The question of macroscopic occupation and the spontaneous emergence of coherence for exciton ensembles has gained renewed attention due to the rise of van der Waals heterostructures made of atomically thin semiconductors. The hosted interlayer excitons exhibit nanosecond lifetimes, long enough to allow for excitonic thermalization in time. Several experimental studies reported signatures of macroscopic occupation effects at elevated exciton densities. With respect to theory, excitons are composite particles formed by fermionic constituents, and a general theoretical argument for a bosonic thermalization of an exciton gas beyond the linear regime is still missing. Here, we derive an equation for the phonon mediated thermalization at densities above the classical limit, and identify which conditions favor the thermalization of fermionic or bosonic character, respectively. In cases where acoustic, quasielastic phonon scattering dominates the dynamics, our theory suggests that transition metal dichalcogenide excitons might be bosonic enough to show bosonic thermalization behavior and decreasing dephasing for increasing exciton densities. This can be interpreted as a signature of an emerging coherence in the exciton ground state, and thus provides an explanation for the unexpected recent experimentally observed feature of a decreasing linewidth for increasing densities [Phys. Rev. Res. 2, 042044(R) (2020)]. Also, this interpretation would be in line with a recently observed long coherence length in the same material [Phys. Rev. Lett. 131, 036902 (2023)].

We report a comprehensive spin wave analysis of the semiconducting honeycomb van der Waal antiferromagnet ${\mathrm{NiPS}}_3$. Using single-crystal inelastic neutron scattering, we map out the full Brillouin zone and fit the observed modes to a spin wave model with rigorously defined uncertainty. We find that the third-nearest-neighbor exchange $J_3$ dominates the Hamiltonian, a feature which we fully account for by ab initio density functional theory calculations. We also quantify the degree to which the threefold rotation symmetry is broken and account for the $Q=0$ excitations observed in other measurements, yielding a spin exchange model which is consistent across multiple experimental probes. We also identify a strongly reduced static ordered moment and reduced low-energy intensity relative to the linear spin wave calculations, signaling unexplained features in the magnetism which requires going beyond the linear spin wave approximation.

Establishing the fundamental relation between the homotopy invariants and the band topology of Hamiltonians has played a critical role in the recent development of topological phase research. In this work, we establish the homotopy invariant and the related band topology of three-dimensional (3D) real-valued Hamiltonians with two occupied and two unoccupied bands. Such a real Hamiltonian generally appears in $\mathcal{PT}$-symmetric spinless fermion systems where $\mathcal{P}$ and $\mathcal{T}$ indicate the inversion and time-reversal symmetries, respectively, and ${\left(\mathcal{PT}\right)}^2=1$ due to the spinless condition. We show that the 3D band topology of the system is characterized by two independent Hopf invariants when the lower-dimensional band topology is trivial. Thus, the corresponding 3D band insulator with nonzero Hopf invariants can be called a real Hopf insulator (RHI). In sharp contrast to all the other topological insulators discovered up to now, the topological invariants of RHI can be defined only when the fixed number of both the occupied and unoccupied states are simultaneously considered. Thus, the RHI belongs to the category of delicate topological insulators proposed recently. We show that finite-size systems with slab geometry support surface states with nonzero Chern numbers in a $\mathcal{PT}$-symmetric manner independent of the Fermi level position, and establish the bulk-boundary correspondence. We also discuss the bulk-boundary correspondence of rotation-symmetric RHIs using the reverting Thouless pump.

We present a systematic Raman spectroscopy study on a series of pyrochlore ruthenates, a system which is not yet clearly settled on its magnetic origin and structure. Apart from the Raman-active phonon modes, new peaks that appear in the energy range of 15–35 meV below the Néel temperature $\left(T_{\mathrm{N}}\right)$ are assigned as one-magnon modes. The temperature evolution of one-magnon modes displays no significant thermal dependence in mode frequencies, while the intensities decrease monotonically. Remarkably, one magnons from all compounds show similar characteristics with a single dominant peak at lower energy and weaker side peaks at a couple of meV higher energy. Most importantly, we uncover a striking proportionality between the dominant magnon mode energies and the $T_{\mathrm{N}}$ values. Our results suggest the Ru ions may have similar or the same magnetic phase in all pyrochlore ruthenates of our study. We have thus found an avenue for directly tuning the magnetic exchange interaction by the selection of the $A$-site ion.

