By employing inelastic neutron scattering (INS) and Raman scattering (RS) experiments, we comprehensively investigate crystalline electric field (CEF) excitations in ${\mathrm{NaYbSe}}_2$, a new quantum spin liquid candidate that belongs to a large family of triangular-lattice rare-earth chalcogenides with a high-symmetry structure and negligible structural, spin, and charge disordering effects. We can identify CEF excitations at 15.8, 24.3, and 30.5 meV at 5 K. The selected cuts of the INS spectra are well reproduced with a large anisotropy of $g_{ab}$ = 2.9 and $g_c$ = 1. The CEF excitations are further confirmed by our calculations based on the point charge model. Interestingly, ${\mathrm{NaYbSe}}_2$ exhibits an unusual shift of CEF levels to higher energies with increasing temperatures. Further, the Raman mode close to the first CEF excitation shows an anomalously large softening with decreasing temperatures. The absence of these anomalies in the nonmagnetic isostructural material ${\mathrm{NaLuSe}}_2$ allows us to argue that ${\mathrm{NaYbSe}}_2$ incorporates an unusually strong CEF-phonon resonancelike coupling not reported in any of the triangular-lattice rare-earth chalcogenides. The determination of the CEF excitations suggests the validity of the picture of an effective spin 1/2 at low temperatures.

We have used resonant inelastic x-ray scattering to reveal optical magnons in a honeycomb lattice iridate $\alpha -{\mathrm{Li}}_2{\mathrm{IrO}}_3$. The spectrum in the energy region 20–25 meV exhibits momentum dependence, of which energy is highest at the location of the magnetic Bragg peak, $(h,k)=(\pm 0.32,0)$, and lowered toward $(0,0)$ and $(\pm 1,0)$. We compare our data with a linear spin-wave theory based on a generic nearest-neighbor spin model. We find that a dominant bond-directional Kitaev interaction of order 20 meV is required to explain the energy scale observed in our study. The observed excitations are understood as stemming from optical magnon modes whose intensity is modulated by a structure factor, resulting in the apparent momentum dependence. We also observed diffuse magnetic scattering arising from the short-range magnetic correlation well above $T_N$. In contrast to ${\mathrm{Na}}_2{\mathrm{IrO}}_3$, this diffuse scattering lacks the $C_3$ rotational symmetry of the honeycomb lattice, suggesting that the bond anisotropy is far from negligible in $\alpha -{\mathrm{Li}}_2{\mathrm{IrO}}_3$.

Discovering new materials with ultrahigh thermal conductivity has been a critical research frontier and driven by many important technological applications ranging from thermal management to energy science. Here we have rigorously investigated the fundamental lattice vibrational spectra in ternary compounds and determined the thermal conductivity using a predictive ab initio approach. Phonon transport in $\mathrm{B}-X-\mathrm{C}$ $(X=\mathrm{N},\mathrm{P},\mathrm{As})$ groups is systematically quantified with different crystal structures and high-order anharmonicity involving a four-phonon process. Our calculation found an ultrahigh room-temperature thermal conductivity through strong carbon-carbon bonding up to $2100{\mathrm{Wm}}^{-1}{\mathrm{K}}^{-1}$ beyond most common materials and the recently discovered boron arsenide. This study provides fundamental insight into the atomistic design of thermal conductivity and opens up opportunities in new materials searching towards complicated compound structures.

Monolayer and few-layer phosphorene are anisotropic quasi-two-dimensional (quasi-2D) van der Waals (vdW) semiconductors with the linear-dichroic light-matter interaction and the widely tunable direct band gap in the infrared frequency range. Despite recent theoretical predictions of strongly bound excitons with unique properties, it remains experimentally challenging to probe excitonic quasiparticles due to the severe oxidation that occurs during device fabrication. In this study, we report the observation of strongly bound excitons and trions with highly anisotropic optical properties in intrinsic bilayer phosphorene, which are protected from oxidation by encapsulation with hexagonal boron nitride (hBN) in a field-effect transistor (FET) geometry. Reflection contrast and photoluminescence spectroscopy clearly reveal the linear-dichroic optical spectra from anisotropic excitons and trions in the hBN-encapsulated bilayer phosphorene. The optical resonances from the exciton Rydberg series indicate that the neutral exciton binding energy is over 100 meV even with the dielectric screening from hBN. The electrostatic injection of free holes enables an additional optical resonance from a positive trion (charged exciton) ∼30 meV below the optical band gap of the charge-neutral system. Our work shows exciting possibilities for monolayer and few-layer phosphorene as a platform to explore many-body physics and photonics and optoelectronics based on strongly bound excitons with twofold anisotropy.

