Taking advantage of recent progress in neutron instrumentation and in the understanding of magnetic-field-dependent small-angle neutron scattering, we study the three-dimensional magnetization distribution within an isotropic Nd-Fe-B bulk magnet. The magnetic neutron scattering cross section of this system features the so-called spike anisotropy, which points toward the presence of a strong magnetodipolar interaction. This experimental result combined with the damped oscillatory behavior of the corresponding correlation function and recent micromagnetic simulation results on spherical nanoparticles suggest an interpretation of the neutron data in terms of vortexlike flux-closure patterns. The field-dependent correlation length $L_{\mathrm{c}}$ is well reproduced by a phenomenological power-law model. While the experimental neutron data for $L_{\mathrm{c}}$ are described by an exponent close to unity $(p=0.86)$, the simulation results yield $p=1.70$, posing a challenge to theory to include vortex-vortex interaction effects.

By first-principles calculations, this paper reveals that the antiferromagnetic (AFM) ground state of CrOCl with an orthorhombic structure observed in experiment is mainly due to the competition between the neighboring magnetic exchange interactions ($J_i$'s). The AFM ground state requires that the first and second nearest neighbors ($J_1$ and $J_2$) are AFM couplings with comparable magnitudes, and that the seventh nearest neighbor $\left(J_7\right)$ has a certain size of AFM coupling. By using Liechtenstein's density functional theory $(\mathrm{DFT}+U)$ method and appropriately adjusting the on-site exchange $\left(\mathcal{J}\right)$ parameter, the $J_i$'s can satisfy these conditions and lead to the experimentally observed AFM ground state (AFM-Exp). The $\mathcal{J}$ parameter changes the energy levels of the Cr-$3d$ orbitals and consequently the energy differences between the orbitals, which in turn impacts the sign and magnitude of $J_i$'s. By decomposing the $J_i$'s into orbital contributions, it is shown that introducing the $\mathcal{J}$ parameter significantly reduces the ferromagnetic parts of $J_1$ and $J_2$ and converts them to AFM coupling, which leads to the AFM-Exp ground state.

Normally, in superconductors, as in conductors, in a state with zero current $I$ the momentum of superconducting electrons $\hslash q=0$. Here we demonstrate theoretically and present experimental evidence that in a superconducting/normal metal (SN) hybrid strip placed in an in-plane magnetic field $B_{\mathrm{in}}$ a finite momentum state ($\hslash q\ne 0$) is realized when $I=0$. This state is characterized by current-momentum dependence $I\left(q\right)\ne -I(-q)$, nonreciprocal kinetic inductance $L_k\left(I\right)\ne L_k(-I)$, and different values of depairing currents $I_{\mathrm{dep}}^{\pm }$ flowing along the SN strip in opposite directions. The properties found have orbital nature and originate from the density gradient of superconducting electrons $\nabla n$ across the thickness of the SN strip and field-induced Meissner currents. We argue that this type of finite momentum state should be a rather general phenomenon in superconducting structures with artificial or intrinsic inhomogeneities.

We investigate non-Hermitian skin modes in laser arrays with spatially localized excitation. Intriguingly, we observe an unusual threshold behavior when selectively pumping either the head or the tail of these modes: both cases exhibit the same lasing threshold and hence defy the conventional principle of selective pumping, which aims to maximize the overlap between the pump profile and the target lasing mode. To shed light on this enigma, we reveal a previously overlooked phenomenon, i.e., energy exchange at non-Hermitian coupling junctions with the photonic environment, which does not occur with uniform gain or loss. Utilizing a transfer matrix approach, we elucidate the mechanism of this anomalous threshold behavior, which is determined by the specific physical realization of the non-Hermitian gauge field (i.e., using gain, loss, or their mixture). Finally, we derive a generalized principle of selective pumping in non-Hermitian arrays, which shows that the decisive spatial overlap is given by the triple product of the pump, the lasing mode, and its biorthogonal partner. Our study provides a glimpse into how the two forms of non-Hermiticity, i.e., asymmetric couplings and a complex on-site potential, interact synergetically in laser arrays, which may stimulate further explorations of their collective effects in photonics and related fields.