We study the interplay between collective electronic and lattice modes in polar metals in an applied magnetic field aligned with the polar axis. Static spin-orbit coupling leads to the appearance of a particle-hole spin-flip continuum that is gapped at low energies in a finite field. We find that a weak spin-orbit assisted coupling between electrons and polar phonons induces the emergence of electronic collective modes. The strength of the applied magnetic field tunes the number of modes and their energies, which can lie both above and below the particle-hole continuum. For a range of field values, we identify Fano-type interference between the electronic continuum and phonons. We show that signatures of these collective modes can be observed in optical spectroscopy experiments, and we provide the corresponding theoretical predictions.

Powder samples have been suggested as a pathway to fabricate isotropic magnetoelectric (ME) materials which effectively only have a pseudoscalar or monopole ME response. We demonstrate that random distribution of ME grains alone does not warrant isotropic ME response because the activation of a nonvanishing ME response requires a ME field cooling protocol which tends to induce preferred axes. We investigate the evolution of ME susceptibility in powder chromia samples for various ME field cooling protocols both theoretically and experimentally. In particular, we work out the theoretical expressions for ME susceptibility for powder chromia in the framework of statistical mechanics where Boltzmann factors weigh the orientation of the Néel vector relative to the local orientation of the $c$ axis of a grain. Previous approximations oversimplified the thermodynamic nature of the annealing process giving rise to misleading conclusions on the role of the magnitude of the applied product of electric and magnetic fields on the ME response. In accordance with our refined theory, a strong dependence of the functional form of α vs $T$ of chromia powders on the ME field cooling protocol is observed. It shows that chromia powder is not generically an isotropic ME effective medium but provides a pathway to realize the elusive isotropic ME response.

Magnetic Weyl semimetals (mWSMs) are characterized by linearly dispersive bands with chiral Weyl node pairs associated with broken time-reversal symmetry. One of the hallmarks of mWSMs is the emergence of large intrinsic anomalous Hall effect. On heating the mWSM above its Curie temperature, the magnetism vanishes while exchange-split Weyl point pairs collapse into doubly degenerate gapped Dirac states. Here, we reveal the attractive potential of these Dirac nodes in paramagnetic state for efficient spin current generation at room temperature via the spin Hall effect. Ni and In are introduced to separately substitute Co and Sn in a prototypal mWSM ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$ shandite film and tune the Fermi level. Composition dependence of spin Hall conductivity for paramagnetic shandite at room temperature resembles that of anomalous Hall conductivity for ferromagnetic shandite at low temperature; exhibiting peak-like dependence centering around the Ni-substituted ${\mathrm{Co}}_2{\mathrm{Ni}}_1{\mathrm{Sn}}_2{\mathrm{S}}_2$ and undoped ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$ compositions, respectively. The observed spin Hall and anomalous Hall conductivity maxima at different compositions reflect optimum Fermi-level positioning relative to the paramagnetic Dirac and magnetic Weyl states, suggesting the common origin and intercorrelation between the two Hall effects. Our findings highlight a strategy for the quest of spin Hall materials, guided by the abundant experimental anomalous Hall-effect data of ferromagnets in the literature.

The nitrogen-vacancy (NV) center in diamond is well known in quantum metrology and quantum information for its favorable spin and optical properties, which span a wide temperature range from near zero to over 600 K. Despite its prominence, the NV center's photophysics is incompletely understood, especially at intermediate temperatures between 10–100 K where phonons become activated. In this paper, we present a rate model able to describe the crossover from the low-temperature to the high-temperature regime. Key to the model is a phonon-driven hopping between the two orbital branches in the excited state (ES), which accelerates spin relaxation via an interplay with the ES spin precession. We extend our model to include magnetic and electric fields as well as crystal strain, allowing us to simulate the population dynamics over a wide range of experimental conditions. Our model recovers existing descriptions for the low- and high-temperature limits and successfully explains various sets of literature data. Further, the model allows us to predict experimental observables, in particular the photoluminescence (PL) emission rate, spin contrast, and spin initialization fidelity relevant for quantum applications. Lastly, our model allows probing the electron-phonon interaction of the NV center and reveals a gap between the current understanding and recent experimental findings.

