We investigate the crystal electric field (CEF) excitations of ${\mathrm{Yb}}^{3+}$ ions in powder samples of the triangular-lattice rare-earth-based antiferromagnet ${\mathrm{YbZn}}_2{\mathrm{GaO}}_5$ using inelastic neutron scattering (INS). Three CEF excitations from the ground-state Kramers doublet were observed, each exhibiting significant broadening beyond instrumental resolution. Combining temperature-dependent INS and neutron powder diffraction, we identify a significant static contribution to this broadening and attribute it to heterogeneous coordination of ${\mathrm{Yb}}^{3+}$ ions due to ${\mathrm{Ga}}^{3+}/{\mathrm{Zn}}^{2+}$ site mixing. Rietveld refinement of neutron powder diffraction indicates that 35% of Ga occupies the Zn site and 60% of Zn occupies the Ga site. We show with a point charge model for the CEF Hamiltonian that heterogeneous coordination of ${\mathrm{Yb}}^{3+}$ ions leads to broadened CEF peaks. First-principles calculations demonstrate that the random ${\mathrm{Ga}}^{3+}/{\mathrm{Zn}}^{2+}$ distribution can produce the distortions of the ${\mathrm{YbO}}_6$ octahedra observed from neutron diffraction. Because the documented heterogeneity will extend to exchange interactions, our results suggest that disorder is a significant factor in the unusual magnetism previously reported in ${\mathrm{YbZn}}_2{\mathrm{GaO}}_5$, including broad low-energy magnetic excitations and the absence of magnetic ordering down to $0.3\mathrm{K}$.

Light elements, such as hydrogen and boron, provide a possible route to high-temperature superconductivity. However, the pursuit of light-element superconductors with critical temperature ($T_{\mathrm{c}}$) exceeding the McMillan limit at ambient pressure still remains challenging. Here, we design a two-dimensional (2D) superconductor, ${\mathrm{ScB}}_2{\mathrm{H}}_2$, via hydrogenation. This material features a unique sandwich structure where the scandium layer is asymmetrically functionalized with boron and hydrogen on opposite surfaces. The boron layer plays the key role in high-temperature superconductivity of ${\mathrm{ScB}}_2{\mathrm{H}}_2$ with the metallization of σ bands that induce a Van Hove singularity near the Fermi level. This enables strong coupling between the σ band and the in-plane vibration of B atoms, yielding a large electron-phonon coupling of 1.69. Remarkably, the monolayer ${\mathrm{ScB}}_2{\mathrm{H}}_2$ exhibits three-gap superconductivity with $T_{\mathrm{c}}$ of $51-61\mathrm{K}$ (${\mu }^{*}=0.1-0.2$), surpassing the McMillan limit. Although hydrogen is lighter than boron, the electron-phonon coupling of hydrogen layer is weaker than that of the boron layer. Nevertheless, hydrogen not only stabilizes ${\mathrm{ScB}}_2{\mathrm{H}}_2$ via hydrogenating naked activity Sc sites of monolayer ${\mathrm{ScB}}_2$, but also controls the metallization of the σ band in the honeycomb B sublattice. Our finding provides a theoretical guidance for regulating the 2D superconductivity through functionalizing two faces of 2D materials with light elements.

Topological phases of matter are known to be unstable against strong onsite disorder in one dimension. In this work, however, we propose that in the case of a topological ladder, an onsite quasiperiodic potential under proper conditions, first destroys the initial topological phase (TP) and subsequently, induces another TP through a gap-closing point. Remarkably, by allowing a staggered flux piercing through the plaquettes of the ladder, the gapless critical point bifurcates into two gapless critical lines, resulting in a gapped trivial phase sandwiched between the two TPs. This results in a scenario where the system first undergoes a transition from one TP to a trivial phase and then to the other TP as a function of the quasiperiodic potential strength. Such quasiperiodicity-induced reentrant topological phase transition reveals a phenomenon of direction reversal in the topological transport, which we identify through Thouless charge pumping.