We investigate the nonequilibrium dynamics of quantum spin chains during a round-trip protocol that slowly drives the system across a quantum first-order transition. Out-of-equilibrium scaling behaviors à la Kibble-Zurek for the single-passage protocol across the first-order transition have been previously determined. Here, we show that such scaling relations persist when the driving protocol is inverted and the transition is approached again by a far-from-equilibrium state. This results in a quasi-universality of the scaling functions, which keep some dependence on the details of the protocol at the inversion time. We explicitly determine such quasi-universal scaling functions by employing an effective two-level description of the many-body system near the transition. We discuss the validity of this approximation and how this relates to the observed scaling regime. Although our results apply to generic systems, we focus on the prototypical example of a 1D transverse field Ising model in the ferromagnetic regime, which we drive across the first-order transitions through a time-dependent longitudinal field.

Surface acoustic waves (SAWs) have significant potential for the energy-efficient control of magnetic domain walls (DWs). This study investigates the influence of SAW frequency (50, 100, and 200 MHz) on DW dynamics in magnetic thin films. Micromagnetic simulations are performed to examine the effects of SAWs on DW velocity. The results demonstrate that SAWs enhance DW motion by promoting the depinning of DWs from pinning sites through SAW-induced spin rotation. This spin rotation exhibits the same frequency as the applied SAWs. The impact of SAW frequency varies depending on the level of anisotropy disorder in thin films. In films with 3% anisotropy disorder, the DW velocity increases with SAW frequency, highlighting the amplifying effect of spin rotation and enhanced DW depinning. Conversely, in thin films with 1% anisotropy disorder, the DW velocity decreases with SAW frequency owing to significant SAW-induced energy dissipation via spin rotation. These findings underscore the intricate interplay between SAWs, spin rotation, and DW dynamics, emphasizing the role of anisotropy disorder in governing the response of DWs to SAWs. The study contributes to the understanding of SAW-assisted DW motion and provides insights into the optimization of and energy-efficient control of DWs in spintronic applications using SAWs.

Magnons, as elementary excitations in magnetic systems, can carry and transfer angular momentum. Due to the absence of Joule heat during magnon transport, research on magnon transport has gained considerable interest over the past decade. Recently, a full quantum theory has been employed to investigate magnon transport in ferromagnetic insulators (FMIs). However, the most commonly used magnetic insulating material in experiments, yttrium iron garnet (YIG), is a ferrimagnetic insulator (FIMI). Therefore a full quantum theory for magnon transport in FIMIs needs to be established. Here, we propose a Green's function formalism to compute the bulk and interface magnon currents in both FIMIs and antiferromagnetic insulators (AFMIs). We investigate the spatial distribution and temperature dependence of magnon currents in FIMIs and AFMIs generated by a temperature or spin chemical potential step. In AFMIs, magnon currents generated by a temperature step in the two sublattices cancel each other out. Subsequently, we numerically simulate the magnon junction effect using the Green's function formalism, and the results show a near 100% magnon junction ratio. This study demonstrates the potential of using a full quantum theory to investigate magnon transport in specific magnonic devices.

We have performed magnetic susceptibility, magnetization, and specific heat measurements on a chiral magnet ${\mathrm{GdNi}}_3{\mathrm{Ga}}_9$, belonging to the trigonal space group $R32$ (No. 155). A magnetic phase transition takes place at $T_{\mathrm{N}}=19.5$ K. By applying a magnetic field along the $a$ axis at 2 K, the magnetization curve exhibits two jumps at $\sim 3$ kOe and = 45 kOe. To determine the magnetic structure, we performed a resonant x-ray diffraction experiment by utilizing a circularly polarized beam. It is shown that a long-period antiferromagnetic (AFM) helical order is realized at zero field. The Gd spins in the honeycomb layer are coupled in an antiferromagnetic manner in the $c$ plane and rotate with a propagation vector $\mathit{q}=(0,0,1.485)$. The period of the helix is 66.7 unit cells $(\sim 180$ nm). In magnetic fields above 3 kOe applied perpendicular to the helical $c$ axis, the AFM helical order changes to an AFM order with $\mathit{q}=(0,0,1.5)$.