Hyperferroelectrics are observing a growing interest thanks to their unique property to retain a spontaneous polarization even in the presence of a depolarizing field, corresponding to zero macroscopic displacement field $(\mathbf{D}=0)$ conditions. Hyperferroelectricity is ascribed to the softening of a polar $LO$ mode, but the microscopic mechanisms behind this softening are not totally resolved. Here, by means of phonon calculations and force constants analysis, performed in two classes of hyperferroelectrics, the ${\mathrm{ABO}}_3$-LiNbO3-type systems and the hexagonal-ABC systems, we unveil the common features in the dynamical properties of a hyperferroelectric that lead the $LO$ instability: negative or vanishing on-site force constant associated to the cation driving the $LO$ polar mode and a destabilizing cation-anion interaction; both induced by short-range forces. We also predict a possible enhancement of the hyperferroelectric properties under increasing external positive pressures: pressure strengthens the destabilizing short-range interactions, inducing a stronger $LO$ mode instability and the increase of the longitudinal mode effective charges associated to the unstable $LO$ mode. This suggests an eventual enhancement of the $\mathbf{D}=0$ polarization under compressive strain.

First principles are used and combined with Monte Carlo simulations to predict that the application of compressive uniaxial strains in two-dimensional $\mathrm{CrGe}{\mathrm{Te}}_3$ can tune the underlying magnetic parameters in such a way that the system undergoes a phase transition from ferromagnetic (FM) to antiferromagnetic (AFM) phases, with the border between these being a region of high frustration. Such latter region can be the playground for fine-tuning the system to exhibit exotic states. Moreover, we also show and explain that such uniaxial strains also dramatically tune the critical temperatures, namely Néel temperatures for the AFM phase and Curie temperatures for the FM phase, which is of obvious technological and fundamental significance.

We study the coexistence of superconductivity (SC) and density-wave state and reconcile various puzzling experimental data in organic superconductors ${\left(\mathrm{TMTSF}\right)}_2{\mathrm{PF}}_6$ and ${\left(\mathrm{TMTSF}\right)}_2{\mathrm{ClO}}_4$. The anisotropic resistance drop above $T_c$ is qualitatively described by nascent isolated SC islands within a bulk analytical model. However, the observed anisotropic SC onset is explained only when the finite size and flat needle shape of samples is considered. Our results pave a way to estimate the volume fraction and the typical size of SC islands far from the sample surface and apply to many inhomogeneous superconductors, including high-$T_c$ cuprate or Fe-based ones.

We studied the in-plane angular magnetoresistance (AMR), in the normal state, of underdoped superconducting ${\mathrm{Sr}}_{1–x}{\mathrm{La}}_x\mathrm{Cu}{\mathrm{O}}_2$, which has the simplest crystal structure among cuprates. The measurements of two underdoped thin films with different dopings were performed in intense magnetic field $H$ (up to 22 T). The longitudinal magnetoresistance at temperature $T$ is negative and scales with $H/T$. For both samples, the AMR is anisotropic and shows an unexpected dependence on $H$ intensity. While at the low magnetic field, one observes essentially twofold AMR oscillations for the more doped sample, fourfold ones start to grow under the high magnetic field, resulting in the coexistence of the two. For the less doped film at the low magnetic field, both twofold and fourfold AMR components exist. With the increase of the magnetic field, the fourfold component survives a π/4 phase shift, during which its amplitude vanishes, at a magnetic field $H_c$ such as $16\mathrm{T}<H_c<17\mathrm{T}$. As a result, at the high magnetic field above $H_c$, the angular dependence of the in-plane magnetoresistance turns out to be the same for both samples. We tentatively ascribe the above features to the presence of antiferromagnetism in the ${\mathrm{CuO}}_2$ planes of underdoped ${\mathrm{Sr}}_{1–x}{\mathrm{La}}_x\mathrm{Cu}{\mathrm{O}}_2$.

