We analyze orbital effects of an in-plane magnetic field on the spin structure of states of a gated quantum dot based in a two-dimensional electron gas. Starting with a $k·p$ Hamiltonian, we perturbatively calculate these effects for the conduction band of GaAs, up to the third power of the magnetic field. We quantify several corrections to the $g$-tensor and reveal their relative importance. We find that for typical parameters, the Rashba spin-orbit term and the isotropic term, $H_{43}\propto {\mathbf{P}}^2\mathbf{B}·\mathbf{\sigma }$, give the largest contributions in magnitude. The in-plane anisotropy of the $g$-factor is, on the other hand, dominated by the Dresselhaus spin-orbit term. At zero magnetic field, the total correction to the $g$-factor is typically 5%–10% of its bulk value. In strong in-plane magnetic fields, the corrections are modified appreciably.

We present a consistent theory of the topological Hall effect (THE) in two-dimensional magnetic systems with a disordered array of chiral spin textures, such as magnetic skyrmions. We focus on the scattering regime when the mean-free path of itinerant electrons exceeds the spin texture size, and THE arises from the asymmetric carrier scattering on individual chiral spin textures. We calculate the resistivity tensor on the basis of the Boltzmann kinetic equation taking into account the asymmetric scattering on skyrmions via the collision integral. Our theory describes both the adiabatic regime when THE arises from a spin Hall effect and the nonadiabatic scattering when THE is due to purely charge transverse currents. We analyze the dependence of THE resistivity on a chiral spin texture structure, as well as on material parameters. We discuss the crossover between spin and charge regimes of THE driven by the increase of skyrmion size, the features of THE due to the variation of the Fermi energy, and the exchange interaction strength; we comment on the sign and magnitude of THE.

We present a functional renormalization-group calculation of the properties of a quantum critical metal in $d=2$ spatial dimensions. Our theory describes a general class of Pomeranchuk instabilities with $N_b$ flavors of boson. At small $N_b$ we find a family of fixed points characterized by weakly non-Fermi-liquid behavior of the conduction electrons and $z\approx 2$ critical dynamics for the order-parameter fluctuations, in agreement with the scaling observed by Schattner et al. [Phys. Rev. X 6, 031028 (2016)] for the Ising-nematic transition. Contrary to recent suggestions that this represents an intermediate regime en route to the scaling limit, our calculation suggests that this behavior may persist all the way to the critical point. As the number of bosons $N_b$ is increased, the model's fixed-point properties cross over to $z\approx 1$ scaling and non-Fermi-liquid behavior similar to that obtained by Fitzpatrick et al. [Phys. Rev. B 88, 125116 (2013)].

When two two-dimensional electron gas layers, each at Landau-level filling factor $\nu =1/2$, are sufficiently close together, a condensate of interlayer excitons emerges at low temperature. Although the excitonic phase is qualitatively well understood, the incoherent phase just above the critical layer separation is not. Using a combination of tunneling spectroscopy and conventional transport, we explore the incoherent phase in samples both near the phase boundary and further from it. In the more closely spaced bilayers we find the electronic spectral functions narrower and the Fermi energy of the $\nu =1/2$ composite fermion metal smaller than in the more widely separated bilayers. We attribute these effects to a softening of the intralayer Coulomb interaction due to interlayer screening.

In the study of strongly interacting systems, correlations on the two-particle level are receiving more and more attention. In this work, we study a particular two-particle correlation function: the fermion-boson vertex. It describes the response of the Green's function when an external field is applied and is an important ingredient of diagrammatic extensions of dynamical mean-field theory. We provide several perspectives on this object, using Ward identities, sum rules, perturbative analysis, and asymptotic relations. We then use these tools to study the vertex across the doping-driven metal-insulator transition and find a divergent imaginary part.

We study the current of chiral charge density in a Dirac semimetal with two Dirac points in momentum space, subjected to an externally applied time dependent electric field and in the presence of a magnetic field. Based on the kinetic equation approach, we find contributions to the chiral charge current that are proportional to the second power of the electric field and to the first and second powers of the magnetic field, describing the interplay of the chiral anomaly and the drift motion of electrons moving under the action of electric and magnetic fields.

