Nitinol (NiTi), the most widely used shape-memory alloy, exhibits an austenite phase that has yet to be identified. The usually assumed austenitic structure is cubic B2, which has imaginary phonon modes, hence it is unstable. We suggest a stable austenitic structure that “on average” has B2 symmetry (observed by x-ray and neutron diffraction), but it exhibits finite atomic displacements from the ideal B2 sites. The proposed structure has a phonon spectrum that agrees with that from neutron scattering, has diffraction spectra in agreement with x-ray diffraction, and has an energy relative to the ground state that agrees with calorimetry data.

The disorder-induced superconductor-to-insulator transition in amorphous ${\mathrm{Nb}}_x{\mathrm{Si}}_{1-x}$ two-dimensional thin films is studied for different niobium compositions $x$ through a variation of the sample thickness $d$. We show that the critical thickness $d_c$, separating a superconducting regime from an insulating one, increases strongly with diminishing $x$, thus attaining values of over 100 Å. The corresponding phase diagram in the $(d,x)$ plane is inferred and related to the three-dimensional situation. The two-dimensional superconductor-to-insulator transition well connects with the three-dimensional superconductor-to-metal transition.

We present a flexible density-matrix renormalization group approach to calculate finite-temperature spectral functions of one-dimensional strongly correlated quantum systems. The method combines the purification of the finite-temperature density operator with a moment expansion of the Green's function. Using this approach, we study finite-temperature properties of dynamical spectral functions of spin-$1/2$ XXZ chains with Dzyaloshinskii-Moriya interactions in magnetic fields and analyze the effect of these symmetry breaking interactions on the nature of the finite-temperature dynamic spin structure factor.

The magnetic properties of vanadate-carbonate compound, ${\mathrm{K}}_2{\mathrm{Mn}}_3\left({\mathrm{VO}}_4\right){}_2{\mathrm{CO}}_3$, has been discussed in recent literature in terms of unusual low-spin configuration of ${\mathrm{Mn}}^{2+}$. Using first-principles density functional theory-based microscopic analysis of electronic and magnetic structure of this compound, we find that contrary to previous suggestion, two inequivalent ${\mathrm{Mn}}^{2+}$ ions in this compound, one in octahedral and another in trigonal-bipyramidal coordination of oxygen atoms, are both in high-spin state. Our first-principles determination of Mn-Mn magnetic exchanges leads to a spin-5/2 model consisting of alternate layers of honeycomb and triangular spin lattices, with vastly differing strengths of magnetic interactions in the two layers, which are very weakly connected among themselves. The computed magnetic susceptibility and magnetization, based on the derived spin model, are compared with measured thermodynamic data.

In structures made up of alternating superconducting and ferromagnet layers (S/F/S heterostructures), it is known that the macroscopic quantum wave function of the ground state changes its phase difference across the F layer from 0 to $\pi$ under certain temperature and geometrical conditions, hence the name “$0–\pi$” for this crossover. We present here a joint experimental and theoretical demonstration that $0–\pi$ is a true thermodynamic phase transition. Microwave measurements of the temperature dependence of the London penetration depth in $\mathrm{Nb}/{\mathrm{Pd}}_{0.84}{\mathrm{Ni}}_{0.16}/\mathrm{Nb}$ trilayers reveal a sudden, unusual decrease of the density of the superconducting condensate (square modulus of the macroscopic quantum wave function) with decreasing temperature, which is predicted by the theory here developed as a transition from the 0 state to the $\pi$ state. Our result for the jump of the amplitude of the order parameter is a thermodynamic manifestation of such a temperature-driven quantum transition.

Confinement generally has the effect of suppressing order in condensed matter. Indeed, phase transitions such as freezing, or the superfluid transition in liquid helium, occur at lower temperatures in confinement than they do in bulk. We provide here an illustration of a physical setting in which the opposite takes place. Specifically, the enhancement of the superfluid response of parahydrogen confined to nanoscale size cavities is demonstrated by means of first-principle computer simulations. Prospects to stabilize and observe the long sought but yet elusive bulk superfluid phase of parahydrogen in purposefully designed porous media are discussed.