The assembly consisting of ${\mathrm{Fe}}_3{\mathrm{O}}_4$ nanoparticles deposited on a plasma-treated polypropylene substrate was structurally characterized and its static and dynamic magnetic properties were investigated. The ${\mathrm{Fe}}_3{\mathrm{O}}_4$ nanoparticles of the nominal size of 10 nm create agglomerates of the size ranging from nominally 10 to 400 nm which were deposited on the polypropylene substrate using the grafting technique. The behavior of zero field cooled and field cooled susceptibility was found to be consistent with the onset of the super spin-glass state with glassy temperature $T_g=211\mathrm{K}$. The formation of the super spin-glass state was also supported by the relative shift of maximum in alternating susceptibility Γ ≈ 0.06 and $z\varepsilon =8.6$ obtained from critical slowing down analysis as well as by the study of memory and aging effects. Significant renormalization of $T_g$ and zυ for obtaining universal behavior in dynamic scaling of alternating susceptibility is tentatively ascribed to deviation toward two-dimensional magnetic behavior. The obtained results suggest that the studied assembly may be appropriate for the realization of unique two-dimensional super spin glass with dominant dipolar interaction.

Measuring the resistivity of high-purity single crystals of ${\mathrm{Sr}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$ under pressure, we find strong evidence that the field-induced spin density wave phase at the $H\parallel c$ metamagnetic transition is suppressed at a surprisingly low pressure of $\sim 3\pm 1$ kbar. This offers the possibility of studying a bare quasi-two-dimensional spin density wave quantum critical point, testing the $T\to 0$ K limit of theories of Planckian dissipation and quantum criticality. Preliminary attempts to fit $\rho \left(T\right)$ with a quantum critical spin fluctuation model, while encouraging, reveal a need for further, complementary measurements.

Tamai et al. discovered an unusual electronic state near the Fermi level at the interface of Cu(111) and a molecular layer of ${\mathrm{C}}_{60}$, which was initially attributed to ${\mathrm{C}}_{60}\text{−}\mathrm{Cu}$ interfacial hybridization [Phys. Rev. B 77, 075134 (2008)]. Later on, Yue et al. suggested that the state was due to the reshaping of a two-dimensional electron gas hosted at the Cu(111) surface, into an artificial graphene with Dirac cones by cutting out muffin-tin potentials of adsorbed fullerene molecules [Phys. Rev. B 102, 201401(R) (2020)]. In the present paper, we introduce a different explanation using angle-resolved photoemission and show that the observed conical bands in the ${\mathrm{C}}_{60}/\mathrm{Cu}\left(111\right)$ system are neither Dirac cones nor hybridization states. Rather, they are formed by umklapp scattering of photoelectrons emitted from bulk and surface bands of Cu(111) and diffracted on the ($4\times 4$) fullerene superstructure. The circular contours near the Fermi level, which resemble the low-energy part of graphene Dirac cones, are a result of the backfolding of the bulk band of copper, while the triangular silhouettes observed at higher binding energies, which mimic the threefold symmetric higher-binding-energy part of graphene Dirac cones, are due to an umklapp effect governed by highly coherent photoelectron diffraction of steeply dispersing Cu(111) $\mathit{sp}$-type surface resonances. We also used density functional theory to study the behavior of the two-dimensional electron gas localized within the honeycomb net created by the electron-exclusive potentials of ${\mathrm{C}}_{60}$ and demonstrate doping above the Fermi level ($p$ doping). The results show no presence of the electronic structure of artificial graphene in ${\mathrm{C}}_{60}/\mathrm{Cu}\left(111\right)$.

