We develop a theory that describes the response of nonreciprocal devices employing two-dimensional materials in the quantum Hall regime capacitively coupled to external electrodes. As the conduction in these devices is understood to be associated to the edge magnetoplasmons (EMPs), we first investigate the EMP problem by using the linear response theory in the random phase approximation. Our model can incorporate several cases that were often treated on different grounds in literature. In particular, we analyze plasmonic excitations supported by a smooth and sharp confining potential in a two-dimensional electron gas, and in monolayer graphene, and we point out the similarities and differences in these materials. We also account for a general time-dependent external drive applied to the system. Finally, we describe the behavior of a nonreciprocal quantum Hall device: the response contains additional resonant features, which were not foreseen from previous models.

The temperature-dependent optical response of excitons in semiconductors is controlled by the exciton-phonon interaction. When the exciton-lattice coupling is weak, the excitonic line has a Lorentzian profile resulting from motional narrowing, with a width increasing linearly with the lattice temperature $T$. In contrast, when the exciton-lattice coupling is strong, the line shape is Gaussian with a width increasing sublinearly with the lattice temperature, proportional to $\sqrt{T}$. While the former case is commonly reported in the literature, here the latter is reported for hexagonal boron nitride. Thus the theoretical predictions of Toyozawa [Prog. Theor. Phys. 20, 53 (1958)] are supported by demonstrating that the exciton-phonon interaction is in the strong-coupling regime in this van der Waals crystal.

In ${\mathrm{TmB}}_4$, localized electrons with a large magnetic moment interact with metallic electrons in boron-derived bands. We examine the nature of ${\mathrm{TmB}}_4$ using full-relativistic ab initio density functional theory calculations, approximate tight-binding Hamiltonian results, and the development of an effective Kondo-Ising model for this system. Features of the Fermi surface relating to the anisotropic conduction of charge are discussed. The observed magnetic moment $\sim 6{\mu }_B$ is argued to require a subtle crystal field effect in metallic systems, involving a flipped sign of the effective charges surrounding a Tm ion. The role of on-site quantum dynamics in the resulting Kondo-Ising type “impurity” model are highlighted. From this model, elimination of the conduction electrons will lead to spin-spin (RKKY-type) interaction of Ising character required to understand the observed fractional magnetization plateaus in ${\mathrm{TmB}}_4$.

The consistent chiral kinetic theory accurate to the second order in electromagnetic and pseudoelectromagnetic fields is derived for a relativistic matter with two Weyl fermions. By making use of such a framework, the properties of longitudinal collective excitations, which include both chiral magnetic and chiral pseudomagnetic waves, are studied. It is shown that the proper treatment of dynamical electromagnetism transforms these gapless waves into chiral (pseudo)magnetic plasmons, whose Langmuir (plasma) gap receives corrections quadratic in both magnetic and pseudomagnetic fields.

A wide range of electrochemical reactions of practical importance occur at the interface between a semiconductor and an electrolyte. We present an embedded density-functional theory method using the recently released self-consistent continuum solvation (SCCS) approach to study these interfaces. In this model, a quantum description of the surface is incorporated into a continuum representation of the bending of the bands within the electrode. The model is applied to understand the electrical response of silicon electrodes in solution, providing microscopic insights into the low-voltage region, where surface states determine the electrification of the semiconductor electrode.

Experimental evidence investigating the high-pressure response (0–9 GPa) of aligned single-walled carbon nanotube (SWNT) arrays in the extreme low Raman shift region (10–100 ${\mathrm{cm}}^{-1}$, squash mode region) is provided to verify a predictive model for deformation-related phase transitions. In addition to the well-known radial breathing mode (RBM) and despite the technical challenges associated with the detection of Raman signals very close to the exciting laser frequency, clear SWNT squash mode peaks were identified and used to refine the predictive model. Furthermore, this paper investigates and proposes explanations for the detailed behavior of the pressure dependent cross-sectional transition. The results demonstrate experimentally, and confirm earlier theoretical models, that the critical pressure scales $\propto 1/\mathcal{O}\left(d_t^3\right)$ against the chirality dependent nanotube diameter $d_t$. Finally, the pressure and chirality dependent Raman upshifts of the squash mode, characterizing the phase transition, are found to be larger than those of the RBM, respectively, confirming the general theoretical prediction of greater environmental sensitivity of squash modes.