The damaging effect of strong electron-electron repulsion on regular, electron-phonon superconductivity is a standard tenet. In spite of that, an increasing number of compounds such as fullerides and more recently alkali-doped aromatics exhibit superconductivity despite very narrow bands and very strong electron repulsion. Here, we explore superconducting solutions of a model Hamiltonian inspired by the electronic structure of alkali-doped aromatics. The model is a two-site, two-narrow-band metal with a single intersite phonon, leading to attraction-mediated, two-order parameter superconductivity. On top of that, the model includes a repulsive onsite Hubbard $U$, whose effect on the superconductivity we study. Starting within mean field, we find that $s_{\pm }$ superconductivity is the best solution surviving the presence of $U$, whose effect is canceled out by the opposite signs of the two order parameters. The correlated Gutzwiller study that follows is necessary because without electron correlations, the superconducting state would in this model be superseded by an antiferromagnetic insulating state with lower energy. The Gutzwiller correlations lower the energy of the metallic state, with the consequence that the $s_{\pm }$ superconducting state is stabilized and even strengthened for small Hubbard $U$.

A combination of dynamical mean field theory and density functional theory, as implemented by Haule et al. [Phys. Rev. B 81, 195107 (2010)], is applied to both the early and late transition metal oxides. For a fixed value of the local Coulomb repulsion, without fine tuning, we obtain the main features of these series, such as the metallic character of ${\mathrm{SrVO}}_3$ and the insulating gaps of ${\mathrm{LaVO}}_3$, ${\mathrm{LaTiO}}_3$, and ${\mathrm{La}}_2{\mathrm{CO}}_4$, which are in good agreement with experiment. This study highlights the importance of local physics and high energy hybridization in the screening of the Hubbard interaction and how different low energy behaviors can emerge from the unified treatment of the transition metal series.

We study the stability of Dirac semimetals with $N$ nodes in three spatial dimensions against strong $1/r$ long-range Coulomb interactions. We particularly study the cases of $N=4$ and $N=16$, where the $N=4$ Dirac semimetal is described by the staggered fermions and the $N=16$ Dirac semimetal is described by the doubled lattice fermions. We take into account the $1/r$ long-range Coulomb interactions between the bulk electrons. Based on the U(1) lattice gauge theory, we analyze the system from the strong coupling limit. It is shown that the Dirac semimetals survive in the strong coupling limit when the out-of-plane Fermi velocity anisotropy of the Dirac cones is weak, whereas they change to Mott insulators when the anisotropy is strong. A possible global phase diagram of correlated multinode Dirac semimetals is presented. Implications of our result to the stability of Weyl semimetals and three-dimensional topological insulators are discussed.

The Casimir effect is a fascinating phenomenon where quantum fluctuations of the electromagnetic field give rise to measurable forces between macroscopic systems. Here we propose that the Casimir effect can be used as a tool to detect changes in electronic structures. In particular, we focus here on the Lifshitz transition—a topological change in the Fermi surface—in a planar spin-orbit-coupled semiconductor in a magnetic field and calculate the Casimir force between the semiconductor and another probe system across the magnetic-field-tuned transition. We show that the Casimir force experiences a sharp kink at the topological transition and provide numerical estimates indicating that the effect is well within experimental reach. The simplest experimental realization of the proposed effect would involve a metal-coated sphere suspended from a microcantilever above a thin layer of InSb (or another semiconductor with a large $g$ factor).

A topological Kondo insulator (TKI) is a strongly correlated material, where hybridization between the conduction electrons and localized $f$ electrons gives rise to a crossover from a metallic behavior at high temperatures to a topologically nontrivial insulating state at low temperatures. The existing description of the TKIs is based on a slave-boson mean-field theory, which neglects dynamic fluctuation phenomena. Here, we go beyond the mean-field theory and investigate the role of Kondo fluctuations on the topological surface states. We derive an effective theory of the Dirac surface states coupled to fluctuations and show that the latter mediate strong repulsive interactions between surface excitations. We show that these effects renormalize the plasmon spectrum on the surface. We also argue that Kondo-mediated interactions may drive a magnetic instability of the surface spectrum.

We have investigated the low-temperature phases of ${\mathrm{LuFe}}_2{\mathrm{O}}_4$ by resonant x-ray scattering (RXS) at the Fe $K$ edge to determine both the ordering sequence and magnitude of charge segregation. Two successive charge ordering (CO) phases have been detected. Resonant superlattice $(1/3,1/3,l/2)$ reflections appear below the so-called CO phase at $T_{\mathrm{CO}} \approx 320$ K. Additionally, resonant superlattice $(1/3,1/3,l)$ reflections are observed below 240 K concurrent with the onset of the magnetic ordering. The σ-σ′ polarization dependence for all the measured superlattice reflections indicates the absence of local anisotropy of the electronic density at the Fe atom. The energy dependence of the resonant intensity for these reflections has been quantitatively analyzed following the monoclinic $C2/m$ structure in the CO phase between 320 and 240 K and the triclinic $P\overline{1}$ structure below 240 K. We find a four-modal charge segregation among the Fe atoms in the $C2/m$ phase with formal valences $\mathrm{Fe}{}^{2.77+},\mathrm{Fe}{}^{2.63+},\mathrm{Fe}{}^{2.36+}$, and $\mathrm{Fe}{}^{2.22+}$ whereas the simplest charge distribution that explains successfully all the RXS data in the $P\overline{1}$ phase is the trimodal $\mathrm{Fe}{}^{2.8+},\mathrm{Fe}{}^{2.5+}$, and $\mathrm{Fe}{}^{2.2+}$. Both ordering models imply the lack of charge segregation along the $c$ axis discarding a polar configuration and thus the occurrence of ferroelectricity.

