Angle-dispersive x-ray powder diffraction experiments have been performed on yttrium metal up to 183 GPa. We find that the recently discovered $oF16$ structure observed in the high-$Z$ trivalent lanthanides is also adopted by yttrium above 106 GPa, pressures where it has a superconducting temperature of $\sim 20$ K. We have also refined both tetragonal and rhombohedral structures against the diffraction data from the preceding “distorted-fcc” phase and we are unable to state categorically which of these is the true structure of this phase. Finally, analysis of yttrium's equation of state reveals a marked change in the compressibility upon adoption of the $oF16$ structure, after which the compression is that of a “regular” metal. Electronic structure calculations of $oF16$-Y confirm its stability over $oF8$ structure seen in Nd and Sm, and provide insight into the nature of the shift of orbital character from $s$ to $d$ under compression.

Negative thermal expansion (NTE)—the phenomenon where some materials shrink rather than expand when heated—is both intriguing and useful but remains poorly understood. Current understanding hinges on the role of specific vibrational modes, but in fact thermal expansion is a weighted sum of contributions from every possible mode. Here we overcome this difficulty by deriving a real-space model of atomic motion in the prototypical NTE material scandium trifluoride, $\mathrm{Sc}{\mathrm{F}}_3$, from total neutron scattering data. We show that NTE in this material depends not only on rigid unit modes—the vibrations in which the scandium coordination octahedra remain undistorted—but also on modes that distort these octahedra. Furthermore, in contrast with previous predictions, we show that the quasiharmonic approximation coupled with renormalization through anharmonic interactions describes this behavior well. Our results point the way towards a new understanding of how NTE is manifested in real materials.

Nonequilibrium dynamics in isolated quantum many-body systems displays a number of intriguing features, such as many-body localization (MBL) and prethermalization. Here we investigate a simple ladder system with disorder, in which various distinct dynamical features coexist and interplay. By exact diagonalization, we demonstrate that the system exhibits the signatures of an MBL-ergodic-MBL reentrant transition, metastability, and disorder-free MBL. We give an account of these properties by introducing a quasiparticle picture and interpreting the quasivacuum energy fluctuation as an effective disorder on the quasi-particle dynamics. It is speculated that the weak perturbation behavior is a finite-size effect, but its relaxation timescale increases with the system size.

Asymmetric destruction of the one-third magnetization plateau upon heating is found in the spin-$\frac{1}{2}$ kagome Heisenberg antiferromagnets by using the typical pure quantum state approach. The asymmetry originates from a density of states of low-lying excited states of $N_{\mathrm{s}}$ spin systems with magnetization $(1/3-2/N_{\mathrm{s}})$ that is larger than the density of states of low-lying states with magnetization $(1/3+2/N_{\mathrm{s}})$. The enhanced specific heat and entropy that reflect the larger density of states in the lower-field side of the plateau are detectable in candidate materials of the kagome antiferromagnets. We discuss how the asymmetry originates from the unprecedented preservation of the ice rule around the plateau.

Iron-based alloys are widely used as structural components in engineering applications. This calls for a fundamental understanding of their mechanical properties, including those of pure iron. Under operational temperatures the mechanical and magnetic properties will differ from those of ferromagnetic body-centered-cubic iron at 0 K. In this theoretical work we study the effect of disordered magnetism on the screw dislocation core structure and compare with results for the ordered ferromagnetic case. Dislocation cores control some local properties such as the choice of glide plane and the associated dislocation mobility. Changes in the magnetic state can lead to modifications in the structure of the core and affect dislocation mobility. In particular, we focus on the core properties of the $\frac{1}{2}\langle 111\rangle$ screw dislocation in the paramagnetic state. Using the noncollinear disordered local moment approximation to address paramagnetism, we perform structural relaxations within density functional theory. We obtain the dislocation core structure for the easy and hard cores in the paramagnetic state, and compare them with their ferromagnetic counterparts. By averaging the energy of several disordered magnetic configurations, we obtain an energy difference between the easy- and hard-core configurations, with a lower, but statistically close, value than the one reported for the ferromagnetic case. The magnetic moment and atomic volume at the dislocation core differ between paramagnetic and ferromagnetic states, with possible consequences on the temperature dependence of defect-dislocation interactions.