We present an investigation of the electron spin dynamics in an ensemble of singly charged semiconductor quantum dots subject to an external magnetic field and laser pumping with circularly polarized light. The spectral laser width is tailored such that ensembles with an increasing number of quantum dots are coherently pumped. Surprisingly, the dephasing time $T^{*}$ of the electron spin polarization depends only weakly on the laser spectral width. These findings can be consistently explained by a cluster theory of coupled quantum dots with a long-range electronic spin-spin interaction. We present a numerical simulation of the spin dynamics based on the central spin model that includes a quantum mechanical description of the laser pulses as well as a time-independent Heisenberg interaction between each pair of electron spins. We discuss the individual dephasing contributions stemming from the Overhauser field, the distribution of the electron $g$ factors, and the electronic spin-spin interaction as well as the spectral width of the laser pulse. This analysis reveals counterbalancing effects on the total dephasing time when increasing the spectral laser width. On one hand, the increasing deviations of the electron $g$ factors reduce the dephasing time. On the other hand, more electron spins are coherently pumped and synchronize due to the electronic spin-spin interaction which extends the dephasing time. We find an excellent agreement between the experimental data and the dephasing time in the simulation using an exponential distribution of Heisenberg couplings with a mean value $\overline{J}\approx 0.26\mu \mathrm{eV}$.

The dynamics of a pure low polariton (LP) system created by resonant broadband excitation in a wide range of wave vectors was investigated in a high-$Q$ GaAs-based microcavity. The LP system is shown to inherit the high spatial coherence from the laser pulse and does not lose it during decay. As a result, its dynamics is well controlled by the spatial and momentum distributions of photons in the exciting pulse and described by the Gross-Pitaevskii equations. In particular, the purely dynamic formation of the highly populated coherent LP state was implemented at the LP band bottom in the cavity excited in a large spot by converging ps-long Gaussian laser pulses when the active region of the cavity is in front of its waist. The formed state is found to persist for several picoseconds until the LP-LP repulsion leads to the creation of high-energy LPs dissipating from the ground state with high velocities.

The temperature ($T$) and density ($\rho$) conditions at which hydrogen undergoes a molecular-to-atomic (MA) transition is crucial to our understanding of the gas-giant planets such as Jupiter and Saturn. First-principles (FP) calculations suggest that this transition is coincident with metallization and acts as a catalyst for hydrogen-helium demixing, which has significant consequences for models of planetary interiors. Prediction of this transition boundary has proven to be difficult using FP methods. In particular, detailed comparisons of finite temperature density functional theory (FT-DFT) calculations of the MA transition in both the high-$T$, low-$\rho$ regime, where the transition is largely $T$ driven, and the low-$T$, high-$\rho$ regime, where the transition is largely $\rho$ driven, suggest that the transition is very sensitive to the exchange-correlation (xc) functional used in the calculation. Here we present a detailed comparison of previous multiple-shock electrical conductivity measurements with FT-DFT calculations employing various xc functionals to probe a regime where both $T$ and $\rho$ play an important role in the transition. The measurement results are found to be inconsistent with the semilocal xc functional PBE and are in much better agreement with the nonlocal xc functionals vdW-DF1 and vdW-DF2. Furthermore, we show that the inconsistency with PBE likely stems from pressure errors associated with the PBE xc functional, resulting in calculated pressures that are too low at these $T$ and $\rho$ conditions. Together with previous comparisons at high-$T$, low-$\rho$ and low-$T$, high-$\rho$ these results provide a consistent picture for the MA transition over a wide $T$ and $\rho$ range. This picture may also provide insight into differences in experimental observations of the metallization of liquid hydrogen and deuterium in the low-$T$ regime.

Acoustic materials displaying coupling between pressure and momentum are known as Willis materials. The simplest Willis materials are comprised of subwavelength scatterers that couple monopoles to dipoles and vice versa, with the interaction defined by a polarizability tensor. We propose a method for retrieving the polarizability tensor for subwavelength Willis acoustic scatterers using a finite set of scattering amplitudes. We relate the polarizability tensor to standard T-matrix and S-matrix scattering formalisms. This leads to an explicit method for retrieving the components of the polarizability tensor in terms of a small set of scattered pressure data in the near or far field. Numerical examples demonstrate the retrieval method for one- and two-dimensional configurations.