Rare-earth delafossite compounds, $AR{C}_2$ ($A=\mathrm{alkali}$ or monovalent ion, $R=\mathrm{rare}\mathrm{earth}$, $C=\mathrm{chalcogen}$), have been proposed for a range of novel quantum phenomena. Particularly, the Tm series, $A\mathrm{Tm}{C}_2$, featuring Tm ions on a triangular lattice, serves as a representative group of compounds to illustrate the interplay and competition between spin-orbit coupling, crystal fields, and exchange couplings in the presence of geometric frustration. Here we report the thermodynamic and inelastic neutron scattering studies on the newly discovered triangular-lattice magnet ${\mathrm{KTmSe}}_2$. Both heat capacity and neutron diffraction measurements reveal the absence of long-range magnetic order. Magnetic susceptibility shows strong Ising-like interactions with antiferromagnetic correlations. Furthermore, inelastic neutron scattering measurements reveal a branch of dispersive crystal field excitations. To analyze these observations, we employ both the transverse field Ising model and the full crystal field scheme, along with exchange interactions. Our results suggest a strong competition between spin exchange interactions and crystal field effects. This work is expected to offer a valuable framework for understanding low-temperature magnetism in ${\mathrm{KTmSe}}_2$ and similar materials.

Motivated by the observation of a pair density wave (PDW) in the kagome metal $\mathrm{CsV}{}_3\mathrm{Sb}{}_5$, we consider the fate of electrons near a $p$-type Van Hove singularity (vHS) of the kagome lattice with on-site Hubbard $U$ and the nearest-neighbor repulsion $V$. We study the effect of such interactions on Fermi surface “patches” at the vHS. We show how a feature unique to the kagome lattice known as sublattice interference crucially affects the form of the interactions among the patches. The renormalization group (RG) flow of such interactions results in a regime where the nearest-neighbor interaction $V$ exceeds the on-site repulsion $U$. We identify this condition as being favorable for the formation of charge-density-wave (CDW) and PDW orders. In the weak-coupling limit, we find a complex CDW order as the leading instability, which breaks time-reversal symmetry. Beyond RG, we perform a Hartree-Fock study to a $V$-only model and find the pair-density-wave order indeed sets in at some intermediate coupling.

It is generally believed that in a two-dimensional metal, whose ground state is antiferromagnetically ordered with $\mathbf{Q}=(\pi ,\pi )$, thermal (static) magnetic fluctuations give rise to precursor behavior above $T_N$ in which the spectral function of a hot fermion (the one for which $\mathbf{k}$ and $\mathbf{k}+\mathbf{Q}$ are Fermi surface points) contains two peaks, separated by roughly the same energy as in the antiferromagnetically ordered state. The two peaks persist in some range of $T>T_N$ and eventually merge into a single peak at zero frequency. This behavior is obtained theoretically by departing from free fermions in a paramagnet and evaluating the dressed fermionic Green's function by summing up infinite series of diagrams with contributions from thermal magnetic fluctuations. We show, following [Y. M. Vilk and A.-M. S. Tremblay, J. Phys. I (France) 7, 1309 (1997)] that keeping vertex renormalization diagrams in these series is crucial as other terms only broaden the spectral function of a hot fermion but do not shift its maximum away from zero frequency. As the consequence, the magnetic pseudogap should be treated as an input for theories that neglect vertex corrections, such as, e.g., Eliashberg theory for magnetically mediated superconductivity. We also analyze the potential pseudogap behavior at $T=0$. We argue that it may exist, but only at a finite correlation length, and not as a precursor to antiferromagnetism.

