An analytical formula for the linear complex susceptibility of dipolar assemblies subjected to thermal agitation, long-range interactions, and an externally applied uniform sinusoidal field of weak amplitude is derived using the forced rotational diffusion equation of Cugliandolo et al. [Phys. Rev. E 91, 032139 (2015)] in the virial approximation. If the Kirkwood correlation factor of the dipolar assembly $g_K$ exceeds unity, a thermally activated process arising from the interaction-specific component arises, while for $g_K<1$ the susceptibility spectrum normalized by its static value is practically unaltered with respect to that of the ideal gas phase.

The transport through a domain wall pinned at a nanoconstriction in (Ga,Mn)As wires is investigated theoretically using the Landauer-Büttiker approach by considering the Rashba and Dresselhaus spin-orbit interactions. The local nonequilibrium spin densities produced by electrical spin injection at the nanoconstriction are calculated numerically along the nanowire. The adiabatic and nonadiabatic components of the spin-transfer torque, expressed in terms of the gradient of the spin current density, are also computed. An oscillatory behavior in the spin-transfer torque is observed for the systems containing atomically sharp domain walls, due to the strong reflections at the domain wall caused by the large magnetization gradient. It is demonstrated that the strength of the oscillations for nonadiabatic spin torque increases by the negative Rashba parameter ${\alpha }_x$, while it decreases with increasing positive values of ${\alpha }_x$. However, the nonadiabatic spin torque increases with $\left|{\alpha }_y\right|$, regardless of its sign. Furthermore, it is shown that the Dresselhaus coupling $\beta$ does not considerably alter the $z$ component of the spin torque, while the other two components are effectively changed by the Dresselhaus spin-orbit interaction.

Using the Gorkov equations, we provide an exact solution for a one-dimensional model of superconductivity in the presence of a conical helicoidal exchange field. Due to the special type of symmetry of the system, the superconducting transition always occurs into a nonuniform superconducting phase (in contrast with the Fulde-Ferrell-Larkin-Ovchinnikov state, which appears only at low temperatures). We directly demonstrate that the uniform superconducting state in our model carries a current and thus does not correspond to the ground state. We study in the framework of the Bogoliubov–de Gennes approach the properties of the Josephson junction with a conical ferromagnet as a weak link. In our numerical calculations, we do not use any approximations (such as, e.g., a quasiclassical approach), and we show a realization of an anomalous ${\varphi }_0$ junction (with a spontaneous phase difference ${\varphi }_0$ in the ground state). The spontaneous phase difference ${\varphi }_0$ strongly increases at high values of the exchange field near the borderline with a half-metal, and it exists also in the half-metal regime.

We present a quantum Monte Carlo (QMC) study, based on the Langevin equation, of a Hamiltonian describing electrons coupled to phonon degrees of freedom. The bosonic part of the action helps control the variation of the field in imaginary time. As a consequence, the iterative conjugate gradient solution of the fermionic action, which depends on the boson coordinates, converges more rapidly than in the case of electron-electron interactions, such as the Hubbard Hamiltonian. Fourier acceleration is shown to be a crucial ingredient in reducing the equilibration and autocorrelation times. After describing and benchmarking the method, we present results for the phase diagram focusing on the range of the electron-phonon interaction. We delineate the regions of charge density wave formation from those in which the fermion density is inhomogeneous, caused by phase separation. We show that the Langevin approach is more efficient than the determinant QMC method for lattice sizes $N\gtrsim 8\times 8$ and that it therefore opens a potential path to problems including, for example, charge order in the three-dimensional Holstein model.

An infinite projected entangled pair state (iPEPS) is a tensor network ansatz to represent a quantum state on an infinite 2D lattice whose accuracy is controlled by the bond dimension $D$. Its real, Lindbladian, or imaginary time evolution can be split into small time steps. Every time step generates a new iPEPS with an enlarged bond dimension $D^{\prime }>D$, which is approximated by an iPEPS with the original $D$. In P. Czarnik and J. Dziarmaga, Phys. Rev. B 98, 045110 (2018), an algorithm was introduced to optimize the approximate iPEPS by maximizing directly its fidelity to the one with the enlarged bond dimension $D^{\prime }$. In this paper, we implement a more efficient optimization employing a local estimator of the fidelity. For imaginary time evolution of a thermal state's purification, we also consider using unitary disentangling gates acting on ancillas to reduce the required $D$. We test the algorithm simulating Lindbladian evolution and unitary evolution after a sudden quench of transverse field $h_x$ in the 2D quantum Ising model. Furthermore, we simulate thermal states of this model and estimate the critical temperature with good accuracy: $0.1\%$ for $h_x=2.5$ and $0.5\%$ for the more challenging case of $h_x=2.9$ close to the quantum critical point at $h_x=3.04438\left(2\right)$.

