The existence of nonergodic ground states is considered as a precursor to a first-order long-range magnetostructural transformation. ${\mathrm{Mn}}_3{\mathrm{Ga}}_{0.45}{\mathrm{Sn}}_{0.55}\mathrm{C}$ lies compositionally between two compounds, ${\mathrm{Mn}}_3\mathrm{GaC}$ and ${\mathrm{Mn}}_3\mathrm{SnC}$, undergoing first-order magnetic transformation. However, ${\mathrm{Mn}}_3{\mathrm{Ga}}_{0.45}{\mathrm{Sn}}_{0.55}\mathrm{C}$, which crystallizes in a single-phase cubic structure, exhibits more than one long-range magnetic transition. Using a combination of magnetization, ac susceptibility, neutron diffraction, and x-ray-absorption fine-structure techniques, it is shown that, although ${\mathrm{Mn}}_3{\mathrm{Ga}}_{0.45}{\mathrm{Sn}}_{0.55}\mathrm{C}$ exhibits long-range magnetic order, it presents a cluster glassy ground state due to formation of magnetically ordered Ga-rich and Sn-rich clusters. The clusters are big enough to present signatures of long-range magnetic order but are distributed in a way that limits interactions between two clusters of the same type, leading to a frozen magnetic state at low temperatures. The main reason for such a cluster-glass state is the difference in the local structure of Mn atoms that find themselves in Ga-rich and Sn-rich clusters.

In this contribution, we report that ${\mathrm{CaY}}_2{\mathrm{Co}}_2{\mathrm{Ge}}_3{\mathrm{O}}_{12}$ exhibits an unusual anisotropic and chainlike antiferromagnetic arrangement of spins despite crystallizing in the highly symmetric garnet structure. Using low-temperature powder neutron diffraction and symmetry analysis, we identify a magnetic structure consisting of chainlike motifs oriented along the body diagonals of the cubic unit cell with moments pointing parallel to the chain direction due to the strong Ising character of the Co ions. Antiferromagnetic order sets in below 6 K and exhibits both temperature- and field-induced magnetic transitions at high fields. Combining the results, we present a magnetic phase diagram that suggests ${\mathrm{CaY}}_2{\mathrm{Co}}_2{\mathrm{Ge}}_3{\mathrm{O}}_{12}$ undergoes a quantum phase transition at low temperatures and moderate fields.

We theoretically study the effects of strong magnetoelastic coupling on the transport properties of magnetic insulators. We develop a Boltzmann transport theory for the mixed magnon-phonon modes (“magnon polarons”) and determine transport coefficients and the spin diffusion length. Magnon-polaron formation causes anomalous features in the magnetic field and temperature dependence of the spin Seebeck effect when the disorder scattering in the magnetic and elastic subsystems is sufficiently different. Experimental data by Kikkawa et al. [Phys. Rev. Lett. 117, 207203 (2016)] on yttrium iron garnet films can be explained by an acoustic quality that is much better than the magnetic quality of the material. We predict similar anomalous features in the spin and heat conductivity and nonlocal spin transport experiments.

The crystallographic, magnetic, and transport properties of the van der Waals bonded, layered compound ${\mathrm{CrTe}}_3$ have been investigated on single-crystal and polycrystalline materials. The crystal structure contains layers made up of lozenge-shaped ${\mathrm{Cr}}_4$ tetramers. Electrical resistivity measurements show the crystals to be semiconducting, with a temperature dependence consistent with a band gap of 0.3 eV. The magnetic susceptibility exhibits a broad maximum near 300 K characteristic of low dimensional magnetic systems. Weak anomalies are observed in the susceptibility and heat capacity near 55 K, and single-crystal neutron diffraction reveals the onset of long-range antiferromagnetic order at this temperature. Strongly dispersive spin waves are observed in the ordered state. Significant magnetoelastic coupling is indicated by the anomalous temperature dependence of the lattice parameters and is evident in structural optimization in van der Waals density functional theory calculations for different magnetic configurations. The cleavability of the compound is apparent from its handling and is confirmed by first-principles calculations, which predict a cleavage energy $0.5\mathrm{J}/{\mathrm{m}}^2$, similar to graphite. Based on these results, ${\mathrm{CrTe}}_3$ is identified as a promising compound for studies of low dimensional magnetism in bulk crystals as well as magnetic order in monolayer materials and van der Waals heterostructures.