We study the magnetization process of a one-dimensional extended Heisenberg model, the $J\text{−}Q$ model, as a function of an external magnetic field $h$. In this model, $J$ represents the traditional antiferromagnetic Heisenberg exchange and $Q$ is the strength of a competing four-spin interaction. Without external field, this system hosts a twofold-degenerate dimerized (valence-bond solid) state above a critical value $q_c\approx 0.85$ where $q\equiv Q/J$. The dimer order is destroyed and replaced by a partially polarized translationally invariant state at a critical field value. We find magnetization jumps (metamagnetism) between the partially polarized and fully polarized state for $q>q_{\min }$, where we have calculated $q_{\min }=\frac{2}{9}$ exactly. For $q>q_{\min }$, two magnons (flipped spins on a fully polarized background) attract and form a bound state. Quantum Monte Carlo studies confirm that the bound state corresponds to the first step of an instability leading to a finite magnetization jump for $q>q_{\min }$. Our results show that neither geometric frustration nor spin anisotropy are necessary conditions for metamagnetism. Working in the two-magnon subspace, we also find evidence pointing to the existence of metamagnetism in the unfrustrated $J_1\text{−}{J}_2$ chain $\left(J_1>0,J_2<0\right)$, but only if $J_2$ is spin anisotropic. In addition to the studies at zero temperature, we also investigate quantum-critical scaling near the transition into the fully polarized state for $q\le q_{\min }$ at $T>0$. While the expected “zero-scale-factor” universality is clearly seen for $q=0$ and $q\ll q_{\min }$, for $q$ closer to $q_{\min }$ we find that extremely low temperatures are required to observe the asymptotic behavior, due to the influence of the tricritical point at $q_{\min }$. In the low-energy theory, one can expect the quartic nonlinearity to vanish at $q_{\min }$ and a marginal sixth-order term should govern the scaling, which leads to a crossover at a temperature $T^{*}\left(q\right)$ between logarithmic tricritical scaling and zero-scale-factor universality, with $T^{*}\left(q\right)\to 0$ when $q\to q_{\min }$.

The complete set of hallmarks of the three-dimensional antiferromagnet near the quantum critical point has been recently observed in the spin dimer compound ${\mathrm{TlCuCl}}_3$. Nonetheless, the mechanism, responsible for several distinct features of the experimental data, has remained a puzzle, namely: (i) the paramagnons exhibit remarkable robustness to thermal damping and are stable up to high temperatures, where $k_{B}T$ is comparable with the excitation energy; and (ii) the width to mass ratios of the high-temperature paramagnons are, within the error bars, equal to that of the low-temperature amplitude (or Higgs) mode. We propose such a mechanism and identify two principal factors contributing to the scaling between width to mass ratios of the paramagnon and the amplitude mode: (i) the emergence of the thermal mass scale reorganizing the paramagnon decay processes, and (ii) substantial renormalization of the multimagnon interactions by thermal fluctuations. The study is carried out for the general case of a $D=3+1$ quantum antiferromagnet within the framework of the ${\phi }^4$ model using the hybrid Callan-Symanzik + Wilson thermal renormalization group method. Our approach is tested by demonstrating a good quantitative agreement with available experimental data across the phase diagram of ${\mathrm{TlCuCl}}_3$.

The geometrically frustrated two-dimensional triangular lattice magnets $A{}_4{B}^{\prime }B{}_2{\mathrm{O}}_{12}$ ($A=\mathrm{Ba}$, Sr, La; $B^{\prime }=\mathrm{Co}$, Ni, Mn; $B=\mathrm{W}$, Re) have been studied by x-ray diffraction, ac and dc susceptibilities, powder neutron diffraction, and specific-heat measurements. The results reveal the following: (i) The samples containing ${\mathrm{Co}}^{2+}$ (effective spin-1/2) and ${\mathrm{Ni}}^{2+}$ (spin-1) ions with small spin numbers exhibit ferromagnetic (FM) behavior and ordering, respectively, while the sample containing ${\mathrm{Mn}}^{2+}$ (spin-5/2) ions with a large spin number exhibits antiferromagnetic (AFM) ordering. We ascribe these spin-number-manipulated ground states to the competition between the AFM $B^{\prime }$-O-O-$B^{\prime }$ and FM $B^{\prime }$-O-$B$-O-$B^{\prime }$ superexchange interactions. (ii) The chemical pressure introduced into the Co-containing samples through the replacement of different-size ions on the $A$ site finely tunes the FM behavior temperature of the system. This effect is not simply governed by the lattice parameters and requires more precise structural measurements to elucidate.

