A high-pressure (HP) yttrium allotrope, $hP3$-Y (space group $P6/mmm$), was synthesized in a multi-anvil press at 20 GPa and 2000 K which is recoverable to ambient conditions. Its relative stability and electronic properties were investigated using density functional theory calculations. A $hP3$-Y derivative hydride, $hP3-\mathrm{YH}x$, with a variable hydrogen content ($x=2.8$, 3, 2.4), was synthesized in diamond anvil cells by the direct reaction of yttrium with paraffin oil, hydrogen gas, and ammonia borane upon laser heating to ∼3000 K at 51, 45 and 38 GPa, respectively. Room-temperature decompression leads to gradual reduction and eventually the complete loss of hydrogen at ambient conditions. Isostructural $hP3-\mathrm{NdH}x$ and $hP3-\mathrm{GdH}x$ hydrides were synthesized from Nd and Gd metals and paraffin oil, suggesting that the $hP3$-Y structure type may be common for rare-earth elements. Our results expand the list of allotropes of trivalent lanthanides and their hydrides and suggest that they should be considered in the context of studies of HP behavior and properties of this broad class of materials.

Weyl semimetals realize massless relativistic fermions with two Weyl nodes separated in energy and momentum space, whose low-energy physics is described by Dirac fermions with an axial gauge constant. Here, we study their electromagnetic linear responses based on the effective field theory and on the chiral kinetic theory. Although the static chiral magnetic effect is canceled by the Chern-Simons current under the Pauli-Villars regularization, a dynamical magnetic field is found capable of driving an electric current along its direction, with the total transported charge being independent of temperature and chemical potential for a uniform field. We also incorporate dissipation in the relaxation-time approximation and study collective excitations coupled with Maxwell electromagnetic fields when Weyl node populations deviate from equilibrium. Their dispersion relations at low frequency and long wavelength are determined only by electric, chiral magnetic, and anomalous Hall conductivities, which predict unstable modes leading to anisotropic generation of electromagnetic waves oriented to the direction of Weyl node separation.

We study a one-dimensional Kitaev chain driven by the anisotropy parameter $J_{-}=t/{\tau }_Q$, in the presence of weak Gaussian noise in the parameter $J_{+}$. The system is prepared in the ground state of the Hamiltonian at the initial time $t\to -\infty$. The defect density and the residual energy at the end of the drive protocol reveals anti-Kibble Zurek (AKZ) behavior which deviates from the earlier reported linear in quench time dependence for the defect density. The entropy density at the end of the protocol is found to exhibit the signature of the AKZ behavior. In the context of the Kitaev chain, the two-point spin correlators are short ranged. The hidden topological order across the quantum phase transition point is probed via the nonlocal string order parameters. The two-point Majorana correlator and the hidden string correlator calculated in the final decohered state at the end of the noisy drive protocol is consistent with the AKZ picture. We have analyzed the kink statistics in the dual spin space, the first three cumulants at the end of the noisy drive also exhibit the AKZ scaling behavior for slower sweeps similar to the defect density. The kink distribution function is well approximated with the normal distribution with nonuniversal noise dependent mean and variance.

Here we present a magnetic thin film with a weak ferrimagnetic (FIM) phase above the Néel temperature ($T_N=240\mathrm{K}$) and a noncollinear antiferromagnetic (AFM) phase below, exhibiting a small net magnetization due to strain-associated canting of the magnetic moments. A long-range ordered FIM phase has been predicted in related materials, but without symmetry analysis. We now perform this analysis and use it to calculate the magneto-optical Kerr effect (MOKE) spectra in the AFM and FIM phases. From the good agreement between the form of the measured and predicted MOKE spectra, we propose the AFM and FIM phases share the magnetic space group $C{2}^{\prime }/m^{\prime }$ and that the symmetry-driven magneto-optic and magneto-transport properties are maximized at room temperature in the FIM phase due to the nonzero intrinsic Berry phase contribution present in these materials. A room temperature FIM with large optical and transport signatures, as well as sensitivity to lattice strain and magnetic field, has useful prospects for high-speed spintronic applications.