The subtle interplay between competing degrees of freedom, crystal electric fields, and spin correlations can lead to exotic quantum states in $4f$ ion-based frustrated triangular lattice antiferromagnets. We present the crystal structure, thermodynamic and muon spin relaxation ($\mu \mathrm{SR}$) studies of the $4f$ ion-based frustrated magnet ${\mathrm{Ba}}_4{\mathrm{YbReWO}}_{12}$, wherein ${\mathrm{Yb}}^{3+}$ ions constitute a triangular lattice. The magnetic susceptibility does not show any signature of spin freezing down to 1.9 K or long-range magnetic ordering down to 0.4 K. The low-temperature Curie-Weiss fit to the inverse magnetic susceptibility data reveals a weak antiferromagnetic exchange interaction, which is corroborated by the fit of magnetic specific heat data following the $J_1-J_2$ model with the nearest-neighbor-exchange interaction of $J_1\approx -0.197\mathrm{K}$ between the $J_{\mathrm{eff}}$ = 1/2 states of the ${\mathrm{Yb}}^{3+}$ moments in the lowest Kramers doublet. The lowest Kramers ground state doublet is well separated from the first excited state with a gap of ${\mathrm{\Delta }}_{\text{CEF}}$ = 278 K, as evidenced by our $\mu \mathrm{SR}$ experiments that support the realization of $J_{\text{eff}}=1/2$ at low temperatures. The specific heat experiments do not detect a phase transition down to 56 mK. The magnetic specific heat shows a broad maximum at 90 mK suggesting a disordered ground state with short-range spin correlations. The associated magnetic entropy release at low temperatures is consistent with that expected for the $J_{\mathrm{eff}}$ = 1/2 state. The zero-field $\mu \mathrm{SR}$ measurements show neither the signature of spin freezing nor a phase transition, at least down to 43 mK. Our results suggest a dynamic, disordered ground state in this $J_{\mathrm{eff}}$ = 1/2 frustrated triangular lattice antiferromagnet. ${\mathrm{Ba}}_{4}R{\mathrm{ReWO}}_{12}$ ($R$=rare earth) offers a viable platform to realize intriguing quantum states borne out of spin-orbit coupling and frustration.

We study the effect of symmetry-breaking perturbations in the multiorbital Hubbard model coupled to anisotropic Jahn-Teller phonons, which is relevant for the description of fulleride superconductors. This system is often approximated by a model with static antiferromagnetic (AFM) Hund's coupling, in which the coupling to the Jahn-Teller phonon is effectively described, but the retardation effect associated with phonon propagation is neglected. We compare the properties of the models with static AFM Hund's coupling and dynamical Jahn-Teller electron-phonon interaction by means of the Eliashberg theory. Considering the susceptibilities for the spin, magnetic orbital, electric orbital, and superconductivity, we reveal qualitatively different behavior between the two models in the case of the magnetic orbital susceptibility. We further study the effect of a magnetic field on the $s$-wave spin-singlet superconducting state. In the presence of the field, the magnetic orbital susceptibility becomes nonzero due to a combination of multiorbital and retardation effects, while the spin susceptibility remains zero at low temperatures. By analyzing this phenomenon both numerically and analytically, we clarify that odd-frequency pairs induced by the magnetic field play a crucial role in the spin and orbital magnetic susceptibilities. Thus, the magnetic degrees of freedom produce interesting behaviors in the presence of retardation effects associated with electron-phonon coupling.

We consider a general model of two atomic chains forming a moiré pattern due to a small mismatch in their lattice spacings, given by $\theta =(a_1-a_2)/a_2$. Assuming arbitrary single-band dispersion relations ${\varepsilon }_1\left(p\right)$ and ${\varepsilon }_2\left(q\right)$ for the chains, along with an arbitrary interchain coupling term $T\left(x\right)$, we show that the entire spectrum of such a one-dimensional moiré superchain is governed by a single three-term recurrence (TTR) relation. We analyze this TTR relation using the discrete Wentzel-Kramers-Brillouin method and demonstrate how the entire structure of the spectrum as well as emergence of flat bands can be easily identified from a pair of upper and lower potential functions of the TTR relation. We also comment on the chiral limit of the moiré superchain, which can be viewed, in some sense, as a one-dimensional analog of the chiral limit of twisted bilayer graphene.