Weak ferromagnetism has been widely found in antiferromagnetic systems, including the technically important orthorhombic perovskites $R\mathrm{Fe}{\mathrm{O}}_3$ ($R=\mathrm{rare}\mathrm{earth}$) owing to spin canting associated with crystal symmetry. Antisymmetric exchange interaction (AEI) and single-ion anisotropy (SIA) are the essential mechanisms responsible for the development of noncollinear structures in antiferromagnetic systems. AEI and SIA share the same structural restriction to facilitate the spin canting. While both AEI and SIA originate from the spin-orbit coupling effect, they have sharply different dependences on the local structure. Consideration of the structural dependence of a canted spin motivates us to revisit the orthoferrite family. The spin canting along the $c$ axis measured on precisely oriented crystals increases monotonically from $\mathrm{La}\mathrm{Fe}{\mathrm{O}}_3$ to $\mathrm{Lu}\mathrm{Fe}{\mathrm{O}}_3$. Based on Moriya's model, AEI is sensitive to the octahedral-site distortion in the perovskite structure. However, the site distortion does not exhibit a monotonic change with the rare-earth substitution. Instead, a linear relationship has been found in both the octahedral rotation and the spin canting angle versus the rare-earth ionic radius, which indicates that SIA plays a significant role leading the spin canting in orthoferrites.

An efficient conversion of a charge current into a spin current is a crucial point for application of the spin Hall effect in practical spintronic devices. Recently, we revealed that this goal can be achieved by using ultrathin fcc (111) and (001) noble metal films doped with Bi impurities, which possess spin Hall angles up to 80%. Here, we show that the effect can be further amplified in monolayer films with a strong crystalline anisotropy. This is demonstrated by considering noble metal films with fcc (110) geometry. Our theoretical study predicts related spin Hall angles exceeding 100% especially when the crystalline anisotropy is increased, which tunes the Fermi surface topology.

We present electrical resistance measurements of elemental yttrium on bulk and film samples, and both exhibit superconductivity at very high pressures. We show that the pressure dependence of the superconducting transition temperature above 100 GPa is in good agreement with the predicted $Fddd$ phase by Chen et al. [Phys. Rev. Lett. 109, 157004 (2012)]. This result together with a Rietveld refinement made on x-ray data at 123 GPa from Samudrala et al. [J. Phys. Condens. Matter 24, 362201 (2012)] offer strong evidence that the atomic structure of yttrium above 100 GPa is orthorhombic $Fddd$. Furthermore, our process of evaporating yttrium film directly on a diamond anvil is expected to be a valuable asset for future synthesis of new superhydride superconductors.

We present a comprehensive study of the spin excitations—as measured by the dynamical spin structure factor $S(q,\omega )$—of the so-called block-magnetic state of low-dimensional orbital-selective Mott insulators. We realize this state via both a multi-orbital Hubbard model and a generalized Kondo-Heisenberg Hamiltonian. Due to various competing energy scales present in the models, the system develops periodic ferromagnetic islands of various shapes and sizes, which are antiferromagnetically coupled. The $2\times 2$ particular case was already found experimentally in the ladder material $\mathrm{Ba}{\mathrm{Fe}}_2{\mathrm{Se}}_3$ that becomes superconducting under pressure. Here we discuss the electronic density as well as Hubbard and Hund coupling dependence of $S(q,\omega )$ using density matrix renormalization group method. Several interesting features were identified: (1) An acoustic (dispersive spin-wave) mode develops. (2) The spin-wave bandwidth establishes a new energy scale that is strongly dependent on the size of the magnetic island and becomes abnormally small for large clusters. (3) Optical (dispersionless spin excitation) modes are present for all block states studied here. In addition, a variety of phenomenological spin Hamiltonians have been investigated but none matches entirely our results that were obtained primarily at intermediate Hubbard $U$ strengths. Our comprehensive analysis provides theoretical guidance and motivation to crystal growers to search for appropriate candidate materials to realize the block states, and to neutron scattering experimentalists to confirm the exotic dynamical magnetic properties unveiled here, with a rich mixture of acoustic and optical features.

