The origins of anomalous thermal expansion in the chiral monoclinic solid $\alpha -{\mathrm{BiB}}_3{\mathrm{O}}_6$ have been studied through ab initio calculations. Positive and negative axial thermal expansion are shown to be driven by librations of borate units, elastic anisotropy, and most notably by chiral acoustic phonons involving elliptical motions of bismuth atoms. The chirality of the lattice gives rise to these modes by allowing the transverse acoustic branches to have opposite circular polarizations, only one of which couples strongly to the lattice strains. These results further the understanding of relationships between crystallographic symmetry and physical properties.

We theoretically propose a route to control magnetic and topological orders in a broad class of insulating magnets with a DC electric field. We show from the strong-coupling expansion that magnetic exchange interactions along the electric field direction are generally enhanced in Mott insulators. Employing this DC field effect, we demonstrate that several magnetic or topological ordered phases such as quantum spin liquids and Haldane-gap states can be induced if we apply a strong enough DC electric field to typical frustrated or low-dimensional magnets. Our proposal is effective especially for magnets in the vicinity of magnetic phase transition points, and would also be applicable for magnets under low-frequency AC electric fields such as terahertz laser pulses. A similar strategy of controlling exchange interactions can also be utilized in cold atomic systems.

A pressure-induced collapse of magnetic ordering in $\beta -{\mathrm{Li}}_2{\mathrm{IrO}}_3$ at $P_m\sim 1.5–2$ GPa has previously been interpreted as evidence for possible emergence of spin liquid states in this hyperhoneycomb iridate, raising prospects for experimental realizations of the Kitaev model. Based on structural data obtained at room temperature, this magnetic transition is believed to originate in small lattice perturbations that preserve crystal symmetry, and related changes in bond-directional anisotropic exchange interactions. Here we report on the evolution of the crystal structure of $\beta -{\mathrm{Li}}_2{\mathrm{IrO}}_3$ under pressure at low temperatures ($T\le 50$ K) and show that the suppression of magnetism coincides with a change in lattice symmetry involving Ir-Ir dimerization. The critical pressure for dimerization shifts from 4.4(2) GPa at room temperature to $\sim 1.5–2$ GPa below 50 K. While a direct $Fddd\to C2/c$ transition is observed at room temperature, the low temperature transitions involve new as well as coexisting dimerized phases. Further investigation of the Ir ($L_3$/$L_2$) isotropic branching ratio in x-ray absorption spectra indicates that the previously reported departure of the electronic ground state from a $J_{\mathrm{eff}}=1/2$ state is closely related to the onset of dimerized phases. In essence, our results suggest that the predominant mechanism driving the collapse of magnetism in $\beta -{\mathrm{Li}}_2{\mathrm{IrO}}_3$ is the pressure-induced formation of $\mathrm{Ir}{}_2$ dimers in the hyperhoneycomb network. The results further confirm the instability of the $J_{\mathrm{eff}}=1/2$ moments and related noncollinear spiral magnetic ordering against formation of dimers in the low-temperature phase of compressed $\beta -{\mathrm{Li}}_2{\mathrm{IrO}}_3$.

We have performed angle-resolved photoemission spectroscopy on a heterostructure consisting of topological insulator ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ and iron chalcogenide FeTe fabricated on ${\mathrm{SrTiO}}_3$ substrate by molecular-beam-epitaxy technique. This system was recently found to show the superconductivity albeit nonsuperconducting nature of each constituent material. Upon interfacing FeTe with two quintuple layers of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$, we found that the Dirac-cone surface state of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ is shifted toward higher binding energy, while the holelike band at the Fermi level originating from FeTe moves toward lower binding energy. This suggests that electron charge transfer takes place from FeTe to ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ through the interface. The present result points to the importance of hole-doped FeTe interface for the occurrence of unconventional superconductivity.

We develop a tetrahedron method for the Brillouin-zone integration of expressions that vary a lot as a function of energy. The usual tetrahedron method replaces the continuous integral over the Brillouin zone by a weighted sum over a finite number of $\mathbf{k}$ points. The weight factors are determined under the assumption that the function to be integrated be linear inside each tetrahedron, so the method works best for functions that vary smoothly over the Brillouin zone. In this paper, we describe a new method that can deal with situations where this condition is not fulfilled. Instead of weight factors, we employ weight functions, defined as piecewise cubic polynomials over energy. Since these polynomials are analytic, any function, also strongly varying ones, can be integrated accurately and piecewise analytically. The method is applied to the evaluation of the $GT$ self-energy using two techniques, analytic continuation and contour deformation. (We also describe a third technique, which is a hybrid of the two. An efficient algorithm for the dilogarithm needed for analytic continuation is formulated in Appendix.) The resulting spectral functions converge very quickly with respect to the $\mathbf{k}$-point sampling.