Heisenberg-type spin-chain materials have been extensively studied over the years, yet not much is known about their behavior in the presence of disorder. Starting from ${\mathrm{BaCu}}_2{\mathrm{Si}}_2{\mathrm{O}}_7$, a typical spin-$\frac{1}{2}$ chain system, we investigate a series of compounds with different extents of bond disorder, where the systematic replacement of Si with Ge results in a remodulation of the ${\mathrm{Cu}}^{2+}$ exchange interactions. By combining magnetometry measurements with nuclear magnetic resonance studies, we follow the evolution of the disorder-related properties from the well-ordered ${\mathrm{BaCu}}_2{\mathrm{Si}}_2{\mathrm{O}}_7$ to the maximally disordered ${\mathrm{BaCu}}_2{\mathrm{SiGeO}}_7$. Our data indicate that already a weak degree of disorder of only 5% Ge, apart from reducing the three-dimensional magnetic ordering temperature $T_{\mathrm{N}}$ quite effectively, induces a qualitatively different state in the paramagnetic regime. At maximum disorder our data indicate that this state may be identified with the theoretically predicted random singlet (RS) state. With decreasing disorder the extension of the RS regime at temperatures above $T_{\mathrm{N}}$ is reduced, yet its influence is clearly manifest, particularly in the features of nuclear magnetic resonance relaxation data.

The classical-map hypernetted-chain (CHNC) scheme, developed for treating fermion fluids at strong coupling and at finite temperatures, is applied to electron-electron and electron-hole double quantum wells. The pair-distribution functions and the local field factors needed in linear-response theory are determined for a range of temperatures, carrier densities, and barrier widths typical for experimental double-quantum-well systems in GaAs-GaAlAs. For electron-hole double quantum wells, a large enhancement in the pair-distribution functions is found for small carrier separations. The CHNC equations for electron-hole systems no longer hold at low densities where bound-state formation occurs.

We present an extended version of the projector-based renormalization method that can be used to address not only equilibrium but also nonequilibrium situations in coupled fermion-boson systems. The theory is applied to interacting electrons, holes, and photons in a semiconductor microcavity, where the loss of cavity photons into vacuum is of particular importance. The method incorporates correlation and fluctuation processes beyond mean-field theory in a wide parameter range of detuning, Coulomb interaction, light-matter coupling, and damping, even in the case when the number of quasiparticle excitations is large. This enables the description of exciton and polariton formation and their possible condensation through spontaneous phase symmetry breaking by analyzing the ground-state, steady-state, and spectral properties of a rather generic electron-hole-photon Hamiltonian, which also includes the coupling to two fermionic baths and a free-space photon reservoir. Thereby, the steady-state behavior of the system is obtained by evaluating expectation values in the long-time limit by means of the Mori-Zwanzig projection technique. Tracking and tracing different order parameters, the fully renormalized single-particle spectra and the steady-state luminescence, we demonstrate the Bose-Einstein condensation of excitons and polaritons and its smooth transition when the excitation density is increased.

A direct measurement of the density of Andreev bound states (ABSs) is experimentally investigated in a superconductor–quantum dot–superconductor hybrid nanowire system. A hard proximity-induced superconducting gap is observed, arising from superconducting correlations, in the transport spectrum of the hybrid system. Conductance peaks observed inside the superconducting gap reveal that the subgap states participate in the transport of the hybrid junction. We explore the evolution of low-energy Andreev bound states in a few-electron quantum dot (QD) coupled to superconductors, by probing the magnetic field dependence of exquisite detailed conductance spectra with a small bias voltage applied on the superconducting lead. In the presence of low magnetic fields, the resonance current is enhanced and broadened, attributed to the transport through Andreev bound states in QD, as the energy of ABSs reaches the threshold set by the applied bias voltage with the increase of the magnetic field. The simulated transport spectrum matches the experimentally observed evolution patterns of conductance, further implying the superconducting correlation nature of the observed electron transport. At high magnetic fields, the conductance maxes as the Fermi level reaches the degeneration point of Landau levels, leading to conductance peaks shown in the alternating narrow and wide patterns of Coulomb blockade oscillations.

We investigate the violation of the first Hund's rule in $4d$ and $5d$ transition-metal oxides that form solids of dimers. Bonding states within these dimers reduce the magnetization of such materials. We parametrize the dimer formation with realistic hopping parameters and find not only regimes where the system behaves like a Fermi liquid or as a Peierls insulator, but also strongly correlated regions due to Hund's coupling and its competition with the dimer formation. The electronic structure is investigated using the cluster dynamical mean-field theory for a dimer in the two-plane Bethe lattice with two orbitals per site and $3/8$ filling, which is three electrons per dimer. It reveals dimer-antiferromagnetic order of a high-spin (double-exchange) state and a low-spin (molecular-orbital) state. At the crossover region, we observe the suppression of long-range magnetic order, fluctuation enhancement, and renormalization of electron masses. At certain interaction strengths, the system becomes an incoherent antiferromagnetic metal with well-defined local moments.