In this theoretical Letter, we examine the influence of dimensionality, size reduction, and heat-transport direction on the phonon-drag contribution to the Seebeck coefficient of silicon nanostructures. Phonon-drag contribution, which arises from the momentum transfer between out-of-equilibrium phonon populations and charge carriers, significantly enhances the thermoelectric coefficient. Our implementation of the phonon-drag term accounts for the anisotropy of nanostructures, such as thin films and nanowires through the boundary- and momentum-resolved phonon lifetime. Our approach also takes into account the spin-orbit coupling, which turns out to be crucial for hole transport. We reliably quantify the phonon-drag contribution at various doping levels, temperatures, and nanostructure geometries for both electrons and holes in silicon nanostructures. Our results support the recent experimental findings, showing that a part of phonon-drag contribution survives in 100-nm silicon nanostructures.

We experimentally explore a synthetic-antiferromagnetically (SAF) coupled easy-plane stochastic magnetic tunnel junction (EP-SMTJ) for a baseline device behavior assessment. The tunnel conductance fluctuates over time due to magnetoresistance reflecting the superparamagnetic state of the EP-SMTJ's layers. Binary digitizing at its median gives a stochastic bit stream, which is analyzed using the NIST SP800-22r1a test suite. When the tunnel conductance is sampled at different times for producing the binary bit stream, a best bit rate of $\sim 250$ Mb/s is seen. If two independently taken bit streams are xored, the effective bit rate can be $>1$ Gb/s, provided that the in-plane anisotropy is small compared to ambient temperature ($k_B{T}_a$). The SAF design for both the free and the reference layer allows for a device conductance fluctuation median that is well centered and only weakly voltage-bias dependent. Comparison of experimental observations with a four-moment coupled finite-temperature macrospin model reveals the role of in-plane anisotropy as causing additional telegraphing fluctuations, which is associated with a longer timescale for stochasticity—a behavior that needs to be optimized for high-speed stochastic bit-stream applications.

We demonstrate via exact diagonalization that AA-stacked transition metal dichalcogenide homobilayers host fractional quantum anomalous Hall (FQAH) states with fractionally quantized Hall conductance at fractional fillings $n=\frac{1}{3},\frac{2}{3}$ and zero magnetic field. While both states are most robust at angles close to 2-degree, the $n=\frac{1}{3}$ state gives way to a charge density wave with increasing twist angle whereas the $n=\frac{2}{3}$ state survives across a much broader range of twist angles. We show that the competition between FQAH states and charge density wave or metallic phases is primarily controlled by the wave functions and dispersion of the underlying Chern band, respectively. Additionally, Ising ferromagnetism is found across a broad range of fillings where the system is insulating or metallic alike. The spin gap is enhanced at filling fractions where integer and fractional quantum anomalous Hall states are formed.

We introduce a novel gauge-invariant, quantized interband index in two-dimensional (2D) multiband systems. It provides a bulk topological classification of a submanifold of parameter space (e.g., an electron valley in a Brillouin zone), and therefore overcomes difficulties in characterizing topology of submanifolds. We confirm its topological nature by numerically demonstrating a one-to-one correspondence to the valley Chern number in $k·p$ models (e.g., gapped Dirac fermion model) and the first Chern number in lattice models (e.g., Haldane model). Furthermore, we derive a band-resolved topological charge and demonstrate that it can be used to investigate the nature of edge states due to band inversion in valley systems like multilayer graphene.