We present a framework to systematically address topological phases when finer partitionings of bands are taken into account, rather than only considering the two subspaces spanned by valence and conduction bands. Focusing on $C_2\mathcal{T}$-symmetric systems that have gained recent attention, for example, in the context of layered van-der-Waals graphene heterostructures, we relate these insights to homotopy groups of Grassmannians and flag varieties, which in turn correspond to cohomology classes and Wilson-flow approaches. We furthermore make use of a geometric construction, the so-called Plücker embedding, to induce windings in the band structure necessary to facilitate nontrivial topology. Specifically, this directly relates to the parametrization of the Grassmannian, which describes partitioning of an arbitrary band structure and is embedded in a better manageable exterior product space. From a physical perspective, our construction encapsulates and elucidates the concepts of fragile topological phases beyond symmetry indicators as well as non-Abelian reciprocal braiding of band nodes that arises when the multiple gaps are taken into account. The adopted geometric viewpoint most importantly culminates in a direct and easily implementable method to construct model Hamiltonians to study such phases, constituting a versatile theoretical tool.

We re-examine the zero-temperature phase diagram of the two-leg Hubbard ladder in the small $U$ limit, both analytically and using density-matrix renormalization group (DMRG). We find a ubiquitous Luther-Emery phase, but with a crossover in behavior at a characteristic interaction strength, $U^{★}$; for $U\gtrsim U^{★}$, there is a single emergent correlation length $ln\left[\xi \right]\sim 1/U$, characterizing the gapped modes of the system, but for $U\lesssim U^{★}$, there is a hierarchy of length scales, differing parametrically in powers of $U$, reflecting a two-step renormalization group flow to the ultimate fixed point. Finally, to illustrate the versatility of the approach developed here, we sketch its implications for a half-filled triangular lattice Hubbard model on a cylinder and find results in conflict with inferences concerning the small $U$ phase from recent DMRG studies of the same problem.

Topological properties and topological superconductivity in real materials have attracted intensive experimental and theoretical attention recently. Based on symmetry analysis and first-principles electronic structure calculations, we predict that $R\mathrm{Ru}{\mathrm{B}}_2$ $(R=\mathrm{Y}, \mathrm{Lu})$ are not only topological superconductor (TSC) candidates but also own the hybrid hourglass-type Dirac ring which is protected by the nonsymmorphic space group symmetry. Due to the band inversion around the time-reversal invariant $\mathrm{\Gamma }$ point in the Brillouin zone, $R\mathrm{Ru}{\mathrm{B}}_2$ also have Dirac topological surface states (TSSs). More importantly, their TSSs on the (010) surface are within the band gap of bulk and cross the Fermi level, which form single Fermi surfaces. Considering the fact that both $\mathrm{Y}\mathrm{Ru}{\mathrm{B}}_2$ and $\mathrm{Lu}\mathrm{Ru}{\mathrm{B}}_2$ are superconductors with respective superconducting transition temperatures $\left(T_c\right)$ of 7.6 K and 10.2 K, the superconducting bulks will likely induce superconductivity in the TSSs via the proximity effect. The ternary borides $R\mathrm{Ru}{\mathrm{B}}_2$ may thus provide a very promising platform for studying the properties of topological superconductivity and hourglass fermions in future experiments.

We investigate the radiative heat transport along a Su-Schrieffer-Heeger chain of InSb nanoparticles. We show that in the topological nontrivial phase, the edge modes dominate the radiative heat transport despite their strong localization at the edges of the finite chain due to a long-range coupling of the first and last particles. We further discuss the scaling laws of the heat transfer with respect to the chain length for the longitudinal and transversal band and edge modes and conclude that both types of modes obey the same scaling law.

We performed ${}^{13}\mathrm{C}$-NMR measurements on an organic superconductor $\lambda \text{−}{\left(\mathrm{BETS}\right)}_2{\mathrm{GaCl}}_4$ to investigate the superconducting (SC) gap symmetry, where BETS stands for bis(ethylenedithio)tetraselenafulvalene. As the temperature approaches 0 K, the Knight shift in the SC state decreases; the nuclear spin-lattice relaxation rate $1/T_1$ shows cubic temperature dependence below the SC transition temperature $T_c$ without a coherence peak. These results can be understood in terms of $d$-wave superconductivity. Moreover, an increase in $1/T_{1}T$ at lower temperatures was observed just above $T_c$; this behavior can be interpreted as an effect of spin-density-wave (SDW) fluctuation. We suggest that the $d$-wave superconductivity of $\lambda \text{−}{\left(\mathrm{BETS}\right)}_2{\mathrm{GaCl}}_4$ originates from SDW fluctuation.