The electronic band structure of a solid is a collection of allowed bands separated by forbidden bands, revealing the geometric symmetry of the crystal structures. Comprehensive knowledge of the band structure with band parameters explains intrinsic physical, chemical, and mechanical properties of the solid. Here we report the artificial polaritonic band structures of two-dimensional honeycomb lattices for microcavity exciton-polaritons using GaAs semiconductors in the wide-range detuning values, from cavity photonlike (red-detuned) to excitonlike (blue-detuned) regimes. In order to understand the experimental band structures and their band parameters, such as gap energies, bandwidths, hopping integrals, and density of states, we originally establish a polariton band theory within an augmented plane wave method with two-kind bosons, cavity photons trapped at the lattice sites, and freely moving excitons. In particular, this two-kind band theory is absolutely essential to elucidate the exciton effect in the band structures of blue-detuned exciton-polaritons, where the flattened excitonlike dispersion appears at larger in-plane momentum values captured in our experimental access window. We reach an excellent agreement between theory and experiments in all detuning values.

Hyperbolic propagation offers exciting opportunities in nanophotonics, from subdiffraction imaging to enhanced local density of states. This transport regime is typically induced by strong modulation of conductivity, i.e., with alternating metallic and dielectric material properties. Here, we analyze a moving impedance surface, showing that suitably tailored homogeneous metasurfaces can support one-way hyperbolic propagation when in motion, adding nonreciprocity to hyperbolic propagation phenomena, and without suffering from nonlocal effects stemming from discretization or finite granularity of the surface.

Study of electron transport in chemically synthesized graphene (CSG) under magnetic field and its magnetization due to the presence of a transition-metal (TM) atom on its surface is indeed an interesting area of research in condensed-matter physics. In the last few years, several theoretical along with a few experimental results have been reported on magnetotransport in CSG, however the interaction of conduction electrons with the induced magnetic moments arising due to charge-transfer effect when a TM atom is placed on the CSG surface is poorly understood. In the present paper, CSG has been decorated by dilute concentration of the TM (Ni, Co) atom to investigate the interaction of the conduction electron and induced magnetic moment in CSG due to the presence of the TM atom and their effect on overall magnetotransport behavior. Large positive magnetoresistance (MR) is obtained in CSG over the entire temperature and field range, however in the case of TM decorated CSG (TM-CSG) a dramatic change in MR (from positive to negative) is observed in the high-field region after 30 K. The positive MR in CSG arises due to wave-function shrinkage and the large negative MR is explained in TM-CSG by the electron-hole (e-h) puddle generated in CSG due to charge impurity and disorder created due to the presence of the TM atom on the CSG surface. A magnetic hysteresis loop up to 5 K also confirms the magnetic moment generated in CSG as a result of charge transfer from the TM atom to CSG.

The quantization of the energy in a magnetic field (Landau quantization) at a three-quarter Dirac point is studied theoretically. The three-quarter Dirac point is realized in the system of massless Dirac fermions with the critically tilted Dirac cone in one direction, where a linear term disappears and a quadratic term ${\alpha }_2{q}_x^2$ with a constant ${\alpha }_2$ plays an important role. The energy is obtained as $E_n\propto {\alpha }_2^{\frac{3}{5}}{\left(nB\right)}^{\frac{4}{5}}$, where $n=1,2,3,\cdots$, by means of numerically solving the differential equation. The same result is obtained analytically by adopting an approximation. The result is consistent with the semiclassical quantization rule studied previously. The existence of the $n=0$ state is studied by introducing the energy gap due to the inversion-symmetry-breaking term, and it is obtained that the $n=0$ state exists in one of a pair of three-quarter Dirac points, depending on the direction of the magnetic field when the energy gap is finite.

We report on gigahertz admittance measurements indicative of intrinsic relaxation time for an interacting quantum resistance-inductance ($RL$) circuit formed by a gate-defined quantum point contact in the quantum Hall regime. The dependence of the admittance on intrinsic parameters of the circuit is mapped and found to be highly relevant to the driving frequency, the magnetic field, and the edge potential profile. Remarkably, measurements show that in the coherent limit the relaxation time is universal irrespective of resistance and inductance of the circuit, which is given by the electronic time of flight in the circuit. When the quantum $RL$ circuit is exposed to strong electron interactions, classical laws of electrodynamic response of the whole circuit are recovered, associated with observable variations of relaxation time with resistance. The observed transition of the relaxation time is explained in terms of the effect of Coulomb interaction which is tunable in a gate-defined quantum point contact. Our experiment demonstrates that deviations from the resistance-independent relaxation time are governed by the strength of Coulomb interactions, which provides valuable information to sufficiently determine the interaction parameter.

