Crystal and magnetic structures of the high-pressure-stabilized perovskite modification of ${\mathrm{Mn}}_2{\mathrm{O}}_3$ ($\zeta \text{−}{\mathrm{Mn}}_2{\mathrm{O}}_3$) have been studied by neutron powder diffraction combined with symmetry arguments based on the phenomenological Landau theory. This metastable phase exhibits a unique charge disproportionation phenomenon stabilizing the quadruple perovskite structure (${\mathrm{Mn}}^{2+}{\mathrm{Mn}}_3^{3+}){\mathrm{Mn}}_4^{3.25+}{\mathrm{O}}_{12}$ with an additional charge-ordering and commensurate orbital density wave localized in the B-site perovskite position. The commensurate nature of the orbital density wave is stimulated by a coupling of the orbital ordering to independent structural distortions, which improve poor bonding conditions of ${\mathrm{Mn}}^{2+}$ in the A-site perovskite position. Below $T_1\sim 100$ K, an anharmonic longitudinal spin density wave arises and locks to the structural modulation associated with the orbital density. At $T_2\sim 50$ K, the magnetic subsystem delocks from the structural modulation giving rise to a multi-$k$ phase-modulated ground state admixing cycloidal and helical components. The complex anharmonic and phase-modulated magnetic structures are discussed based on a phenomenological magneto-orbital coupling scheme, previously developed to explain the multi-$k$ helical ground states with modulated spin chirality observed in ${\mathrm{A}}^{2+}{\mathrm{Mn}}_7{\mathrm{O}}_{12}$ (${\mathrm{A}}^{2+}=\text{Ca}$, Sr, Pb, and Cd) quadruple perovskites.

The semiconducting gap in the $\mathrm{FeG}{\mathrm{a}}_3$ intermetallic originates from $\mathrm{Fe}\left(3d\right)/\mathrm{Ga}\left(4p\right)$ hybridization. Pressures of 15–20 GPa initiate a disruption of this semiconducting tetragonal $P{4}_2/mnm$ structure and an emergence of a high-pressure metallic phase, estimated to be fully stabilized just beyond ∼35 GPa. An accompanying pronounced ∼17% volume collapse occurs at the structural transition. The high-pressure metallic phase has a $T^{1/2}$ temperature dependence of the resistivity below its minimum at 8–12 K, symptomatic of disorder. There is a corresponding weak high-temperature dependence of the resistivity and resultant broad maximum at ∼250 K to yield “bad-metal” values of ∼0.5 mΩ cm at room temperature. This is shown to signify that the high-pressure phase is a low carrier density metal on the verge of an Anderson transition. Ga $K$-edge absorption spectroscopy and Fe Mössbauer spectroscopy local probes indicate that the atomic disorder stems from a pressure-instigated rearrangement of the Ga sublattice at the structural transition.

We study a Yang-Baxter integrable quantum spin-1/2 chain with random interactions. The Hamiltonian is local and involves two- and three-spin interactions with random parameters. We show that the energy eigenstates of the model are never localized and in fact exhibit perfect energy and spin transport at both zero and infinite temperatures. By considering the vicinity of a free fermion point in the model we demonstrate that this behavior persists under deformations that break Yang-Baxter integrability but preserve the free fermion nature of the Hamiltonian. In this case the ballistic behavior can be understood as arising from the correlated nature of the disorder in the model. We conjecture that the model belongs to a broad class of models avoiding localization in 1D.

Existence of surface acoustic waves on the boundary of half-infinite one-dimensional phononic crystals is investigated. The structure is formed of perfectly bonded solid nonpiezoelectric layers of general anisotropy. The layers are parallel to the substrate surface. It is shown that at most three surface waves can exist in a stopband. The number of surface waves on the structure with a given order of layers is correlated with the number of surface waves on the structure where the order of layers is reversed, namely, in total at most three surface waves occur within a stopband. However, if the layers are arranged so that the period is symmetric with respect to its midplane, “symmetric” period, then at most one surface wave exists per stopband on the phononic crystal-vacuum boundary. A criterion of the occurrence of such a wave in the lowest stopband is found. No surface acoustic wave exists in the lowest stopband on the mechanically clamped surface but one surface wave can occur in the other stopbands. The case of “symmetric” period has no relation to crystallographic symmetry. In particular, it occurs in half-infinite two-layered phononic crystals where the thickness of the exterior layer of the substrate is half the thickness of the interior layers of the same material.