The bulk-edge correspondence characterizes topological insulators and superconductors. We generalize this concept to the bulk-corner correspondence and the edge-corner correspondence in two dimensions. In the bulk-corner (edge-corner) correspondence, the topological number is defined for the bulk (edge), while the topological phase is evidenced by the emergence of zero-energy corner states. It is shown that the boundary-obstructed topological phases recently proposed are the edge-corner-correspondence type, while the higher-order topological phases are classified into the bulk-corner-correspondence type and the edge-corner-correspondence type. We construct a simple two-dimensional model exhibiting the edge-corner correspondence based on two Chern insulators having the $s$-wave, $d$-wave, and $s_{\pm }$-wave pairings. Here, the emergence of zero-energy corner states in the topological phase is a result of the topological numbers defined for the edge Hamiltonians and not for the bulk Hamiltonian. It is pointed out that the emergence of zero-energy corner states is observable by measuring the impedance resonance in topological electric circuits.

Using finite-temperature determinantal quantum Monte Carlo calculations, we find that the dominant pairing symmetries in the standard Hubbard model on the honeycomb lattice depend on the electron filling. When the electron density $\rho$ is larger than around 0.25, the dominant pairing symmetry is the $d+id$-wave; when the electron density is low enough ($\rho \lesssim 0.25$), the dominant pairing symmetry is the $p+ip$ wave. For two electron densities $\rho =0.1$ and $\rho =0.4$, where the dominant pairing symmetries are $p+ip$ wave and $d+id$ wave separately, we study the effect of two types of hopping inhomogeneities (the plaquette and the quasi-$1\mathrm{D}$ hopping inhomogeneities) on the pairing symmetries. For $\rho =0.1$, the plaquette hopping inhomogeneity drives the dominant $p+ip$-wave pairing symmetry into the $d+id$-wave; while the quasi-$1\mathrm{D}$ hopping inhomogeneity almost does not affect the $p+ip$ wave, and arises the $f$ wave, causing the coexistence of $p+ip$-wave and $f$-wave pairing. For $\rho =0.4$, both the plaquette and the quasi-$1\mathrm{D}$ hopping inhomogeneities destroy the $d+id$-wave pairing symmetry, without causing other pairing symmetry. Our results suggest that the effect of hopping inhomogeneity on the pairing symmetries is robust and depends on the electron filling. This finding may be useful for the design of artificial graphene superconductors with different kinds of pairing, especially for the realization of the $d+id$-wave, $p+ip$-wave, or $f$-wave superconductors at low filling.

We consider a system of weakly coupled one-dimensional wires forming a three-dimensional stack in the presence of a spatially periodic modulation of the chemical potential along the wires, equivalent to a charge density wave (CDW). An external static magnetic field is applied parallel to the wire axes. We show that, for a certain parameter regime, due to interplay between the CDW and magnetic field, the system can support a second-order topological phase characterized by the presence of chiral quasi-1D quantum Hall effect (QHE) hinge modes. Interestingly, we demonstrate that direction of propagation of the hinge modes depends on the phase of the CDW and can be reversed only by electrical means without the need of changing the orientation of the magnetic field. Furthermore, we show that the system can also support 2D chiral surface QHE states, which can coexist with one-dimensional hinge modes, realizing a scenario of a hybrid high-order topology. Performing two-terminal transport simulations in the linear response regime, we confirm quantized QHE resistance plateaus, which are highly robust to disorder giving a clear signature of hinge and surface states.

${\mathrm{Ni}}_2{\mathrm{In}}_{0.9}{\mathrm{Cr}}_{0.1}{\mathrm{SbO}}_6$ is a trigonal chiral polar helimagnet with multiferroic nature. Here, we perform optical absorption measurements on ${\mathrm{Ni}}_2{\mathrm{In}}_{0.9}{\mathrm{Cr}}_{0.1}{\mathrm{SbO}}_6$ in magnetic fields. The observed magnetochromism is attributed to the spin-dependent transition probability of the intra-atomic $d\text{−}d$ excitation from the ${}^3{A}_{2g}$ ground state to the ${}^1{E}_g$ excited state of ${\mathrm{Ni}}^{2+}$ ions with the $3d^8$ electronic configuration. Exploiting the magnetodielectric coupling in optical frequencies, we propose that the boundary between ferrimagnetic and incommensurate magnetic phases of ${\mathrm{Ni}}_2{\mathrm{In}}_{0.9}{\mathrm{Cr}}_{0.1}{\mathrm{SbO}}_6$ in 6 T along the $c$ axis should lie at around 50 K.