We investigate higher-order plasmons in graphene nanoribbons, and we present how electronic edge states and wave-function fine structure influence the graphene plasmons. Based on nearest-neighbor tight-binding calculations, we find that a standing-wave model based on nonlocal bulk plasmon dispersion is surprisingly accurate for armchair ribbons of widths even down to a few nanometers, and we determine the corresponding phase shift upon edge reflection and an effective ribbon width. Wider zigzag ribbons exhibit a similar phase shift, whereas the standing-wave model describes few-nanometer zigzag ribbons less satisfactorily, to a large extent because of their edge states. We directly confirm that also the larger broadening of plasmons for zigzag ribbons is due to their edge states. Furthermore, we report a prominent fine structure in the induced charges of the ribbon plasmons, which for armchair ribbons follows the electronic wave-function oscillations induced by intervalley coupling. Interestingly, the wave-function fine structure is also found in our analogous density-functional theory calculations, and both these and tight-binding numerical calculations are explained quite well with analytical Dirac theory for graphene ribbons.

The anisotropic mechanical response of ZIF-8 and ZIF-67 is investigated as a function of pressure and its main features (including shear-destabilization eventually leading to amorphization) discussed in terms of specific lattice vibrations and structural changes occurring in the framework. At zero pressure, the two ZIFs are characterized by an elastic anisotropy with directions of maximum and minimum stiffness along $\langle 111\rangle$ and $\langle 100\rangle$, respectively. At $P=0.2$ GPa, the framework exhibits a perfectly isotropic mechanical response, while at $P>0.2$ GPa a different (complementary) anisotropic response is observed with directions of maximum and minimum stiffness along $\langle 100\rangle$ and $\langle 111\rangle$, respectively. The bulk modulus of the two ZIFs initially slightly increases up to 0.1 GPa of pressure and then decreases at higher pressures. Amorphization in both ZIF-8 and ZIF-67 is confirmed to be due to the pressure-driven mechanical instability of their frameworks to shear deformations. The directional elastic moduli of the two ZIFs are partitioned into contributions from specific normal modes of vibration. The elastic constants $C_{11}$, and $C_{12}$ [and thus the bulk modulus $K=1/3(C_{11}+2C_{12})]$ are mostly affected by symmetric “gate-opening” vibrations of the imidazolate linkers in the four-membered rings. The $C_{44}$ shear elastic constant (and thus the mechanical instability and amorphization of the framework) are instead related to asymmetric “gate-opening” vibrations of the four-membered rings.

We study magnetic properties of spin glass (SG) systems under a random field (RF), based on the suggestion that RFs can be induced by a weak transverse field in the compound ${\mathrm{LiHo}}_x{\mathrm{Y}}_{1-x}{\mathrm{F}}_4$. We consider a cluster spin model that allows long-range disordered interactions among clusters and short-range interactions inside the clusters, besides a local RF for each spin following a Gaussian distribution with standard deviation $\mathrm{\Delta }$. We adopt the one-step replica symmetry breaking approach to get an exactly solvable single-cluster problem. We discuss the behavior of order parameters, specific heat $C_m$, nonlinear susceptibility ${\chi }_3$, and phase diagrams for different disorder configurations. In the absence of RF, the ${\chi }_3$ exhibits a divergence at $T_f$, while the $C_m$ shows a broad maximum at a temperature $T^{**}$ around $30\%$ above $T_f$, as expected for conventional SG systems. The presence of RF changes this scenario. The $C_m$ still shows the maximum at $T^{**}$ that is weakly dependent on $\mathrm{\Delta }$. However, the $T_f$ is displaced to lower temperatures, enhancing considerably the ratio $T^{**}/T_f$. Furthermore, the divergence in ${\chi }_3$ is replaced by a rounded maximum at a temperature $T^{*}$, which becomes increasingly higher than $T_f$ as $\mathrm{\Delta }$ is enhanced. As a consequence, the paramagnetic phase is unfolded in three regions: (i) a conventional paramagnetism ($T>T^{**}$); (ii) a region with formation of short-range order with frozen spins ($T^{*}<T<T^{**}$); (iii) a region with slow growth of free-energy barriers slowing down the spin dynamics before the SG transition ($T_f<T<T^{*}$) suggesting an intermediate Griffiths phase before the SG state. Our results reproduce qualitatively some findings of ${\mathrm{LiHo}}_x{\mathrm{Y}}_{1-x}{\mathrm{F}}_4$ as the rounded maximum of ${\chi }_3$ behavior triggered by RF and the deviation of the conventional relationship between the $T_f$ and $T^{**}$.