We study the spin-orbit torques (SOTs) in Co/Cu/Co magnetic trilayers based on first-principles density-functional theory calculations in the case where the applied electric field lies in-plane, i.e., parallel to the interfaces. We assume that the bottom Co layer has a fixed in-plane magnetization, while the top Co layer can be switched. We find that the SOT on the top ferromagnet can be controlled by the bottom ferromagnet because of the nonlocal character of the SOT in this system. As a consequence the SOT is anisotropic, i.e., its magnitude varies with the direction of the applied electric field. We show that the Dzyaloshinskii-Moriya interaction (DMI) in the top layer is anisotropic as well, i.e., the spin-spiral wavelength of spin spirals in the top layer depends on their in-plane propagation direction. This effect suggests that DMI can be tuned easily in magnetic trilayers via the magnetization direction of the bottom layer. In order to understand the influence of the bottom ferromagnet on the SOTs and the DMI of the top ferromagnet, we study these effects in Co/Cu magnetic bilayers for comparison. We find the SOTs and the DMI to be surprisingly large despite the small spin-orbit interaction of Cu.

The chiral compound ${\mathrm{YbNi}}_3{\mathrm{Ga}}_9$ with a trigonal structure, which is a valence fluctuating material at ambient pressure, undergoes a magnetic order under pressure above $P_{\mathrm{c}}$ of 9 GPa. We have studied the temperature-pressure-magnetic-field phase diagrams of this compound by ac calorimetric measurements under pressures $P$ up to 12 GPa. The specific heat $C$ divided by temperature $T,C/T$, markedly increases with applying pressure up to $P_{\mathrm{c}}$. At 8.6 GPa, just below $P_{\mathrm{c}},C/T$ exhibits $-lnT$ dependence in the temperature range 2–9 K. At $T<1.5\mathrm{K},C/T$ is saturated to a large value $1\mathrm{J}/{\mathrm{K}}^2\mathrm{mol}$, i.e., heavy Fermi-liquid behavior. At $P=9.3\mathrm{GPa}$, just above $P_{\mathrm{c}}$, a broad maximum in $C/T$ appears at 1.6 K, indicating the onset of the magnetic order. With increasing $P$ further, the maximum shifts to a higher temperature and transforms to a sharp $\lambda$-type peak at 5 K for $P\ge 11\mathrm{GPa}$. By using the temperature, pressure, and magnetic field $B$ dependences of the specific-heat data $C\left(T,P,B\right)$ obtained under $B\perp c$ and $B\parallel c$, we have constructed $B\text{−}T$ phase diagrams at each $P$ value. Another field-induced ordered phase appears in the diagrams only for $B\perp c$ at $P\ge 11\mathrm{GPa}$. The origin of this induced phase is discussed in relation to the chiral soliton lattice and the skyrmion lattice reported for chiral magnets.

In this paper, we report the transport, thermodynamic, and superconducting properties of a new filled skutterudite ${\mathrm{CaOs}}_4{\mathrm{P}}_{12}$ synthesized under high pressure and high temperature. The electrical resistivity of 3.4–4.$8\mathrm{m}\mathrm{\Omega }\mathrm{cm}$, carrier concentration of 3.8–6.$1\times {10}^{20}{\mathrm{cm}}^{-3}$, and positive Hall coefficient suggest that ${\mathrm{CaOs}}_4{\mathrm{P}}_{12}$ is a semimetal with hole carriers. An anomaly due to low-energy optical modes corresponding to an Einstein temperature of 150 K was observed in the specific heat. Resistivity, dc magnetic susceptibility, and specific heat measurements indicate bulk superconductivity below 2.5 K. The specific heat anomaly at $T_{\mathrm{c}},\mathrm{\Delta }C/\gamma T_{\mathrm{c}}\approx 1.4$, is in agreement with the Bardeen-Cooper-Schrieffer (BCS) value of 1.43. The electron-phonon coupling constant ${\lambda }_{ep}$ is estimated to be 0.47. ${\mathrm{CaOs}}_4{\mathrm{P}}_{12}$ is classified as a BCS-type, weakly coupled type-II superconductor with an upper critical field of $H_{\mathrm{c}2}\approx 22$ kOe and Ginzburg-Landau coherence length of $\xi \approx 12$ nm.