A numerical and analytical study of the dynamics of domain walls (DWs) in a magnetic tunnel junction with perpendicular magnetic anisotropy in a free layer is presented. Equilibrium states of the domain wall are obtained for various widths of the structure. The corresponding symmetries of the components of spin transfer torques ${\mathbf{T}}_{STT}$ and the polarizer directions favoring stable DW motion under perpendicular current injection are obtained. The DW steady motion with velocities up to 200 m/s at current densities below ${10}^6\mathrm{A}/{\mathrm{cm}}^2$ is reported. The Walker breakdown is demonstrated, and the dynamics of the postthreshold DW motion is investigated for various configurations of torques and polarizer directions. To have analytical insight into the investigated regimes of DW dynamics a theoretical model is developed and verified by micromagnetic simulations.

It has been predicted that transverse spin current can propagate coherently (without dephasing) over a long distance in antiferromagnetically ordered metals. Here, we estimate the dephasing length of transverse spin current in ferrimagnetic CoGd alloys by spin pumping measurements across the compensation point. A modified drift-diffusion model, which accounts for spin-current transmission through the ferrimagnet, reveals that the dephasing length is about 4–5 times longer in nearly compensated CoGd than in ferromagnetic metals. This finding suggests that antiferromagnetic order can mitigate spin dephasing—in a manner analogous to spin echo rephasing for nuclear and qubit spin systems—even in structurally disordered alloys at room temperature. We also find evidence that transverse spin current interacts more strongly with the Co sublattice than the Gd sublattice. Our results provide fundamental insights into the interplay between spin current and antiferromagnetic order, which are crucial for engineering spin torque effects in ferrimagnetic and antiferromagnetic metals.

The pressure dependence of the magnetic ordering temperatures for the lanthanide ferromagnets Gd, Tb, Dy, and Ho has been investigated in the pressure region up to 18 GPa by two types of magnetic measurements using a superconducting quantum interference device (SQUID). The present magnetic measurements enabled us to investigate the pressure dependence of the magnetization intensity at low magnetic fields as well as the magnetic ordering temperatures. Their results are interpreted in the light of such previous experiments as magnetic susceptibility, magnetization, electrical resistance, neutron diffraction, and Mössbauer spectroscopy measurements. All of the magnetic orderings in the above four elements were suppressed down to less than the detection level, being related to the structural transition. The ferromagnetic ordering in Gd, Tb, Dy, and Ho is stabilized in the hcp structure. The magnetic anomalies due to the helimagnetic ordering of Tb and Dy disappear at the Sm-to-dhcp transition and the hcp-to-Sm transition, respectively, while that of Ho disappears in the Sm-type phase near the Sm-to-dhcp transition.

We present a general stochastic Liouville theory of electrical transport across a barrier between two conductors that occurs via sequential hopping through a single defect's spin-0 to spin-$1/2$ transition. We find magnetoconductances similar to Hanle features (pseudo Hanle features) that originate from Pauli blocking without spin accumulation, and also predict that evolution of the defect's spin modifies the conventional Hanle response, producing an inverted Hanle signal from spin center evolution. We propose studies in oblique magnetic fields that would unambiguously determine if a magnetoconductance results from spin-center assisted transport.

Solid-state spin qubits are promising candidates for the realization of a quantum computer due to their long coherence times and easy electrical manipulation. However, spin-spin interactions, which are needed for entangling gates, have only limited range as they generally rely on tunneling between neighboring quantum dots. This severely constrains scalability. Proposals to extend the interaction range generally focus on coherent electron transport between dots or on extending the coupling range. Here, we study a setup in which such an extension is obtained by using a superconductor as a quantum mediator. Because of its gap, the superconductor effectively acts as a long tunnel barrier. We analyze the impact of spin-orbit (SO) coupling, external magnetic fields, and the geometry of the superconductor. We show that while spin-nonconserving tunneling between the dots and the superconductor due to SO coupling does not affect the exchange interaction, strong SO scattering in the superconducting bulk is detrimental. Moreover, we find that the addition of an external magnetic field decreases the strength of the exchange interaction. Fortunately, the geometry of the superconducting link offers a lot of room to optimize the interaction range, with gains of over an order of magnitude from a two-dimensional (2D) film to a quasi-1D strip. We estimate that for superconductors with weak SO coupling (e.g., aluminum), exchange rates of up to 100 MHz over a micron-scale range can be achieved with this setup in the presence of magnetic fields of the order of 100 mT.