We present a theory for the lightly doped $t\text{−}J$ model that is of possible relevance for the normal state of underdoped cuprates. Starting from an arbitrary dimer covering of the plane, an exact representation of the $t\text{−}J$ Hamiltonian in terms of bond bosons and fermions can be derived. Since all dimer coverings must give identical results for observable quantities, we construct an approximate but translationally invariant Hamiltonian by averaging the bond particle Hamiltonian over all possible dimer coverings. Treating the resulting Hamiltonian in the mean-field approximation, we find Fermi pockets centered near $(\pi /2,\pi /2)$ with a total area of $x/2$ (with $x$ the hole concentration) and a gapped spin-wave-like band of triplet excitations.

We formulate the multiband $k·p$ theory of hyperfine interactions for semiconductor nanostructures in the envelope function approximation. We apply this theoretical description to the fluctuations of the longitudinal and transverse Overhauser field experienced by a hole for a range of InGaAs quantum dots of various compositions and geometries. We find that for a wide range of values of $d$-shell admixture to atomic states forming the top of the valence band, the transverse Overhauser field caused by this admixture is of the same order of magnitude as the longitudinal one, and band mixing adds only a minor correction to this result. In consequence, the $k·p$ results are well reproduced by a simple box model with the effective number of ions determined by the wave-function participation number, as long as the hole is confined in the compositionally uniform volume of the dot, which holds in a wide range of parameters, excluding very flat dots.

Magnetically launched flyer plates were used to investigate the shock response of beryllium between 90 and 300 GPa. Solid aluminum flyer plates drove steady shocks into polycrystalline beryllium to constrain the Hugoniot from 90 to 190 GPa. Multilayered copper/aluminum flyer plates generated a shock followed by an overtaking rarefaction which was used to determine the sound velocity in both solid and liquid beryllium between 130 and 300 GPa. Disappearance of the longitudinal wave was used to identify the onset of melt along the Hugoniot and measurements were compared to density functional theory calculations to explore the proposed hcp-bcc transition at high pressure. The onset of melt along the Hugoniot was identified at $\sim 205\mathrm{GPa}$, which is in good agreement with theoretical predictions. These results show no clear indication of an hcp-bcc transition prior to melt along the beryllium Hugoniot. Rather, the shear stress, determined from the release wave profiles, was found to gradually decrease with stress and eventually vanish at the onset of melt.

A two-dimensional electron gas in a high magnetic field displays macroscopically degenerate Landau levels, which can be split into Hofstadter subbands by means of a weak periodic potential. By carefully engineering such a potential, one can precisely tune the number, bandwidths, band gaps, and Chern character of these subbands. This allows a detailed study of the interplay of disorder, interaction, and topology in two-dimensional systems. We first explore the physics of disorder and single-particle localization in subbands derived from the lowest Landau level, that nevertheless may have a topological nature different from that of the entire lowest Landau level. By projecting the Hamiltonian onto subbands of interest, we systematically explore the localization properties of single-particle eigenstates in the presence of quenched disorder. We then introduce electron-electron interactions and investigate the fate of many-body localization in subbands of varying topological character.

Ferromagnetic particles of nano- and micrometer-size range are in high demand owing to their promising technological applications in magnetic memory storage, biomedical treatment, and microwave devices. New absorbing materials are required to possess the properties of low density and strong absorption in a broad frequency band. The dynamic susceptibility spectra of nanosized spherical shells supporting onion and vortex magnetization configurations are studied as a function of particle radius $40–125\mathrm{nm}$ and shell thickness $10–115\mathrm{nm}$ by means of numerical micromagnetic simulation. For the saturated shell, the frequencies of higher-order spin-wave modes are found to rapidly approach the subterahertz frequency regime ($200–300\mathrm{GHz}$) when the geometry is transformed from the single-domain sphere to the spherical shell. The frequency of each mode is proportional to $R_2^{-2}$ when the sphere radius outer $R_2$ is below 30 nm in reasonable agreement with the exchange approximation. Thickness-dependent vortex core gyrotropic modes were studied in the absence of an external bias field in the frequency range $0.1–30\mathrm{GHz}$. The size dependence of the vortex spin excitations is complicated by mixing of the modes at different particle sizes. For shells with large thickness and outer radii above $R_2\ge 100\mathrm{nm}$ the amplitudes of both $n=0$ and $n=1$ vortex flexural modes are less intense than even higher-frequency resonances. However, the $n=0$ mode becomes the dominant mode in the dynamic susceptibility when either the radius or thickness of the particle becomes sufficiently small.