From microscopic point-dipole model calculations of the screening of the Coulomb interaction in nonpolar systems by polarizable atoms, it is known that screening strongly depends on dimensionality. For example, in one-dimensional systems, the short-range interaction is screened, while the long-range interaction is antiscreened. This antiscreening is also observed in some zero-dimensional structures, i.e., molecular systems. By means of ab initio calculations in conjunction with the random-phase approximation (RPA) within the FLAPW method, we study screening of the Coulomb interaction in ${\mathrm{Fe}}_x{\mathrm{O}}_y$ clusters. For completeness, these results are compared with their bulk counterpart magnetite. It appears that the on-site Coulomb interaction is very well screened both in the clusters and bulk. On the other hand, for the intersite Coulomb interaction, the important observation is made that it is almost constant throughout the clusters, while for the bulk it is almost completely screened. More precisely and interestingly, in the clusters antiscreening is observed by means of ab initio calculations.

Spin-orbit coupling in crystals is known to lead to unusual direction-dependent exchange interactions; however understanding of the consequences of such effects in molecular crystals is incomplete. Here we perform four-component relativistic density functional theory computations on the multinuclear molecular crystal ${\mathrm{Mo}}_3{\mathrm{S}}_7$(dmit)${}_3$ and show that both intra- and intermolecular spin-orbit coupling are significant. We describe a powerful Wannier spin-orbital construction technique and use it to determine a long-range relativistic single-electron Hamiltonian from first principles. We analyze the various contributions to this Hamiltonian through the lens of group theory, building on our previous work. Intermolecular spin-orbit couplings like those found here are known to lead to quantum spin-Hall and topological insulator phases on the 2D lattice formed by the tight-binding model predicted for a single layer of ${\mathrm{Mo}}_3{\mathrm{S}}_7$(dmit)${}_3$.

We present an ab initio study of core excitations of solid-state materials focusing on the role of electron-hole correlation. In the framework of an all-electron implementation of many-body perturbation theory into the $\mathtt{exciting}$ code, we investigate three different absorption edges of three materials, spanning a broad energy window, with transition energies between a few hundred to thousands of eV. Specifically, we consider excitations from the Ti $K$ edge in rutile and anatase ${\mathrm{TiO}}_2$, from the Pb $M_4$ edge in ${\mathrm{PbI}}_2$, and from the Ca $L_{2,3}$ edge in CaO. We show that the electron-hole attraction rules x-ray absorption for deep core states when local fields play a minor role. On the other hand, the local-field effects introduced by the exchange interaction between the excited electron and the hole dominate excitation processes from shallower core levels, separated by a spin-orbit splitting of a few eV. Our approach yields absorption spectra in good agreement with available experimental data and allows for an in-depth analysis of the results, revealing the electronic contributions to the excitations, as well as their spatial distribution.

The gapless Bogoliubov-de Gennes (BdG) quasiparticles of a clean three-dimensional spinless $p_x+i{p}_y$ superconductor provide an intriguing example of a thermal Hall semimetal (ThSM) phase of Majorana-Weyl fermions; such a phase can support a large anomalous thermal Hall conductivity and protected surface Majorana-Fermi arcs at zero energy. We study the effects of quenched disorder on such a gapless topological phase by carrying out extensive numerical and analytical calculations on a lattice model for a disordered, spinless $p_x+i{p}_y$ superconductor. Using the kernel polynomial method, we compute both average and typical density of states for the BdG quasiparticles, from which we construct the phase diagram of three-dimensional dirty $p_x+i{p}_y$ superconductors as a function of disorder strength and chemical potential of the underlying normal state. We establish that the power law quasilocalized states induced by rare statistical fluctuations of the disorder potential give rise to an exponentially small density of states at zero energy, and even infinitesimally weak disorder converts the ThSM into a thermal diffusive Hall metal (ThDM). Consequently, the phase diagram of the disordered model only consists of ThDM and thermal insulating phases. We show the existence of two types of thermal insulators: (i) a trivial thermal band insulator (ThBI) [or BEC phase] with a smeared gap that can occur for suitable band parameters and all strengths of disorder, supporting only exponentially localized Lifshitz states (at low energy) and (ii) a thermal Anderson insulator that only exists for large disorder strengths compared to all band parameters. We determine the nature of the two distinct localization-delocalization transitions between these two types of insulators and ThDM. Additionally, we establish the scaling properties of an avoided (or hidden) quantum critical point for moderate disorder strengths, which govern the crossover between ThSM and ThDM phases over a wide range of energy scales. We also discuss the experimental relevance of our findings for three-dimensional, time reversal symmetry breaking, triplet superconducting states.