We report the synthesis and comprehensive ac and dc susceptibility measurements of ${\mathrm{K}}_x\mathrm{R}{\mathrm{u}}_{4-y}\mathrm{N}{\mathrm{i}}_y{\mathrm{O}}_8$ hollandite. The value of the relative frequency shift, $\delta T_f$, has been determined as 0.025 which is within the range expected for spin-glass systems (0.005–0.06). Additionally, the characteristic flipping time of a single spin flip, ${\tau }_0$, and the dynamical critical exponent, $-zv$, were determined to have values of $5.82\times {10}^{-8}\mathrm{s}$ and 6.1(3), respectively from the power law. While the value of ${\tau }_0$ is comparatively very large, $-zv$ is consistent with what is expected for spin-glass systems. Field-cooled hysteresis behavior demonstrates a small increase in the remnant magnetization (at 2 K) on increasing the strength of the cooling field, suggesting that the degree of short-range correlations increases consistent with the formation of larger spin clusters. Thermoremnant magnetization data indicate an exponential-like decay of the magnetization as a function of time with the remnant magnetization remaining nonzero. However, it is clear from these data that multiple components contribute to the decay behavior. Collectively, these data confirm spin-glass character for ${\mathrm{K}}_{0.73\left(3\right)}\mathrm{N}{\mathrm{i}}_{1.9\left(5\right)}\mathrm{R}{\mathrm{u}}_{2.1\left(5\right)}{\mathrm{O}}_8$ and clearly demonstrate that the magnetic behavior of this material is far from simplistic.

We report a spin-driven ferroelectricity below the helical spin ordering temperature in a nonoxide mineral $\mathrm{MnS}{\mathrm{b}}_2{\mathrm{S}}_4$, which crystallizes in a centrosymmetric monoclinic $\mathrm{HgB}{\mathrm{i}}_2{\mathrm{S}}_4$ structure. At $\mathrm{T}\sim 25\mathrm{K}$, the dielectric constant shows an anomaly and the spontaneous electric polarization begins to develop. The polarization obtained from pyroelectric current measurement can be switched by reversing the direction of poling electric field. Interestingly, upon applying magnetic field parallel to the electric field, the polarization is enhanced nearly six times above a critical field $H\left|\right|E\sim 5\mathrm{T}$, demonstrating large magnetoelectric effect in this system. Below $\mathrm{T}\sim 25\mathrm{K}$, this compound is known to exhibit an unusual magnetic structure where the spiral spins lie in the ac plane but propagate along the $b$ axis with an angle of $\sim {133}^{\circ }$ between the adjacent magnetic moments. The origin of ferroelectric ordering has been attributed to the inverse Dzyaloshinskii-Moriya type interaction in accordance with local symmetry considerations.

We present an extensive study of the ferromagnetic heavy-fermion compound ${\mathrm{U}}_4{\mathrm{Ru}}_7{\mathrm{Ge}}_6$. Measurements of electrical resistivity, specific heat, and magnetic properties show that ${\mathrm{U}}_4{\mathrm{Ru}}_7{\mathrm{Ge}}_6$ orders ferromagnetically at ambient pressure with a Curie temperature $T_C=6.8\pm 0.3$ K. The low-temperature magnetic behavior of this soft ferromagnet is dominated by the excitation of gapless spin-wave modes. Our results on the transport properties of ${\mathrm{U}}_4{\mathrm{Ru}}_7{\mathrm{Ge}}_6$ under pressures up to 2.49 GPa suggest that ${\mathrm{U}}_4{\mathrm{Ru}}_7{\mathrm{Ge}}_6$ has a putative ferromagnetic quantum critical point (QCP) at $P_c\approx 1.7\pm 0.02$ GPa. In the ordered phase, ferromagnetic magnons scatter the conduction electrons and give rise to a well-defined power law temperature dependence in the resistivity. The coefficient of this term is related to the spin-wave stiffness, and measurements of the very low temperature resistivity show the behavior of this quantity as the ferromagnetic QCP is approached. We find that the spin-wave stiffness decreases with increasing pressure, implying that the transition to the nonmagnetic Fermi liquid state is driven by the softening of the magnons. The observed quantum critical behavior of the magnetic stiffness is consistent with the influence of disorder in our system. At quantum criticality ($P=P_c\approx 1.7\pm 0.02$ GPa), the resistivity shows the behavior expected for an itinerant metallic system near a ferromagnetic QCP.