The anisotropic spin-glass transition, in which spin freezing is observed only along the $c$ axis in pseudobrookite ${\mathrm{Fe}}_2\mathrm{Ti}{\mathrm{O}}_5$, has long been perplexing because the ${\mathrm{Fe}}^{3+}$ moments $\left(d^5\right)$ are expected to be isotropic. Recently, neutron diffraction demonstrated that surfboard-shaped antiferromagnetic nanoregions coalesce above the glass transition temperature $T_g\approx 55\mathrm{K}$, and a model was proposed in which the freezing of the surfboard magnetization fluctuations leads to the anisotropic spin-glass state. Given this model, we have carried out high-resolution inelastic neutron scattering measurements of the spin-spin correlations to understand the temperature dependence of the intrasurfboard spin dynamics on neutron (picosecond) timescales. Here, we report on the temperature-dependence of the spin fluctuations measured from single-crystal ${\mathrm{Fe}}_2\mathrm{Ti}{\mathrm{O}}_5$. Strong quasi-elastic magnetic scattering, arising from intrasurfboard correlations, is observed well above $T_g$. The spin fluctuations possess a steep energy–wave vector relation and are indicative of strong exchange interactions, consistent with the large Curie-Weiss temperature. As the temperature approaches $T_g$ from above, a shift in spectral weight from inelastic to elastic scattering is observed. At various temperatures between 4 and 300 K, a characteristic relaxation rate of the fluctuations is determined. Despite the freezing of most of the spin correlations, an inelastic contribution remains even at base temperature, signifying the presence of fluctuating intrasurfboard spin correlations to at least $T/T_g\approx 0.1$, consistent with an energy landscape that is a hybrid between conventional and geometrically frustrated spin glasses.

Layered quaternary fluorochalcogenides with natural superlattice structures have received particular attention in the thermoelectric (TE) field due to their favorable electronic properties. However, further exploration of these materials in the TE field is hindered by the lack of studies on the lattice thermal transport properties. Here, we investigate the microscopic mechanism of lattice thermal transport in natural superlattice materials $\mathrm{Ba}\mathit{XY}\mathrm{F}$ ($\mathit{X}$ = Cu, Ag; $\mathit{Y}$ = Se, Te) using first-principles calculations combined with self-consistent phonon theory, compressive sensing techniques, and Boltzmann transport equations. We consider three- and four-phonon scattering as well as temperature-driven phonon energy shifts due to quartic anharmonicity. We find that the strong quartic anharmonicity plays a crucial role in the lattice stability of BaCuSeF and BaCuTeF. Furthermore, the ${\kappa }_{\mathrm{L}}$ of the four materials exhibits significant anisotropy due to different bonding types along the $a$($b$) and $c$ axes. The calculations indicate that the four materials have low lattice thermal conductivities ${\kappa }_{\mathrm{L}}$, e.g., 0.85–$1.61{\mathrm{Wm}}^{-1}{\mathrm{K}}^{-1}$ at 300 K. Additionally, the results exhibit that a reasonable ${\kappa }_{\mathrm{L}}$ and corresponding temperature dependence can be obtained by considering the fully quartic anharmonicity. Our findings not only fill the gap in the lattice thermal transport research of natural superlattice materials, but also deepen a comprehensive understanding of the low ${\kappa }_{\mathrm{L}}$ of natural superlattice materials.

The magnetic ground state of oxynitride pyrochlore ${\mathrm{Lu}}_2{\mathrm{Mo}}_2{\mathrm{O}}_{5-y}{\mathrm{N}}_2$, a candidate compound for the quantum spin liquid ($S=1/2$, ${\mathrm{Mo}}^{5+}$), was studied by muon spin rotation/relaxation experiment. In contrast to ${\mathrm{Lu}}_2{\mathrm{Mo}}_2{\mathrm{O}}_7$ ($S=1$, ${\mathrm{Mo}}^{4+}$) which exhibits the spin-glass behavior with a freezing temperature $T_g\simeq 16\mathrm{K}$, no such spin freezing or long-range magnetic order was observed down to 0.3 K. Moreover, two separate magnetic domains were detected below $\sim 13\mathrm{K}$, which were characterized by differences in spin dynamics. The first is the “sporadic” spin fluctuation seen in frustrated antiferromagnets, where the amplitude of the hyperfine fields suggests that the excitation comprises a local cluster of unpaired spins. The other is rapid paramagnetic fluctuation, which is only weakly suppressed at low temperatures. In place of the paramagnetic fluctuation, the volume fraction for the sporadic fluctuation steadily increases with decreasing temperature, indicating the formation of an excitation gap with a broad distribution of the gap energy involving a null gap.