Zintl-phase materials exhibit great potential in thermoelectric applications owing to their high electrical conductivity and low thermal conductivity induced by unique structural and electronic characteristics. In this work, we investigate the electrical and thermal transport properties and evaluate the thermoelectric performance of Zintl-phase tellurides $XY{\mathrm{Te}}_2$ $(X=\mathrm{Na}, \mathrm{K}, \mathrm{Rb}, \mathrm{Cs}; Y=\mathrm{Al}, \mathrm{Ga}, \mathrm{In})$ through systematic analysis based on density functional theory, the Boltzmann transport equation, and unified transport theory. Results show that these tellurides can exhibit ultralow lattice thermal conductivity ${\kappa }_L;$ in particular, ${\mathrm{NaInTe}}_2$ shows a value as low as 0.33 W/mK, which is mainly attributed to strong anharmonicity introduced by bonding heterogeneity and the rattling vibrations of the weakly bonded X-site cations. The multiband degeneracy and flat band characteristic near the valence band maximum $(\mathrm{\Gamma }\to M,$ $\mathrm{Z}\to P)$ render into a high power factor of $1.5-2.3\mathrm{mW}/\mathrm{m}{\mathrm{K}}^2$ ($p$ type). By optimizing carrier concentration, exceptional thermoelectric performance with ZT values up to 4.0 ($p$ type) is achieved at 900 K. In contrast, ${\mathrm{NaGaTe}}_2$ shows inferior thermoelectric performance due to the narrow band gap induced bipolar effect that reduces the Seebeck coefficient significantly. In addition, by considering wavelike tunneling effects and higher-order phonon scattering, the tellurides $XY{\mathrm{Te}}_2$ maintain favorable thermoelectric performance with ZT values around 3.3. This work showcases the importance of bond heterogeneity and rattling mode induced ultralow lattice thermal conductivity for enhancing thermoelectric performance in Zintl-phase tellurides.

The quantum interference effect arises from conduction electrons moving along time-reversed electron scattering paths in disordered systems, which leads to weak localization (WL) and antilocalization (WAL) behaviors that suppress and enhance the conductivity, respectively. The research on this effect has so far primarily focused on Hermitian systems, where it is dictated by multiple factors such as symmetry class, dimensionality, interband coupling, band topology, and the nature of disorder potential. However, many realistic systems are intrinsically non-Hermitian due to loss and/or gain from interactions with the external environment. This non-Hermiticity induces complex energy phases into scattering processes, fundamentally modifying electron phase coherence and thereby necessitating a reexamination of quantum interference. Here we investigate the impact of non-Hermiticity on WL and WAL through dissipative topological surface states, and we uncover an underlying connection between symmetry classification and quantum interference behavior. In symmetry class AIII, the geometry of scattering paths, distinguished by their real and imaginary energy components, induces negative Cooperon gaps absent in conventional Hermitian counterparts, whose combination with non-Hermitian topology gives rise to multiple crossovers between WL and WAL, manifesting as alternating variations in positive and negative magnetoconductivities. The corresponding WL and WAL phase diagram reveals the robustness of these crossovers against variations in temperature and magnetic field. As the system transitions from class AIII to class ${\mathrm{AII}}^{†}$ or class A, the entanglement of the real and imaginary energy scatterings obliterates the crossover structure, leaving only WAL-type magnetoconductivity. Using realistic material parameters, we show that the non-Hermiticity-induced quantum interference phenomena should be experimentally observable.

Some magnetic materials exhibit a pronounced peak or upturn in resistivity upon cooling through the ordering transition, while others do not. To explain and quantify this phenomenon, we develop a simple model in which the carrier lifetime is calculated from the scattering of conduction electrons by local magnetic moments. Within this framework, we derive a general formula for the electron scattering rate, applicable to arbitrary energy dispersions and magnetic structure factors. Using ferromagnets as an example, we show that a resistivity anomaly occurs at $T_c$, arising from the rapid increase in the magnetic correlation length $\xi$. We find that this behavior is robust across dimensionality $d$ and persists in anisotropic systems. The maximum resistivity is reached when $1/\xi$ is comparable to the Fermi momentum ${\mathrm{k}}_{\mathrm{F}}$, and the peak height scales inversely with ${\mathrm{k}}_{\mathrm{F}}^{d+2}$. As a result, the effect is most pronounced for smaller Fermi surfaces, where the anomaly can dominate over other resistivity contributions such as electron-phonon or electron-impurity scattering.