Density-functional-theory-based electronic-structure calculations are carried out to elucidate the mechanism of the metal-insulator transition (MIT) of a Wadsley-phase vanadium oxide ${\mathrm{V}}_6{\mathrm{O}}_{13}$. We show that, at the MIT, the orbitals occupied by electrons are reconstructed in the single trellis layers of the V(1) ions, which occurs simultaneously with the transfer of electrons from the V(2) to V(3) ions in the double trellis layers, leaving the V(2) ions nonmagnetic. We discuss that these changes lead to the formation of spin-singlet state associated with the ordering of the $d_{yz}$ and $d_{xz}$ orbitals in the V(1) zigzag chain, together with the formation of the Mott-insulator state with frustrated spin degrees of freedom in the zigzag ladder of the $d_{xy}$ orbitals of the V(3) ions; possible antiferromagnetic ordering patterns are predicted for the latter state. Thus, the spin-singlet and antiferromagnetic states coexist in spatially separated regions at lowest temperatures. The band Jahn-Teller–type instability hidden in the single trellis layer, which is the orbital ordering instability in the strong correlation limit, is suggested to cause the MIT.

We calculate the area, edge, and corner Renyi entanglement entropies in the ground state of the transverse-field Ising model, on a simple-cubic lattice, by high-field and low-field series expansions. We find that while the area term is positive and the line term is negative as required by strong subadditivity, the corner contributions are positive in three dimensions. Analysis of the series suggests that the expansions converge up to the physical critical point from both sides. The leading area-law Renyi entropies match nicely from the high- and low-field expansions at the critical point, forming a sharp cusp there. We calculate the coefficients of the logarithmic divergence associated with the corner entropy and compare them with conformal field theory results with smooth interfaces and find a striking correspondence.

The origin of the colossal magnetodielectric (CMD) effect in ${\mathrm{DyMn}}_2{\mathrm{O}}_5$ [N. Hur et al., Phys. Rev. Lett. 93, 107207 (2004)] has been an outstanding question in multiferroics. Here, we report the activation of the electric dipole mode at 4–5 ${\mathrm{cm}}^{-1}$ in an applied magnetic field, which fully accounts for the CMD effect. We examine two alternative explanations of this mode: an electromagnon and transitions between $f$-electron levels of ${\mathrm{Dy}}^{3+}$ ions. The experimental and theoretical evidence supports an electromagnon origin of the CMD effect.

We have performed an angle-resolved photoemission spectroscopy study of the ${\mathrm{BiS}}_2$-based superconductor $\mathrm{Nd}(O,F){\mathrm{BiS}}_2$. Two small electronlike Fermi surfaces around $X(\pi ,0)$ are observed, which enclose $2.4\%$ and $1.1\%$ of the Brillouin zone area, respectively, corresponding to an electron doping of $7\%$ per Bi site. The Fermi surface topology is far from the nesting scenario proposed in most of the theoretical models for the ${\mathrm{BiS}}_2$-based superconductors. The conduction bands show significant anisotropic splitting along $XM$ and $\Gamma X$, which is attributed to the cooperative effects of large spin-orbit coupling and interlayer coupling. The low-energy spectrum exhibits a weakly dispersing broad hump near the bottom of the conduction bands. This hump is drastically suppressed with increasing temperature, while the spectral weight at the Fermi level is essentially unaffected. These anomalous spectral features indicate that the electrons could be strongly coupled with the lattice in the low-temperature normal state of this superconductor.

In three-dimensional materials with a Dirac spectrum, weak short-ranged disorder is essentially irrelevant near the Dirac point. This is manifestly not the case for Coulomb disorder, where the long-ranged nature of the potential produced by charged impurities implies large fluctuations of the disorder potential even when impurities are sparse, and these fluctuations are screened by the formation of electron/hole puddles. In this paper, I present a theory of such nonlinear screening of Coulomb disorder in three-dimensional Dirac systems, and I derive the typical magnitude of the disorder potential, the corresponding density of states, and the size and density of electron/hole puddles. The resulting conductivity is also discussed.