We present the topology of magnons on the triangular kagome lattice (TKL) by calculating its Berry curvature, Chern number, and edge states. In addition to the ferromagnetic state, the TKL hosts ferrimagnetic ground state as its two sublattices can couple with each other either ferromagnetically or antiferromagnetically. Using Holstein-Primakoff (HP) boson theory and Green's function approach, we find that the TKL has a rich topological band structure with added high Chern numbers compared with the kagome and honeycomb lattices. The magnon edge current allows a convenient calculation of thermal Hall coefficients and the orbital angular momentum gives correlation to the Einstein–de Haas effect. We apply the calculations to the TKL and derive the topological gyromagnetic ratio showing a nonzero Einstein–de Haas effect in the zero-temperature limit. Our results render the TKL as a potential platform for quantum magnonics applications including high-precision mechanical sensors and information transmission.

Van der Waals (vdW) heterostructures provide a rich playground to engineer electronic, spin, and optical properties of individual two-dimensional materials. We investigate the twist-angle and gate dependence of the proximity-induced exchange coupling in the monolayer transition-metal dichalcogenides (TMDCs) ${\mathrm{MoSe}}_2$ and ${\mathrm{WSe}}_2$ due to the vdW coupling to the ferromagnetic semiconductor ${\mathrm{CrI}}_3$, from first-principles calculations. A model Hamiltonian, which captures the relevant band edges at the $K/K^{\prime }$ valleys of the proximitized TMDCs, is employed to quantify the proximity-induced exchange. Upon twisting from $0^{\circ }$ to ${30}^{\circ }$, we find a transition of the TMDC valence band (VB) edge exchange splitting from about $-2$ to $2\mathrm{meV}$, while the conduction band (CB) edge exchange splitting remains nearly unchanged at around $-3\mathrm{meV}$. For the VB of ${\mathrm{WSe}}_2$ (${\mathrm{MoSe}}_2$) on ${\mathrm{CrI}}_3$, the exchange coupling changes sign at around $8^{\circ }$ (${16}^{\circ }$). We find that even at the angles with almost zero spin splittings of the VB, the real-space spin polarization profile of holes at the band edge is highly nonuniform, with alternating spin up-and spin-down orbitals. Furthermore, a giant tunability of the proximity-induced exchange coupling is provided by a transverse electric field of a few V/nm. Within our first-principles framework, we are limited to commensurate structures, and thus considered a maximum strain of 2.2% for the individual monolayers. By investigating different atomic stacking configurations of the strained supercells, we demonstrate that proximity exchange varies locally in space within experimentally realistic setups. We complement our ab initio results by calculating the excitonic valley splitting to provide experimentally verifiable optical signatures of the proximity exchange. Specifically, we predict that the valley splitting increases almost linearly as a function of the twist angle. Furthermore, the proximity exchange is highly tunable by gating, allowing to tailor the valley splitting in the range of 0 to 12 meV in ${\mathrm{WSe}}_2/{\mathrm{CrI}}_3$, which is equivalent to external magnetic fields of up to about 60 T. Our results highlight the important impact of the twist angle and gating when employing magnetic vdW heterostructures in experimental geometries.

The interplay of topology and non-Hermiticity has led to diverse, exciting manifestations in a plethora of systems. In this work, we systematically investigate the role of non-Hermiticity in the Chern insulating Haldane model on a dice lattice. Due to the presence of a nondispersive flat band, the dice-Haldane model hosts a topologically rich phase diagram with the nontrivial phases accommodating Chern numbers $\pm 2$. We introduce non-Hermiticity into this model in two ways—through balanced non-Hermitian gain and loss, and by nonreciprocal hopping in one direction. Both these types of non-Hermiticity induce higher-order exceptional points of order three. Remarkably, the exceptional points at high-symmetry points occur at odd integer values of the non-Hermiticity strength in the case of balanced gain and loss, and at odd integer multiples of $1/\sqrt{2}$ for nonreciprocal hopping. We substantiate the presence and the order of these higher-order exceptional points using the phase rigidity and its scaling. Furthermore, we construct a phase diagram to identify and locate the occurrence of these exceptional points in the parameter space. Non-Hermiticity has yet more interesting consequences on a finite-sized lattice. Unlike for balanced gain and loss, in the case of nonreciprocal hopping, the nearest-neighbor lattice system under periodic boundary conditions accommodates a finite, nonzero spectral area in the complex plane. This manifests as the non-Hermitian skin effect when open boundary conditions are invoked. In the more general case of the dice-Haldane lattice model, the non-Hermitian skin effect can be caused by both gain and loss or nonreciprocity. Fascinatingly, the direction of localization of the eigenstates depends on the nature and strength of the non-Hermiticity. We establish the occurrence of the skin effect using the local density of states, inverse participation ratio, and the edge probability and demonstrate its robustness to disorder. Our results place the dice-Haldane model as an exciting platform to explore non-Hermitian physics.