We report a detailed analysis of the energy spectra, second- and high-order structure functions of velocity differences in superfluid ${}^4\mathrm{He}$ counterflow turbulence, measured in a wide range of temperatures and heat fluxes. We show that the one-dimensional energy spectrum $E_{xz}\left(k_y\right)$ (averaged over the $xz$ plane, parallel to the channel wall), directly measured as a function of the wall-normal wave vector $k_y$, gives more detailed information on the energy distribution over scales than the corresponding second-order structure function $S_2\left({\delta }_y\right)$. In particular, we discover two intervals of $k_y$ with different apparent exponents: $E_{xz}\left(k_y\right)\propto k_y^{-m_{\mathrm{C}}}$ for $k\lesssim k_{\times }$ and $E_{xz}\left(k_y\right)\propto k_y^{-m_{\mathrm{F}}}$ for $k\gtrsim k_{\times }$. Here $k_{\times }$ denotes the wave number that separates scales with relatively strong (for $k\lesssim k_{\times }$) and relatively weak (for $k\gtrsim k_{\times }$) coupling between the normal-fluid and superfluid velocity components. We interpret these $k$ ranges as cascade-dominated and mutual-friction-dominated intervals, respectively. The general behavior of the experimental spectra $E_{xz}\left(k_y\right)$ agrees well with the predicted spectra [L'vov and Pomyalov, Phys. Rev. B 97, 214513 (2018)]. Analysis of the $n\mathrm{th}$-order structure functions statistics shows that in the energy-containing interval, the statistics of counterflow turbulence is close to Gaussian, similar to the classical hydrodynamic turbulence. In the cascade- and mutual-friction-dominated intervals, we found some modest enhancement of intermittency with respect to its level in classical turbulence. However, at small scales, the intermittency becomes much stronger than in the classical turbulence.

We investigated a LaFeAsO single crystal by means of synchrotron Mössbauer spectroscopy under pressure up to 7.5 GPa and down to 13 K and provide a microscopic phase diagram. We found a continuous suppression of the magnetic hyperfine field with increasing pressure, and it completely vanishes at $\sim 7.5\mathrm{GPa}$, which is in contrast to the behavior in polycrystalline samples where the magnetic order vanishes at $\sim 20\mathrm{GPa}$. The different behaviors of the polycrystalline samples might be due to As vacancies. Our results are in qualitative agreement with density functional theory calculations where a reduction of the magnetic moment with increasing pressure was found. We found that among different samples the magnetic phase-transition temperature as well as the low-temperature magnetic hyperfine field decrease with increasing unit-cell volume.

It is well known that superconducting films made of a type-I material can demonstrate a type-II magnetic response, developing stable vortex configurations in a perpendicular magnetic field. Here we show that the superconducting state of a type-I nanowire undergoes more complex transformations, depending on the nanowire thickness. Sufficiently thin nanowires deviate from type I and develop multiquantum vortices and vortex clusters similar to intertype (IT) vortex states in bulk superconductors between conventional superconductivity types I and II. When the nanowire thickness decreases further, the quasi-one-dimensional vortex matter evolves towards type II so that the IT vortex configurations gradually disappear in favor of the standard Abrikosov lattice (chain) of single-quantum vortices. However, type II is not reached. Instead, an ultrathin nanowire re-enters abruptly the type-I regime while vortices tend to be suppressed by the boundaries, eventually becoming one-dimensional phase-slip centers. Our results demonstrate that arrays of nanowires can be used to construct composite superconducting materials with a widely tunable magnetic response.

In this work, we first formulate an effective theory, which generally captures long-range proximity effects of a surface system weakly coupled to an $s$-wave superconductor. The long-range proximity effects include both the emergent long-range pairing and hopping interactions in the surface system. We then model the Rashba spin-orbit-coupled nanowire in proximity with an $s$-wave superconductor by taking into account the emergent nonlocal effects in the weak-coupling limit. In this limit, the induced superconducting pair potential is found to be much smaller than that of the host superconductor, which is in good agreement with recent experiments. Compared with the previously considered strong-coupling limit with local proximity effects, the long-range interactions can significantly modify the topological phase diagram and considerably lower the threshold magnetic field for the emergence of Majorana zero modes.

Heterostructures made of itinerant ferromagnets and superconductors are studied. In contrast to most previous models, ferromagnetism is not enforced by an effective Zeeman field but induced in a correlated single-band model (CSBM) that displays itinerant ferromagnetism as a mean-field ground state. In this model superconductivity and magnetism are both calculated self-consistently. We calculate the magnitude of the magnetization, the superconducting correlations, and variations of the charge density self-consistently for a superconducting-magnetic bilayer by solving the Bogoliubov–de Gennes equations on a two-dimensional lattice. We determine all three quantities as a function of the Coulomb repulsion $U$ and the ferromagnetic exchange interaction $J$. The CSBM displays a variety of features not present in the Zeeman exchange model—for example, the occurrence of electronic phase separation and the competition of magnetic and superconducting orders far away from the interface.