The need to increase data transfer rates constitutes a key challenge in modern information-driven societies. Taking advantage of the transverse spatial modes of light to encode more information is a promising avenue for both classical and quantum photonics. However, to ease access to the encoded information, it is essential to be able to sort spatial modes into different output channels. Here, we introduce a way to customize the sorting of arbitrary spatial light modes. Our method relies on the high degree of control over random scattering processes by preshaping of the phase structure of the incident light. We demonstrate experimentally that various sets of modes, irrespective of their specific modal structure, can be transformed to a broad range of output channel arrangements. Thus, our method enables full access to all of the information encoded in the transverse structure of the field; for example, azimuthal and radial modes. We also demonstrate that coherence is retained in this complex mode transformation, which opens up applications in quantum and classical information science.

We experimentally show that the spin relaxation in degenerate $n$-type germanium ($n^{+}$-Ge) depends strongly on the concentration of the donor impurity ($N_{\mathrm{d}}$) at low temperatures. From measuring nonlocal spin signals for various lateral spin-valve devices at 8 and 77 K, the spin diffusion length (${\lambda }_{\mathrm{Ge}}$) of $n^{+}$-Ge can be estimated as a function of carrier concentration ($n$), i.e., $N_{\mathrm{d}}\approx n$ ($\sim {10}^{18}$ ${\mathrm{cm}}^{-3}$). We clearly find a large change in ${\lambda }_{\mathrm{Ge}}$ from 1.43 to 0.56 $\mu \mathrm{m}$ within a relatively narrow range of $n$ at 8 K. The experimental findings are interpreted quantitatively in terms of a recent theory based on donor-driven spin scattering in multivalley conduction bands in Ge.

Spin transport in thin-film materials can be studied by ferromagnetic resonantly (FMR) driven spin pumping of a charge-free spin current which induces an electromotive force through the inverse spin Hall effect (ISHE). For quantitative ISHE experiments, precise control of the FMR driving-field amplitude $B_1$ is crucial. This study exploits in situ monitoring of $B_1$ by utilization of electron paramagnetic resonantly (EPR) induced transient nutation of paramagnetic molecules (a 1:1 complex of $\alpha ,\gamma$-bisdiphenylene-$\beta$-phenylallyl and benzene, BDPA) placed as a $B_1$ probe in proximity to a NiFe/Pt-based ISHE device. Concurrent to an ISHE experiment, $B_1$ is obtained from the inductively measured BDPA Rabi-nutation frequency. Higher reproducibility is achieved by renormalization of the ISHE voltage to $B_1^2$ with an accuracy that is determined by the homogeneity of the FMR driving field and thus by the applied microwave resonator and ISHE device setup.

Using the anisotropic electron transport and susceptibility measurements we demonstrate the appearance of inhomogeneous gossamer superconductivity in FeSe single crystals at ambient pressure and at temperature five times higher than its zero resistance $T_c$. We also find and quantitatively describe a general property: If inhomogeneous superconductivity in a anisotropic conductor first appears in the form of isolated superconducting islands, it reduces electric resistivity anisotropically with maximal effect along the least conducting axis. This gives a simple tool to study inhomogeneous superconductivity in various anisotropic compounds, which helps to investigate the onset of high-temperature superconductivity.

Small lattice polarons driven by strong external electric fields are considered. A time-dependent Schrödinger equation is integrated directly in time domain. The field agitates ions both directly and through modulation of carrier density. It is found that when the field is in resonance with local ion oscillations, the polaron is liberated from its self-induced trap and the tunneling frequency increases exponentially.

The origin of the anisotropic, paramagnetic phase associated with electronic nematicity in the iron pnictides is yet to be resolved. Furthermore, the detwinning technique used to study the nematic order in single crystals is known to introduce extra anisotropy into the sample, which can smear out the transition and even modify intrinsic characteristics associated with “spontaneous” Ising, $Z_2$, symmetry breaking. Here we use a strain- and stress-free twinned sample to show that there is a significant reduction in the energy relaxation times of the hot electrons following nonequilibrium femtosecond laser excitation on both the high- and low-temperature sides of the nematic phase transition. This femtosecond critical speeding-up behavior provides an alternative way to study complex, electronically driven nematicity, neither invoking external strain nor measuring a small anisotropy in twinned crystals. Particularly, a detailed analysis of the observed ultrafast decay time and the amplitude associated with an initial electronic relaxation provides compelling implications on the physical origin of nematicity in iron pnictides: (1) nematic fluctuations strongly influence the dynamics of electron cooling, and (2) spin fluctuations determine the part of amplitude arising from the nematicity. Finally, we discuss ultrafast coherent phonon generation which may contribute to the measured transition temperature in our ultrafast measurements.