We establish, in the spirit of the Lieb-Schultz-Mattis theorem, lower bounds on the spectral degeneracy of quantum systems with higher (gaugelike) symmetries with rather generic physical boundary conditions in an arbitrary number of spatial dimensions. Contrary to applying twists or equivalent adiabatic operations, we exploit the effects of modified boundary conditions. When a general choice of boundary geometry is immaterial in determining spectral degeneracies while approaching the thermodynamic limit, systems that exhibit rigid noncommuting gaugelike symmetries, such as the orbital compass model, must have an exponential (in the size of the boundary) degeneracy of each of their spectral levels.

Universal scaling relations for dissipative Tomonaga-Luttinger (TL) liquids with $\mathrm{SU}(N$) spin symmetry are obtained for both fermions and bosons, by using asymptotic Bethe-ansatz solutions and conformal field theory (CFT) in one-dimensional non-Hermitian quantum many-body systems with $\mathrm{SU}(N$) symmetry. We uncover that the spectrum of dissipative TL liquids with $\mathrm{SU}(N$) spin symmetry is described by the sum of one charge mode characterized by a complex generalization of $c=1$ U(1) Gaussian CFT, and $N-1$ spin modes characterized by level-$1\mathrm{SU}(N$) Kac-Moody algebra with the conformal anomaly $c=N-1$, and thereby dissipation only affects the charge mode as a result of spin-charge separation in one-dimensional non-Hermitian quantum systems. The derivation is based on a complex generalization of Haldane's ideal-gas description, which is implemented by the $\mathrm{SU}(N$) Calogero-Sutherland model with inverse-square long-range interactions.

Metal intercalation of graphene is a promising method to tune its electronic band structure and generate novel electronic and topological phases. The tuning depends critically on the ability to bond the intercalated atoms at predesigned, subsurface interlayer locations because the emerging band structure depends on metal location. We have studied Dy intercalation under single-layer graphene (SLG) on SiC using spot profile analysis–low-energy electron diffraction and scanning tunneling microscopy (STM). The experimental work is complemented with density-functional theory (DFT) analysis. Because different diffraction spots originate from different subsurface interlayer regions, it is possible to identify changes in the intercalation location by monitoring the spot intensity as a function of growth conditions. DFT calculations of the chemical potential as a function of intercalated Dy coverage support the variation of the stability of the intercalated phase at different intercalated locations. The preferred location is confirmed from STM studies showing the removal of the $6\times 6$ moiré corrugation at the preferred location, observed at higher Dy coverage.

Non-Hermitian quantum many-body systems are attracting widespread interest for their exotic properties, including unconventional quantum criticality and topology. Here, we study how quantum information and correlations spread under a quantum quench generated by a prototypical non-Hermitian spin chain. Using the mapping to fermions, we solve exactly the problem and compute the entanglement entropy and the correlation dynamics in the thermodynamic limit. Depending on the quench parameters, we identify two dynamical phases. One is characterized by rapidly saturating entanglement and correlations. The other presents a logarithmic entanglement growth in time and correlations spreading faster than the associated unitary system, with collapses and revivals giving rise to a modulated light-cone structure. Here, in the long-time limit, we analytically compute the entanglement entropy that follows a logarithmic law, with an effective central charge that we obtain in closed form. Our results provide an example of an exactly solvable non-Hermitian many-body problem that shows rich physics, including entanglement and spectral transitions.