We develop a theoretical framework for probing moiré phonon modes using Raman spectroscopy, and illustrate it with the example of twisted bilayer graphene (TBG). These moiré phonons arise from interlayer sliding motion in twisted two-dimensional materials and correspond to fluctuations of the stacking order in reconstructed moiré superlattices. These include both acousticlike phason modes and a set of low-energy optical modes originating from the zone folding of monolayer graphene's acoustic modes, which are accessible via Raman spectroscopy. We show that the Raman response of TBG exhibits a series of low-frequency peaks that clearly distinguish it from that of decoupled layers. We further examine how anharmonicity and angle disorder affect the Raman spectra. Our findings establish Raman spectroscopy as a powerful tool for exploring moiré phonons in a broad class of twisted van der Waals systems.

The topological quasiparticle states and superconductivity have attracted recent interest, especially when they coexist in a single material, offering a promising approach towards intrinsic topological superconductivity. Based on the first-principles calculations and symmetry analyses, we reveal that the topological nodal line state exists in the superconductor TaOsSi. Interestingly, the nodal line state is composed of hourglass fermions, which are robust against spin-orbit coupling due to the protection of the nonsymmorphic symmetries. Furthermore, the nodal line state gives rise to unique surface states, significantly enhancing the density of electronic states near the Fermi level. By combining the intrinsic superconductivity with nontrivial surface states, TaOsSi emerges as an ideal platform for exploring the rich interplay between superconductivity and nontrivial electronic topology.

The charge dynamics of silicon-vacancy (SiV) centers have been investigated for the first time in high-pressure high-temperature nanodiamonds (NDs) with varying concentrations of substitutional nitrogen (${\mathrm{N}}_{\mathrm{S}}$). We demonstrate a controlled sixfold enhancement of ${\mathrm{SiV}}^{-}$ photoluminescence (PL) under dual-color excitation, consisting of strong red (∼660 nm) illumination combined with weak green (∼530 nm) excitation. The measured dependencies of ${\mathrm{SiV}}^{-}$ PL lifetime and intensity on excitation wavelength, together with the enhancement dependence on ${\mathrm{N}}_{\mathrm{S}}$ concentration in the studied nanodiamonds, provide unambiguous evidence of the involvement of donor nitrogen in ${\mathrm{SiV}}^{-}$ emission dynamics. Saturation curves and second-order PL intensity correlation measurements further indicate suppression of the population of the optically inactive ${\mathrm{SiV}}^{2-}$ state upon the addition of green excitation. These results unlock a practical pathway toward engineering optically controlled and scalable quantum emitters based on SiV-luminescent diamond nanoparticles.

This work proposes a grain boundary (GB) structure prediction framework for A15 [001] symmetric tilt GBs, combining a modified Farey diagram (MFD) with an interface computational method based on the conserved Landau-Brazovskii model. We use MFD to predict GB structures for misorientation angles from $0^{\circ }$ to ${90}^{\circ }$, addressing the challenge of describing high-$\mathrm{\Sigma }$ coincidence site lattice (CSL) GBs. When the Farey fraction of a GB is irreducible, MFD predicts a unique structure. When reducible, it predicts two possible structures. The concrete GB structure is determined by the distance between lattice spheres and the GB plane, influenced by environmental parameters, such as temperature. Computer simulations demonstrate excellent agreement with MFD's theoretical predictions. The developed framework can also be used to predict CSL GB structures in other crystalline materials.

Using a pulse electric heating technique, measurements of temperature, specific volume, pressure, specific enthalpy, and electrical resistivity of highly oriented pyrolytic graphite samples in quasistatic processes were carried out. In these processes the samples were subjected to controlled thermal expansion along the hexagonal axis of graphite and melting occurred at pressures of 0.2–2.5 GPa. The melting line of graphite was determined directly from the Clapeyron-Clausius equation by measuring the pressure dependence of the jumps in specific volume and enthalpy of the graphite samples on melting. It was found that the melting temperature of graphite in the specified pressure range increases with increasing pressure from approximately 6000 to 6200 K. Extrapolation of this dependence to higher pressures shows that the melting temperature reaches its maximum at a pressure of about 4 GPa. Based on the measurement results, the values of specific heat capacity, speed of sound, and isochoric temperature coefficient of resistivity of graphite and liquid carbon near the melting line were determined. It is shown that liquid carbon in the studied ranges of density and pressure is a metal for which the isochoric temperature coefficient of resistivity is negative. The obtained results are compared with literature data, and a detailed discussion of the discrepancies between the results is given. It is argued that the currently used phase diagram of carbon requires significant improvement.