Materials with low thermal conductivity, which can minimize heat dissipation, play a central role in many practical applications, such as thermoelectric devices and thermal barrier coatings. We report the synthesis and low thermal conductivities of narrow band gap semiconductors NaSnAs and NaSnP, in which the measured minimum lattice thermal conductivities are 0.62 and 0.58 W/m/K. The values are close to the calculated minimum of 0.46 and 0.40 W/m/K, respectively, and obviously lower than that for their closely related compound $\mathrm{NaS}{\mathrm{n}}_2\mathrm{A}{\mathrm{s}}_2$, which has fewer lone-pair electrons. Such low thermal conductivities in NaSnAs and NaSnP are due to double lone $s^2$ electron pairs that are present in both Sn sites and As or P sites. Double lone-pair electrons can induce greater Grüneisen parameters $\gamma$ and lower thermal conductivity further than that in $\mathrm{NaS}{\mathrm{n}}_2\mathrm{A}{\mathrm{s}}_2$. Our findings provide hints for exploring new materials with low thermal conductivity.

We study plasmon modes of the two-dimensional electron gas residing at the interface of band insulators ${\mathrm{LaAlO}}_3$ and ${\mathrm{SrTiO}}_3$ (LAO/STO) and the plasmon excitations of graphene-LAO/STO double layer as well. Considering the electron-electron interaction within random phase approximation, we calculate the plasmon dispersions of both systems numerically and in the long-wavelength limit analytical expressions for collective modes are found. One optical mode and two (three) acoustic modes are predicted for the LAO/STO (graphene-LAO/STO) system where only the uppermost acoustic mode of both systems can emerge above the electron-hole continuum depending on the characteristics of each system. In the case of LAO/STO interface, thanks to the spatial separation between ${\mathrm{t}}_{2\mathrm{g}}$ orbitals, the upper acoustic mode might be undamped at the long-wavelength limit depending on the exact value of the dielectric constant of ${\mathrm{SrTiO}}_3$. Same as other double layer systems, the interlayer distance for the graphene-LAO/STO system plays a crucial role in damping the upper acoustic mode. Faster damping of all plasmon modes of the present double layer system in comparison with the ones with conventional two-dimensional electron gas instead of ${\mathrm{t}}_{2\mathrm{g}}$ electron gas is also found due to heavier effective masses of the gas and also stronger interlayer Coulomb interaction.

We study time-dependent electron transport and quantum noise in a ballistic graphene field effect transistor driven by an ac gate potential. The nonlinear response to the ac signal is computed through Floquet theory for scattering states and Landauer-Büttiker theory for charge current and its fluctuations. Photon-assisted excitation of a quasibound state in the top-gate barrier leads to resonances in transmission that strongly influence the noise properties. For strong doping of graphene under source and drain contacts, when electrons are transmitted through the channel via evanescent waves, the resonance leads to a substantial suppression of noise. The Fano factor is then reduced well below the pseudodiffusive value, $F<1/3$, also for strong ac drive. The good signal-to-noise ratio (small Fano factor) on resonance suggests that the device is a good candidate for high-frequency (THz) radiation detection. We show analytically that Klein tunneling (total suppression of back-reflection) persists for perpendicular incidence also when the barrier is driven harmonically. Although the transmission is inelastic and distributed among sideband energies, a sum rule leads to total suppression of shot noise.

Both metaphysical and practical considerations related to time inversion have intrigued scientists for generations. Physicists have strived to devise and implement time-inversion protocols, in particular different forms of “time mirrors” for classical waves. Here we propose an instantaneous time mirror for quantum systems, i.e., a controlled time discontinuity generating wave function echoes with high fidelities. This concept exploits coherent particle-hole oscillations in a Dirac spectrum in order to achieve population reversal, and can be implemented in systems such as (real or artificial) graphene.

We examine in this work the role of disorder in contributing to the sluggish relaxation observed in intrinsic electron glasses. Our approach is guided by several empirical observations: First and foremost, Anderson localization is a pre-requisite for observing these nonequilibrium phenomena. Secondly, sluggish relaxation appears to favor Anderson insulators with relatively large Fermi energies (hence proportionally large disorder). These observations motivated us to consider a way to measure the underlying disorder in a realistic Anderson insulator. Optical studies using a series of amorphous indium oxide (${\mathrm{In}}_x\mathrm{O}$) establish a simple connection between carrier concentration and the disorder necessary to approach the metal-insulator transition from the insulating side. This is used to estimate the typical magnitude of the quenched potential fluctuation in the electron-glass phase of this system. The implications of our findings on the slow dynamics of Anderson insulators are discussed. In particular, the reason for the absence of a memory dip and the accompanying electron-glass effects in lightly-doped semiconductors emerges as a natural consequence of their weak disorder.