${\mathrm{Mn}}_3{\mathrm{Si}}_2{\mathrm{Te}}_6$ is a rare example of a layered ferrimagnet. It has recently been shown to host a colossal angular magnetoresistance as the spin orientation is rotated from the in- to out-of-plane direction, proposed to be underpinned by a topological nodal-line degeneracy in its electronic structure. Nonetheless, the origins of its ferrimagnetic structure remain controversial, while its experimental electronic structure, and the role of correlations in shaping this, are little explored to date. Here, we combine x-ray and photoemission-based spectroscopies with first-principles calculations to probe the elemental-selective electronic structure and magnetic order in ${\mathrm{Mn}}_3{\mathrm{Si}}_2{\mathrm{Te}}_6$. Through these, we identify a marked Mn-Te hybridization, which weakens the electronic correlations and enhances the magnetic anisotropy. We demonstrate how this strengthens the magnetic frustration in ${\mathrm{Mn}}_3{\mathrm{Si}}_2{\mathrm{Te}}_6$, which is key to stabilizing its ferrimagnetic order, and find a crucial role of both exchange interactions extending beyond nearest-neighbors and antisymmetric exchange in dictating its ordering temperature. Together, our results demonstrate a powerful methodology of using experimental electronic structure probes to constrain the parameter space for first-principles calculations of magnetic materials, and through this approach, reveal a pivotal role played by covalency in stabilizing the ferrimagnetic order in ${\mathrm{Mn}}_3{\mathrm{Si}}_2{\mathrm{Te}}_6$.

Here we report a study of the spin dynamics in the ferromagnetic chain compound ${\mathrm{Li}}_2{\mathrm{CuO}}_2$. Inelastic neutron scattering measurements allow for the spin Hamiltonian to be determined using a $J_1-J_2$ XXZ-Heisenberg spin chain model with weak interchain interactions. The primary exchange parameters determined from our data are qualitatively consistent with those of Lorenz et al. [Europhys. Lett. 88, 37002 (2009)], and our data allow for the resolution of additional interchain exchange interactions. We also observe the formation of two- and, potentially, three-magnon bound states. The two-magnon bound state exists only in the magnetically ordered phase of this material, consistent with stabilization by the weak, Ising-like exchange anisotropy of the nearest-neighbor intrachain interaction. In contrast, the potential three-magnon state persists in a finite temperature regime above $T_N$, indicating an unconventional character. Our results establish ${\mathrm{Li}}_2{\mathrm{CuO}}_2$ as an experimental platform for the study of exchange anisotropy-stabilized bound states in a ferromagnetic chain.

We investigate the non-Hermitian Haldane model on hyperbolic $\{8,3\}$ and $\{12,3\}$ lattices and showcase its intriguing topological properties in the simultaneous presence of non-Hermitian effect and hyperbolic geometry. From bulk descriptions of the system, we calculate the real-space non-Hermitian Chern numbers by generalizing the method from its Hermitian counterpart and present a corresponding phase diagram of the model. For boundaries, we find that skin-topological modes appear in the range of the bulk energy gap under certain boundary conditions, which can be explained by an effective one-dimensional zigzag chain model mapped from a hyperbolic lattice boundary. Remarkably, these skin-topological modes are localized at specific corners of the boundary, constituting a hybrid higher-order skin-topological effect on hyperbolic lattices.

The transfer of information between two physical locations is an essential component of both classical and quantum computing. In quantum computing the transfer of information must be coherent to preserve quantum states and hence the quantum information. We establish a simple protocol for transferring one- and two-electron encoded logical qubits in quantum dot arrays. The theoretical energetic cost of this protocol is calculated—in particular, the cost of freezing and unfreezing tunneling between quantum dots. Our results are compared with the energetic cost of shuttling qubits in quantum dot arrays and transferring classical information using classical information buses. Only our protocol can manage constant dissipation for any chain length. This protocol could reduce the cooling requirements and constraints on scalable architectures for quantum dot quantum computers.

Despite significant achievements in characterizing the properties of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ over the last three decades, the precise nature of its electronic ground state is still unresolved. In this work, we provide a missing piece of the puzzle by uncovering evidence of electronic nematic order in the normal state of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$, revealed by ultrafast time-resolved optical dichroism measurements of uniaxially strained thin films. This nematic order, whose domains are aligned by the strain, spontaneously breaks the fourfold rotational symmetry of the crystal. The temperature dependence of the dichroism resembles an Ising-like order parameter, and optical pumping induces a coherent oscillation of its amplitude mode. The existence of electronic nematic order in the normal state of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ may have consequences for the form and mechanism of superconductivity in this material.