This work investigates the high-pressure structure of freestanding superconducting $(T_c=4.3$ K) boron-doped diamond (BDD) and how it affects the electronic and vibrational properties using Raman spectroscopy and x-ray diffraction in the 0–30 GPa range. High-pressure Raman scattering experiments revealed an abrupt change in the linear pressure coefficients, and the grain boundary components undergo an irreversible phase change at 14 GPa. We show that the blueshift in the pressure-dependent vibrational modes correlates with the negative pressure coefficient of $T_c$ in BDD. The analysis of x-ray diffraction data determines the equation of state of the BDD film, revealing a high bulk modulus of $B_0=510\pm 28$ GPa. The comparative analysis of high-pressure data clarified that the $s{p}^2$ carbons in the grain boundaries transform into hexagonal diamond.

In several unconventional superconductors, the highest superconducting transition temperature $T_c$ is found in a region of the phase diagram where the antiferromagnetic transition temperature extrapolates to zero, signaling a putative quantum critical point. The elucidation of the interplay between these two phenomena—high-$T_c$ superconductivity and magnetic quantum criticality—remains an important piece of the complex puzzle of unconventional superconductivity. In this paper, we combine sign-problem-free quantum Monte Carlo simulations and field-theoretical analytical calculations to unveil the microscopic mechanism responsible for the superconducting instability of a general low-energy model, called the spin-fermion model. In this approach, low-energy electronic states interact with each other via the exchange of quantum critical magnetic fluctuations. We find that even in the regime of moderately strong interactions, both the superconducting transition temperature and the pairing susceptibility are governed not by the properties of the entire Fermi surface, but instead by the properties of small portions of the Fermi surface called hot spots. Moreover, $T_c$ increases with increasing interaction strength, until it starts to saturate at the crossover from hot-spots-dominated to Fermi-surface-dominated pairing. Our work provides not only invaluable insights into the system parameters that most strongly affect $T_c$, but also important benchmarks to assess the origin of superconductivity in both microscopic models and actual materials.

Perpendicularly magnetized nanowires exhibit distinct domain wall types depending on the geometry. Wide wires contain Bloch walls, and narrow wires Néel walls. Here, the transition region is investigated by direct imaging of the wall structure using high-resolution spin-polarized scanning electron microscopy. An achiral intermediate wall type is discovered that is unpredicted by established theoretical models. With the help of micromagnetic simulations, the formation of this wall type is explained.

We investigate the effects of non-Hermiticity on topological pumping and uncover a connection between a topological edge invariant based on topological pumping and the winding numbers of exceptional points. In Hermitian lattices, it is known that the topologically nontrivial regime of the topological pump only arises in the infinite-system limit. In finite non-Hermitian lattices, however, topologically nontrivial behavior can also appear, and we show that this can be understood as the effect of encircling a pair of exceptional points during a pumping cycle. This phenomenon is observed experimentally in a non-Hermitian microwave network containing variable-gain amplifiers.

We propose field theories for a deconfined quantum critical point in SU(3) antiferromagnets on the triangular lattice. In particular we consider the continuous transition between a magnetic, three-sublattice color-ordered phase and a trimerized SU(3) singlet phase. Starting from the magnetically ordered state we derive a critical theory in terms of fractional bosonic degrees of freedom, in close analogy to the well-developed description of the SU(2) Néel—valence bond solid (VBS) transition on the square lattice. Our critical theory consists of three coupled $C{P}^2$ models and we study its fixed point structure using a functional renormalization group approach in a suitable large $N$ limit. We find a stable critical fixed point and estimate its critical exponents, thereby providing an example of deconfined criticality beyond the universality class of the $C{P}^N$ model. In addition we present a complementary route towards the critical field theory by studying topological defects of the trimerized SU(3) singlet phase.

Using thermal quantum field theory, we derive an expression for the exchange constant that resembles Fukuyama's formula for orbital magnetic susceptibility (OMS). Guided by this formal analogy between the exchange constant and OMS, we identify a contribution to the exchange constant that arises from the geometrical properties of the band structure in mixed phase space. We compute the exchange constants for free electrons and show that the geometrical contribution is generally important. Our formalism allows us to study the exchange constants in the presence of spin-orbit interaction. Thereby, we find sizable differences between the exchange constants of helical and cycloidal spin spirals. Furthermore, we discuss how to calculate the exchange constants based on a gauge-field approach in the case of the Rashba model with an additional exchange splitting, and we show that the exchange constants obtained from this gauge-field approach are in perfect agreement with those obtained from the quantum field theoretical method.