We study pseudogap behavior in a metal near a spin density wave (SDW) instability due to thermal magnetic fluctuations. We consider the $t-t^{\prime }$ Hubbard model on a square lattice at a finite doping, at intermediate coupling strength, and analyze the thermal evolution of the electron spectral function between a SDW ordered state at low temperatures and a normal Fermi liquid at high temperatures. We argue that for proper description of the pseudogap one needs to sum up infinite series of diagrams for both the fermionic self-energy and the SDW order parameter in the SDW state or the magnetic correlation length in the paramagnetic state. We use the eikonal approach to sum up an infinite series of diagrammatic contributions from thermal fluctuations. Earlier studies found that in the SDW state the spectral function $A_{\mathit{k}}\left(\omega \right)$ of a hot fermion at a finite $T$ is exponentially small below the energy scale $\mathrm{\Delta }\left(T\right)$, which scales with SDW order and vanishes at the ordering temperature $T_N$, and has a hump at a larger frequency ${\mathrm{\Delta }}_{\mathrm{PG}}$, comparable to the zero-temperature SDW gap $\mathrm{\Delta }(T=0)$. We argue that the hump, which we associate with the pseudogap, survives in some $T$ range above $T_N$. We show that this range is split by regions of strong and weak pseudogap behavior. In the first region, ${\mathrm{\Delta }}_{\mathrm{PG}}$ is weakly temperature dependent, despite the fact that it comes from thermal fluctuations. Such a behavior has been seen in numerical studies of the Hubbard model. We show that to obtain it one needs to go beyond the one-loop approximation and sum up the infinite series of diagrams. In the second regime, ${\mathrm{\Delta }}_{\mathrm{PG}}$ decreases with increasing $T$ and eventually vanishes. We further argue that a magnetic pseudogap at a finite $T$ emerges only if the ground state is magnetically ordered. We present the phase diagram and apply the results to high-$T_c$ cuprates.

The magnetic, noncollinear parametrization of Dudarev's $\mathrm{DFT}+U$ method is generalized to fully relativistic ultrasoft pseudopotentials. We present the definition of the $\mathrm{DFT}+U$ total energy functional and the calculation of forces and stresses in the case of orthogonalized atomic orbitals defining the localized Hubbard manifold, where additional contributions arising from the derivative of the inverse square root of the overlap matrix appear. We further extend the perturbative calculation of the Hubbard $U$ parameters within density-functional perturbation theory to the noncollinear relativistic case, by exploiting an existing and recently developed theoretical approach that takes advantage of the time-reversal operator to solve a second Sternheimer equation. We validate and apply the new scheme by studying the electronic structure and the thermodynamics of the binary compounds $\text{Eu}X$ (where $X$ = O, S, Se, Te is a chalcogen atom), as representative simple crystals, and of the pyrochlore ${\mathrm{Cd}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$, representative of a more structurally complex oxide.

We propose highly efficient photon pair generation via down-conversion in a polaritonic cyclic three-level system formed via coupling an exciton and a localized plasmon polariton. Pure dephasing of the exciton enables optical transitions from the upper to lower polariton state via symmetry breaking and thus opens a pathway for the cascaded emission of two photons. The strength of this down-conversion pathway is determined by the coupling strength between exciton and plasmon polariton and the exciton dephasing rate. In the case of strong up to ultrastrong coupling and strong dephasing, this results in a nanoscopic ${\chi }^{\left(2\right)}$-system capable of few-photon down-conversion. A semiconductor quantum dot embedded in a gap plasmon, which supports strong coupling between plasmon and exciton, in combination with fast pure dephasing in the quantum dot exciton, allows one to reach conversion efficiencies down to the single-photon limit under experimentally achievable parameters at ambient conditions.

In this paper, we derive corrections to the subleading logarithmic term of the entanglement entropy in systems with spontaneous broken continuous symmetry. Using quantum Monte Carlo simulations, we show that the improved scaling formula leads to much better estimations of the number of Goldstone modes in the two-dimensional square lattice spin-1/2 Heisenberg model and bilayer spin-1/2 Heisenberg model in systems of rather small sizes, compared with previous results. In addition, the universal geometry-dependent finite constant in the entanglement entropy scaling is also obtained in good agreement with the theoretical value.