We report the magnetoresistance of ScSb, which is a semimetal with a simple rocksalt-type structure. We found that the magnetoresistance reaches $\sim 28000\%$ at 2 K and 14 T in our best sample, and it exhibits a resistivity plateau at low temperatures. The Shubnikov–de Haas oscillations extracted from the magnetoresistance data allow the full construction of the Fermi surface, including the so-called ${\alpha }_3$ pocket which has been missing in other closely related monoantimonides, and an additional hole pocket centered at $\mathrm{\Gamma }$. The electron concentration $\left(n\right)$ and the hole concentration $\left(p\right)$ are extracted from our analysis, which indicates that ScSb is a nearly compensated semimetal with $n/p\approx 0.93$. The calculated band structure indicates the absence of a band inversion, and the large magnetoresistance in ScSb can be attributed to the nearly perfect compensation of electrons and holes, despite the existence of the additional hole pocket.

In past work we introduced a method which allows for exact computations of entanglement Hamiltonians. The method relies on computing the resolvent for the projected (on the entangling region) Green's function using a solution to the Riemann-Hilbert problem combined with finite-rank perturbation theory. Here we analyze in detail several examples involving excited states of chiral fermions (Dirac and Majorana) on a spatial circle. We compute the exact entanglement Hamiltonians and an exact formula for the change in entanglement entropy due to the introduction of a particle above the Dirac sea. For Dirac fermions, we give the first-order temperature correction to the entanglement entropy in the case of a multiple-interval entangling region.

Interaction in a flat band is magnified due to the divergence in the density of states, which gives rise to a variety of many-body phenomena such as ferromagnetism and Wigner crystallization. Until now, however, most studies of flat-band physics have been based on model systems, making their experimental realization something that would occur in the distant future. Here, we propose a class of systems made of real atoms, namely carbon atoms with realistic physical interactions (dubbed here “kagome graphene/graphyne”). Density functional theory calculations reveal that these kagome lattices offer a controllable way to realize robust flat bands sufficiently close to the Fermi level. Upon hole doping, they split into spin-polarized bands at different energies to result in a flat-band ferromagnetism. At half-filling, this splitting reaches its highest level of 768 meV. At smaller fillings, e.g., when $\nu =\frac{1}{6}$, on the other hand, a Wigner crystal spontaneously forms, where the electrons form closed loops localized on the grid points of a regular triangular lattice. It breaks the translational symmetry of the original kagome lattice. We further show that the kagome lattices exhibit good mechanical stabilities, based on which a possible route for experimental realization of the kagome graphene is also proposed.

We report a study on the evolution of a charge density wave and antiferromagnetism in the series of the polycrystalline solid solution ${\mathrm{Nd}}_{1-x}{\mathrm{Gd}}_x{\mathrm{NiC}}_2 (0\le x\le 1)$ by means of magnetic and transport properties measurements. The experimental results reveal the violation of the de Gennes law and a strong correlation between the Peierls, Néel, and Curie-Weiss temperatures, which strongly suggests a cooperative interaction between the charge density wave state and antiferromagnetism due to Fermi surface nesting enhancement of the Ruderman-Kittel-Kasuya-Yosida interaction. We also find that the obtained results for the ${\mathrm{Nd}}_{1-x}{\mathrm{Gd}}_x{\mathrm{NiC}}_2$ $(0\le x\le 1)$ series overlap with the $T_{\mathrm{CDW}}$ trend line in the phase diagram for ${\mathrm{RNiC}}_2$ family.

We present a field theory description of a quantum phase transition in two spatial dimensions between a $\text{U}\left(1\right)$ algebraic spin liquid with $N$ flavors of gapless two-component Dirac fermionic spinons and a gapped ${\mathbb{Z}}_2$ spin liquid. This transition is driven by spinon pairing and concomitant Higgsing of the emergent U(1) gauge field. For sufficiently large $N$, we find a quantum critical point with non-Gaussian exponents that is stable against instanton proliferation. We compute critical exponents using either $1/N$ or $\epsilon$ expansions, and give estimates of the critical value of $N$ below which the quantum critical point disappears.

As many-body Floquet theory becomes more popular, it is important to find ways to connect theory with experiment. Theoretical calculations can have a periodic driving field that is always on, but experiment cannot. Hence, we need to know how long a driving field is needed before the system starts to look like the periodically driven Floquet system. We answer this question here for noninteracting band electrons in the infinite-dimensional limit by studying the properties of the system under pulsed driving fields and illustrating how they approach the Floquet limit. Our focus is on determining the minimal pulse lengths needed to recover the qualitative and semiquantitative Floquet theory results.