We study the nucleation and collective dynamics of Shankar skyrmions [Shankar, J. Phys. 38, 1405 (1977)] in the class of frustrated magnetic systems described by a SO(3) order parameter, including multilattice antiferromagnets and amorphous magnets. We infer the expression for the spin-transfer torque that injects skyrmion charge into the system and the Onsager-reciprocal pumping force that enables its detection by electrical means. The thermally assisted flow of topological charge gives rise to an algebraically decaying drag signal in nonlocal transport measurements. We contrast our findings to analogous effects mediated by spin supercurrents.

Orbital magnetization is known empirically to play an important role in several magnetic phenomena, such as permanent magnetism and ferromagnetic superconductivity. Within the recently developed “modern theory of orbital magnetization,” theoretical insight has been gained into the nature of this often neglected contribution to magnetism but is based on an underlying mean-field approximation. From this theory, a few treatments have emerged which also take into account correlations beyond the mean-field approximation. Here, we apply the scheme developed in a previous work [F. Aryasetiawan et al., Phys. Rev. B 93, 161104(R) (2016)] to the spin-$\frac{1}{2}$ Haldane-Hubbard model to investigate the effect of charge fluctuations on the orbital magnetization within the $GW$ approximation. Qualitatively, we are led to distinguish between two quite different situations: (i) When the lattice potential is larger than the nearest-neighbor hopping, the correlations are found to boost the orbital magnetization. (ii) If the nearest-neighbor hopping is instead larger than the lattice potential, the correlations reduce the magnetization. The boost and reduction are identified to stem from interband and intraband correlations, respectively, and the relative importance of the two varies with the strength of the lattice potential. We finally study graphene with parameters obtained from first principles.

Magnetic skyrmions are particlelike topological excitations that have recently generated much interest as candidates for future spintronic devices based on small skyrmion size, enhanced topological stability, and/or mutual interaction. Here, we examine the properties of isolated skyrmions in a frustrated chiral magnet with competing Dzyaloshinskii-Moriya and frustrated exchange interactions. We show that the skyrmion size drastically decreases even for small values of competing stabilization mechanisms. Skyrmion mutual interaction remains attracting as is inherent for frustrated skyrmions, but the value of the Dzyaloshinskii constant regulates the number of minima in the interaction potentials. Moreover, the constructed phase diagrams for a chiral helimagnet contain a distorted spiral state that can be considered as a buffer between the helicoidal and conical one-dimensional modulations. The formulated concepts may further enhance the functionalities of spintronic devices. In particular, the controlled instability of skyrmions with respect to the conical state allows one to obtain bimeronlike structures. Moreover, our results provide physical insight into the chiral states in the magnetic systems, e.g., in ${\mathrm{MnSi}}_{1-x}{\mathrm{Ge}}_x$.

An electron paramagnetic resonance study for an organic superconductor ${\beta }^{\prime \prime }\text{−}{\left(\text{BEDT-TTF}\right)}_4\left[\left({\mathrm{H}}_3\mathrm{O}\right)\mathrm{Ga}{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_3\right]·{\mathrm{C}}_6{\mathrm{H}}_5{\mathrm{NO}}_2$ reveals that superconductivity coexists uniformly with the charge ordered state in one material. In the charge ordered state, the interplane spin exchange is gapped, while the in-plane conductivity is not significantly modified. This anisotropic behavior is explained by an exotic charge ordered state, in which molecular-site-selective carrier localization coexists with conducting carriers on other molecules. The relationship between superconductivity and this conductive charge ordered state is investigated.