High-$T_c$ cuprate strange metals are characterized by a DC resistivity that scales linearly with $T$ from the onset of superconductivity to the crystal melting temperature, characterized by a current life time ${\tau }_{\hslash }\simeq \hslash /\left(k_{B}T\right)$, the “Planckian dissipation”. At the same time, the optical conductivity ceases to be of the Drude form at high temperatures, suggesting a change of the underlying dynamics that surprisingly leaves the $T$-linear DC resistivity unaffected. We use the AdS/CFT correspondence that describes strongly coupled, densely many-body entangled metallic states of matter to study the DC thermoelectrical transport properties and the optical conductivities of the local quantum critical Gubser-Rocha holographic strange metal in $2+1$ dimensions in the presence of a lattice potential, a prime candidate to compare with experiment. We find that the electrical DC resistivity is linear in $T$ at low temperatures for a large range of potential strengths and wave vectors, even as it transitions between different dissipative regimes. At weak lattice potential the optical conductivity evolves as a function of increasing temperature from a Drude form to a “bad metal” characterized by a mid-IR resonance without changing the DC transport, similar to that seen in cuprate strange metals. This mid-IR peak and notably its temperature evolution can be fully understood as a consequence of umklapp hydrodynamics: i.e., hydrodynamic perturbations are Bloch modes in the presence of a lattice. At strong lattice potential an “incoherent metal” is realized instead where momentum conservation no longer plays a role in the transport. We confirm that in this regime the thermal diffusivity appears to be insensitive to the breaking of translations and can be explained by Planckian dissipation originating in universal microscopic chaos. A similar behavior has been found for holographic metals with strong homogeneous momentum relaxation. The charge diffusivity does not submit to this chaos explanation, even though the continuing linear-in-$T$ DC resistivity saturates to an apparent universal slope, numerically equal to a Planckian rate.

We formulate the non-Abelian Berry connection (tensor $\mathbb{R}$) and phase (matrix $\mathit{\Gamma }$) for a multiband system and apply them to semiconductor holes in the presence of Rashba and Dresselhaus spin-orbit interactions (SOIs). For this purpose, we focus on heavy-mass holes confined in a SiGe two-dimensional quantum well, whose electronic structure and spin texture are explored by the extended $\mathit{k}·\mathit{p}$ approach. To explore the influence of the nonadiabatic process, we perform the contour integral of $\mathbb{R}$ faithfully along the equienergy surface by combining the time-dependent Schrödinger equation with the semiclassical equation of motion for a cyclotron and then calculate the energy dependence of $\mathit{\Gamma }$ computationally. The intersubband interactions in the valence band strongly modifies the SOIs. Accordingly, holes conserve the spin quasidegeneracy at several specific points, where the interstate hybridization generates off-diagonal components both of $\mathbb{R}$ and $\mathit{\Gamma }$, and the simple $\pi$ quantization found in the Abelian Berry phase is violated. Moreover, these off-diagonal terms cause “resonant repulsion” at the quasidegenerate energy. Consequently, $\mathrm{HH}\pm$ exhibits a discontinuity in the energy dependence of $\mathit{\Gamma }$.

We systematically study the effects of the shear and uniaxial strains, applied along different crystallographic directions, on the electronic spectrum of Lieb and kagome lattices by using the tight-binding model with a general Hamiltonian that describes both lattices by means of only one control parameter. Our findings show that such deformations do not open an energy gap in their electronic spectra but can cause (i) approximation of the energy cones, (ii) anisotropy in the energy levels, and (iii) deformation of the flat band, such that the triply degenerate Dirac point in the Lieb lattice transforms into two doubly degenerate Dirac points. By analyzing hypothetical strain cases in which the values of the hopping parameters do not change, we observe that effects such as deformation in the flat band and division of the triply degenerate Dirac point are only due to the hopping parameter changes caused by the strain. Moreover, we identify cases in which there are non-null strain-induced pseudovector potentials in Lieb and kagome lattices.