We apply a microscopic theory of exciton polaritons in cavity-confined monolayer transition-metal dichalcogenides including both optical polarizations in the monolayer plane, allowing us to describe how chiral cavity photons interact with the valley degrees of freedom of the active material. Upon polariton formation, the degenerate excitons inhabiting the two inequivalent valleys are shown to assume symmetric and antisymmetric superpositions as a result of cavity-mediated intravalley interactions combined with intervalley Coulomb interactions. This is representative of a polariton-induced coherent mixing of the valley polarization. Upon dephasing, this mixing is prone to open a new valley relaxation channel that attains significance with increasing cavity coupling. Importantly, we show that optical cavities with an asymmetric reflectance of left- and right-handed circularly polarized photons offer a considerably more robust platform to realize a conserved valley polarization, as the valley localization of excitons is reinstated by an asymmetric Rabi splitting that lifts their degeneracy. Moreover, we show this degeneracy lifting to allow for wavelength-selective access to the valley pseudospin by means of a polariton-induced chiral Stark effect, offering interesting opportunities for valleytronic applications.

We propose topological semimetals generated by the square-root operation for tight-binding models in two and three dimensions, which we call square-root topological semimetals. The square-root topological semimetals host topological band touching at finite energies, whose topological protection is inherited from the squared Hamiltonian. Such a topological character is also reflected in emergence of boundary modes with finite energies. Specifically, focusing on topological properties of squared Hamiltonian in class AIII, we reveal that a decorated honeycomb (decorated diamond) model hosts finite-energy Dirac cones (nodal lines). We also propose a realization of a square-root topological semimetal in a spring-mass model, where robustness of finite-energy Dirac points against the change of tension is elucidated.

Thermoelectric materials can recover electrical energy from waste heat and vice versa, which are of great significance in green energy harvesting and solid state refrigerators. The thermoelectric figure of merit ($z\mathrm{T}$) quantifies the energy conversion efficiency, and a large Seebeck or Nernst effect is crucial for the development of thermoelectric devices. Here we present a significantly large Nernst thermopower in topological semimetal ${\mathrm{ZrTe}}_5$, which is attributed to both strong Berry curvature and bipolar transport. The largest in-plane $S_{xy}$ (when B//b) approaches 1900 $\mu \mathrm{V}/\mathrm{K}$ at $T=100$ K and B = 13 T, and the out-of-plane $S_{xz}$ (when B//c) reaches 5000 $\mu \mathrm{V}/\mathrm{K}$. As a critical part of $z_N\mathrm{T}$, the linearly increased in-plane $S_{xy}$ and resistivity ${\rho }_{\mathrm{yy}}$ in regard to B induces an almost linear increasing transversal $z_N\mathrm{T}$ without saturation under high fields. The maximum $z_N\mathrm{T}$ of 0.12 was obtained at B = 13 T and $T=120$ K, which significantly surmounts its longitudinal counterpart under the same condition.

As a new topological phase in three-dimensional momentum space, Weyl semimetals extend the repertoire of exotic topological states. In this paper, we find the existence of photonic Weyl points in electromagnetic continuous gyromagnetic metamaterials. The dispersion of the longitudinal mode of one of the two bands forming the Weyl point is flat; thus, the Weyl degeneracy point of the system is exactly at the critical transition between the type-I and type-II Weyl points. The Fermi arc connects the projections of the Weyl points with opposite chirality at the interface between the gyromagnetic metamaterial and vacuum. Full-wave simulation results show that surface waves can bypass sharp defects and achieve robust, unidirectional optical transmission without reflection. Furthermore, taking the spatial nonlocal effects into account, the ideal type-I Weyl points can exist in this class of metamaterials. This paper sheds light on fundamental understanding of topological physics, and the study of Weyl points in metamaterials will greatly enrich the field of topological photonics.