The unambiguous experimental detection of quantum spin liquids and, in particular, of the long-sought Kitaev quantum spin liquid (KQSL) with its Majorana fermion excitations remains an outstanding challenge. One of the major obstacles is the absence of signatures that definitively characterize this phase. Here, we propose the Landau levels known to form in the Majorana excitation spectrum of the KQSL when certain strain fields are applied as a direct signature of Majorana fermions with Dirac-like dispersion. In particular, we show that the Majorana Landau level quantization of strained films of the KQSL can be directly probed by Raman spectroscopy. Such experiments are feasible in thin films of $\alpha -{\mathrm{RuCl}}_3$, which are a promising place to search for the KQSL.

We have performed an angle-resolved photoemission spectroscopy study of ${\mathrm{BaCr}}_2{\mathrm{As}}_2$, which has the same crystal structure as ${\mathrm{BaFe}}_2{\mathrm{As}}_2$, a parent compound of Fe-based superconductors. We determine the Fermi surface of this material and its band dispersion over 5 eV of binding energy. Very moderate band renormalization (1.35) is observed for only two bands. We attribute this small renormalization to enhanced direct exchange as compared to Fe in ${\mathrm{BaFe}}_2{\mathrm{As}}_2$, and to a larger contribution of the $e_g$ orbitals in the composition of the bands forming the Fermi surface, leading to an effective valence count that is reduced by Fe $d$-As $p$ hybridization.

The effect of pinning on self-heating triggering the Larkin-Ovchinnikov (LO) flux-flow instability (FFI) in superconducting thin films is theoretically investigated. The problem is considered relying upon the Bezuglyj-Shklovskij (BS) generalization of the LO theory, accounting for a finite heat removal from the quasiparticles at temperature $T^{*}$ to the bath at temperature $T_0$. The FFI critical parameters, namely the current density $j^{*}$, the electric field $E^{*}$, the power density $P^{*}$, and the vortex velocity $v^{*}$ are calculated and graphically analyzed as functions of the magnetic field and the pinning strength. With increasing pinning strength at a fixed magnetic field value $E^{*}$ decreases, $j^{*}$ increases, while $P^{*}$ and $T^{*}$ remain practically constant. Vortex pinning may hence be the cause for eventual discrepancies between experiments on superconductors with strong pinning and the generalized LO and BS results.

The quasiclassical theory in terms of equations for the Green's functions (Eilenberger equations) is generalized in order to allow for quantitative description of the magnetoelectric effects and proximity-induced triplet correlations in the presence of spin-orbit coupling in hybrid superconducting systems. The formalism is valid under the condition that the spin-orbit coupling is weak with respect to the Fermi energy, but exceeds the superconducting energy scale considerably. On the basis of the derived formalism it is shown that the triplet correlations in the spin-orbit coupled normal metal can be induced by proximity to a singlet superconductor without any exchange or external magnetic field. They contain an odd-frequency even-momentum component, which is stable against disorder. The value of the proximity-induced triplet correlations is on the order of ${\mathrm{\Delta }}_{\mathrm{so}}/{\varepsilon }_F$, which is absent in the framework of the standard quasiclassical approximation, but can be described by our theory. The spin polarization, induced by the Josephson current flowing through the superconductor/Rashba metal/superconductor junction, is also calculated.

We investigate the nonlocal Hanle effect in high mobility two-dimensional electron systems using (Ga,Mn)As/GaAs spin Esaki diodes as spin selective contacts. Spin signals in these systems can be strongly affected by dynamic nuclear polarization, which mimics long spin-relaxation times extracted from the measured Hanle curves. Here, we introduce a method which largely suppresses these effects by using an ac injection-detection setup. This allows us to extract from the measurements realistic spin lifetimes on the order of single nanoseconds. As the detection of Hanle signals is also strongly affected by offset signals we discuss the magnetic field dependence of these background voltages observed in lateral nonlocal spin injection devices. We show how the strength of the background magnetoresistance can be minimized by choosing a proper device geometry.