Graphene plasmons have recently attracted a great deal of attention because of their tunability, long lifetime, and high degree of field confinement in the vertical direction. Nearby metal gates have been shown to modify the graphene plasmon dispersion and further confine their electric field. We study the plasmons of a graphene sheet deposited on a metal, in the regime in which metal bands do not hybridize with massless Dirac fermion bands. We derive exact results for the dispersion and lifetime of the plasmons of such a hybrid system, taking into account metal nonlocalities. The graphene plasmon dispersion is found to be acoustic and pushed down in energy toward the upper boundary of the intraband graphene particle-hole continuum, thereby strongly enhancing the vertical confinement of these excitations. Landau damping of such acoustic plasmons due to particle-hole excitations in the metal gate is found to be surprisingly weak, with quality factors exceeding $Q={10}^2$.

Using first-principles calculations and effective model analysis, we demonstrate that the strained WC-type YN can host an ideal topological semimetal state with an ultraflat nodal ring in the absence of spin-orbit coupling. Distinguished from previous nodal-line semimetals, the nodal ring in YN lies in a single plane of the Brillouin zone with an energy variation less than 0.3 meV; that is, the nodal ring is flat in both momentum and energy space. Interestingly, by applying a uniaxial strain along the $c$ axis, one can effectively tune the size of the nodal ring and even realize a topological phase transition from nodal-ring semimetals to normal insulators. The nodal ring here is topologically nontrivial, as confirmed by directly calculating the topological invariant and topological drumhead surface state. Moreover, due to an anisotropic torus Fermi surface and the nontrivial topology, exotic magnetoresponse effects, including anomalous Landau level structures and direction-dependent phase shifts of quantum oscillations with and without a Berry phase of $\pi$, can be probed experimentally.

The electronic structure of in situ grown ferroelectric ${\mathrm{BaTiO}}_3$(001) films is studied by angle-resolved photoemission spectroscopy. A two-dimensional state is found at the film-vacuum interface with an unexpected checkerboardlike Fermi surface. The observed absence of in-plane dispersion of the states is attributed to the influence of the strong electric fields of the ferroelectric bulk of the film. These fields cause Bloch oscillations and Wannier-Stark localization of free charges in real space, resulting in a smearing in reciprocal space. It is shown that this effect extends to thin ${\mathrm{SrTiO}}_3$ overlayers grown on the ${\mathrm{BaTiO}}_3$ film.

We theoretically study the nonlocal Andreev transport through a normal/superconducting/normal junction formed on bilayer graphene, in which an off-resonant circularly polarized light and interlayer biases are applied to the two normal regions. The effects of the light field, bias voltage, and superconductor length on the crossed Andreev reflection are calculated and analyzed. We show that the pure crossed Andreev reflection can be obtained by modulating the completely opposite valley polarizations in the two terminals under the optical and electrical coaction, and a valley-switch effect can be realized between the pure crossed Andreev reflection and the pure valley polarized elastic cotunneling by reversing the interlayer bias of one terminal. In addition, a very small subthreshold swing can be acquired when the pure nonlocal conductance changes with variation of the Fermi energy. This observation suggests that our device can be operated as a fast nonlocal switching transistor.

We investigate theoretically the electromagnon on the surfaces of the magnetic topological insulator thin films. It is found that when the magnetic asymmetry between the top and bottom surfaces is there, the ferromagnetic resonance is driven by the electric field which is about ten times more efficient compared with that by the magnetic field. The resonant frequency of the electromagnon is also estimated.

We demonstrate the inplane anisotropy of longitudinal thermal conductivities and the weak localization of magnons in a disordered screw-type spiral magnet on a square lattice. We consider a disordered spin system, described by a spin Hamiltonian for the antiferromagnetic Heisenberg interaction and the Dzyaloshinsky-Moriya interaction with the mean-field-type potential of impurities. We derive longitudinal thermal conductivities for the disordered screw-type spiral magnet in the weak-localization regime by using the linear-response theory with the linear-spin-wave approximation and performing perturbation calculations. We show that the inplane longitudinal thermal conductivities are anisotropic due to the Dzyaloshinsky-Moriya interaction. This anisotropy may be useful for experimentally estimating the magnitude of a ratio of the Dzyaloshinsky-Moriya interaction to the Heisenberg interaction. We also show that the main correction term gives a logarithmic suppression with the length scale due to the critical backscattering. This suggests that the weak localization of magnons is ubiquitous for the disordered two-dimensional magnets having global time-reversal symmetry. We finally discuss several implications for further research.