${\mathrm{LaCrGe}}_3$ is a metallic, itinerant ferromagnet with $T_C\approx 86\mathrm{K}$ and an easy magnetic direction along its hexagonal $c$ axis. Here we present studies of the $H\parallel c$ Hall effect in ${\mathrm{LaCrGe}}_3$ at ambient pressure and under pressures up to 24 kbar. At ambient pressure the anomalous Hall conductivity in the ferromagnetic state reaches a large value of $\sim 520$ ${\mathrm{\Omega }}^{-1}$ ${\mathrm{cm}}^{-1}$. The overall data are consistent with the behavior expected for a ferromagnet with magnetic field applied along its easy axis. Measurements under pressure yield an $H=0$, $P\text{−}T$ phase diagram consistent with earlier publications with $T_C$ being suppressed with pressure and nonferromagnetic phases appearing for $P>15$ kbar. The data suggest that the observed $T_C$ suppression is due to the pressure-dependent exchange interactions rather than suppression of the size of the magnetic moment. Field-dependent Hall resistivity in the higher-pressure, nonferromagnetic state is consistent with the net moment being zero for $H=0$. In a finite field there is a sharp anomaly comparable to the one associated with a spin-flip metamagnetic transition. The $H\text{−}T$ phase diagrams corresponding to a pressure-induced magnetic phase are constructed for several pressures. The Hall data suggest the possibility of the pressure-induced nonferromagnetic phase being a combination of multiple small regions of antiferromagnetic phases with different wave vectors.

It is widely accepted that spin-orbit coupling (SOC) generally locks spin and spatial degrees of freedom, as a result, the spin, despite being an axial vector, is fixed and cannot rotate independently, and the magnetic system should be described by magnetic space groups (MSGs). While as a new type of group, spin space groups (SSGs) have been introduced to approximately describe the symmetry of magnetic systems with negligible SOC, and received significant attention recently. In this work, we prove that in two cases of coplanar spin configurations, there are spin-only operations that strictly hold even with considerable Dzyaloshinskii-Moriya interaction (DMI), and the symmetry of their spin models could be described by the spin-coplanar SSG. In addition, we also find that for spin-collinear cases, regardless the strength of DMI, the magnon systems within the framework of linear spin wave theory (LSWT) also preserve the decoupled spin and spatial rotations, but the symmetry does not belong to the conventional definitions of collinear spin groups. We discuss the potential realization of these novel symmetries in rod, layer, and three-dimensional space groups. Our work extends the applicability of SSGs to magnetic materials with heavy elements, and reveals that the coexistence of DMI and SSG symmetries provides new opportunity for exploring novel magnon transport phenomena, and potential material realization had also been discussed.

Noncollinear antiferromagnets (NCAFMs) are appealing for antiferromagnetic spintronics, as they combine the advantages of collinear antiferromagnets with novel emergent phenomena stemming from their complex spin structures. These phenomena are often associated with the intrinsic spin chirality, which characterizes the handedness of the ground-state spin configuration. Here, we investigate a kagome NCAFM interfaced with a normal metal and demonstrate that the ground-state vector spin chirality can be probed through measurements of the spin Seebeck effect (SSE). Starting from a microscopic spin Hamiltonian, we derive the corresponding bosonic Bogoliubov–de Gennes Hamiltonians for the two chiral configurations. Using linear response theory, we obtain a general expression for the spin current thermally pumped into the normal metal by the SSE. We show that a sizable in-plane spin current emerges exclusively in the negative-chiral state, providing a direct signature for real-time detection of chirality switching in kagome NCAFMs. In addition, we find a field-dependent out-of-plane spin current whose magnitude differs between the two chiralities by about 81%, reflecting their distinct magnon band structures.