The excitonic insulator is an elusive electronic phase exhibiting a correlated excitonic ground state. Materials with such a phase are expected to have intriguing properties such as excitonic high-temperature superconductivity. However, compelling evidence on the experimental realization is still missing. Here, we theoretically propose hybrids of two-dimensional semiconductors functionalized by organic molecules as prototypes of excitonic insulators, with the exemplary candidate ${\mathrm{WS}}_2$-F6TCNNQ. This material system exhibits an excitonic insulating phase at room temperature with a ground state formed by a condensate of interlayer excitons. To address an experimentally relevant situation, we calculate the corresponding phase diagram for the important parameters: temperature, gap energy, and dielectric environment. Further, to guide future experimental detection, we show how to optically characterize the different excitonic phases via far-infrared to terahertz spectroscopy valid also for monolayer materials.

We consider interaction-induced broken symmetry states of two Weyl semimetal surfaces with multiple Fermi-arc (FA) states. In the presence of inter- and intrasurface Coulomb interactions, multiple broken symmetries may emerge which coexist and/or compete with one another. Interlayer exciton condensates involving different FA flavors are shown to form, with amplitudes determined by the strength of interactions and the degree of nesting among the arcs. For FA pairs which are well separated in momentum with strong nesting, the resulting state is a particle-hole analog of a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconductor. Intralayer interactions moreover induce charge density wave (CDW) ordering, so that the most general state of the system is a supersolid. These orderings in principle carry signatures in nonlinear behavior and narrow band noise in Coulomb drag transport measurements.

The calculation of free energies from first principles enables the prediction of phase stability of materials with high accuracy; these calculations are complicated in magnetic materials by the interplay of electronic, magnetic, and vibrational degrees of freedom. In this work, we show the feasibility and accuracy of the calculation of phase stability in magnetic systems with $ab\mathit{initio}$ methods and thermodynamic integration by sampling the magnetic and vibrational phase space with coupled atomistic spin dynamics–$ab\mathit{initio}$ molecular dynamics simulations [Stockem et al., PRL 121, 125902 (2018)], where energies and interatomic forces are calculated with density functional theory. We employ the method to calculate the phase stability of Fe at ambient pressure from 800 up to 1800 K. The Gibbs free energy difference between fcc and bcc Fe at zero pressure is calculated with thermodynamic integration over temperature and over stress-strain variables and, for the best set of exchange interactions employed, the Gibbs free energy difference between the two structures is within 5 meV/atom from the CALPHAD estimate, corresponding to an error in transition temperature below 150 K. The present work paves the way to free energy calculations in magnetic materials from first principles with accuracy in the order of 1 meV/atom.

We present several interesting phenomena related to flat-band ferromagnetism in the Hubbard model. The first is a mathematical theorem stating certain conditions under which a flat-band ferromagnetic must necessarily be degenerate with a nonferromagnetic state. This theorem is generally applicable and geometry independent but holds only for a small number of holes in an otherwise filled band. The second phenomenon is a peculiar example where the intuition fails that particles prefer to doubly occupy low-energy states before filling higher-energy states. Lastly, we show a pattern of ferromagnetism which appears in small pentagonal and hexagonal plaquettes at filling factors of roughly 3/10 and 1/4. These examples require only a small number of lattice sites and may be observable in quantum dot arrays currently available as laboratory spin qubit arrays.

We investigated magnetic structures of the centrosymmetric skyrmion material ${\mathrm{Gd}}_2{\mathrm{PdSi}}_3$ by means of polarized neutron scattering near zero field with an isotope-${}^{160}\mathrm{Gd}$-enriched single crystal. In a previous study, magnetic structures in ${\mathrm{Gd}}_2{\mathrm{PdSi}}_3$ at low temperatures were studied by resonant x-ray scattering measurements [T. Kurumaji et al., Science 365, 914 (2019)]. The present polarized neutron results confirm that the magnetic structure in zero field has elliptic screw-type magnetic modulation with a propagation vector of $(q,0,0)$ with $q\sim 0.14$ and its equivalents. As the temperature increases, the system undergoes a magnetic phase transition while keeping the incommensurate $q$-vector of $(q,0,0)$. We found that the thermally induced phase has sinusoidal magnetic modulations with magnetic moments perpendicular both to the $c$ axis and to the $\mathit{q}$-vector. We also investigate the spin-helicity degree of freedom in the ground state by polarized neutrons, revealing that the system contains equal fractions of the left-handed and right-handed screw-type orders as expected from the centrosymmetric crystal structure.