional system. We study states bound to defects in SL ${\mathrm{MoS}}_2$ with three types of vacancy defects (VDs), (i) Mo vacancy, (ii) ${\mathrm{S}}_2$ vacancy, and (iii) $3\times {\mathrm{MoS}}_2$ quantum antidot, and show that odd states play an equally important role as the even states. In particular, we show that odd states bound to VDs lead to resonances in ${\chi }_{\perp }$ inside $E_{g\perp }$ in SL ${\mathrm{MoS}}_2$ with VDs. Additionally, we demonstrate that the states bound to VDs are not necessarily confined to the band gap in the even subsystem, which necessitates extending the energy region affected by the bound states.