${\mathrm{LaPt}}_2{\mathrm{Si}}_2$ in a single-crystalline form was subjected to structure, thermodynamic, thermal, and electron transport studies with a special emphasis on the structure phase transition appearing at $T=85$ K. X-ray diffraction proves the noncentrosymmetric tetragonal structure of ${\mathrm{CaBe}}_2{\mathrm{Ge}}_2$-type (space group P4/nmm). The transition manifests as a small step of opposite sign in both lattice parameters, leaving almost no volume change. $c$ decreases and $a$ increases in the low-$T$ phase, but the change of the $c/a$ ratio does not exceed $0.1\%$. Additional periodicity, related to the supposed charge density wave (CDW) state, can be related to satellites corresponding to the wave vector $q\approx (0.36,0,0)$, which start to grow with temperature decreasing below 175 K and almost vanish (or relocate from the investigated $ab$ plane) below the 85 K transition. Electrical resistivity reveals that the 85 K transition is hysteretic in temperature, with the difference between heating and cooling being almost 10 K, proving the first-order type of the transition. The transition dramatically enhances resistivity in the low-$T$ state, pointing to a formation of a pseudogap. This, however, does allow a superconducting state, arising below $T_c=1.6$ K. The temperature dependence up upper critical field is not compatible with the weak coupling BCS theory. Strong anisotropy of electronic structure and its dramatic changes at the structure transition are manifest also in thermal expansion and thermoelectric power. The Sommerfeld coefficient $\gamma =7.8\mathrm{mJ}{\mathrm{mol}}^{-1}{\mathrm{K}}^{-2}$ and Debye temperature of 205 K could be derived at low temperatures, but the specific heat has a strongly non-Debye like $T$ dependence, which can be ascribed to a low-energy Einstein mode.

We report the novel critical behavior of magnetization in the low carrier concentration systems UTeS and USeS that exhibit the large negative magnetoresistance around the ferromagnetic transition temperatures $T_{\mathrm{C}}\sim 85$ and 23 K, respectively. UTeS and USeS crystallize in the same orthorhombic TiNiSi-type crystal structure as those of uranium ferromagnetic superconductors URhGe and UCoGe. We determine the critical exponents $\beta$ for the spontaneous magnetization $M_{\mathrm{s}}, \gamma$ for the magnetic susceptibility $\chi$, and $\delta$ for the magnetization isotherm at $T_{\mathrm{C}}$ with several methods. The ferromagnetic states in UTeS and USeS have strong uniaxial magnetic anisotropy. However, the critical exponents in the two compounds are different from those in the three-dimensional Ising model with short-range magnetic exchange interactions. Similar sets of the critical exponents have been reported for the uranium ferromagnetic superconductors ${\mathrm{UGe}}_2$ and URhGe, and uranium intermetallic ferromagnets URhSi, UIr, and $\mathrm{U}({\mathrm{Co}}_{0.98}{\mathrm{Os}}_{0.02}$)Al. The universality class of the ferromagnetic transitions in UTeS and USeS may belong to the same one for the uranium compounds. The novel critical phenomenon associated with the ferromagnetic transition is observed not only in the uranium intermetallic ferromagnets with the itinerant $5f$ electrons but also in the low carrier concentration systems UTeS and USeS with the localized $5f$ electrons. The large negative magnetoresistance in UTeS and USeS, and the superconductivity in ${\mathrm{UGe}}_2$ and URhGe, share the similarity of their closeness to the ferromagnetism characterized by the novel critical exponents.

Residual entropy is a key feature associated with emergence in many-body systems. From a variety of frustrated magnets to the onset of spin-charge separation in Hubbard models and fermion-$Z_2$-flux variables in Kitaev models, the freezing of one set of degrees of freedom and the establishment of local constraints are marked by a plateau in entropy as a function of temperature. Yet, with the exception of the rare-earth pyrochlore family of spin-ice materials, evidence for such plateaus is rarely seen in real materials, raising questions about their robustness. Following recent experimental findings of the absence of such plateaus in the triangular-lattice Ising antiferromagnet (TIAF) ${\mathrm{TmMgGaO}}_4$ by Li et al., we explore in detail the existence and rounding of entropy plateaus in TIAF. We use a transfer-matrix method to numerically calculate the properties of the system at different temperatures and magnetic fields, with further neighbor interactions and disorder. We find that temperature windows of entropy plateaus exist only when second-neighbor interactions are no more than a couple of percent of the nearest-neighbor ones, and they are also easily destroyed by disorder in the nearest-neighbor exchange variable, thereby explaining the challenge in observing such effects.

We present a microscopic study of nematicity and magnetism in FeSe over a wide temperature and pressure range using high-energy x-ray diffraction and time-domain Mössbauer spectroscopy. The low-temperature magnetic hyperfine field increases monotonically up to $\sim 6$ GPa. The orthorhombic distortion initially decreases under increasing pressure but is stabilized at intermediate pressures by cooperative coupling to the pressure-induced magnetic order. Close to the reported maximum of the superconducting critical temperature at $p=6.8\mathrm{GPa}$, the orthorhombic distortion suddenly disappears and a new tetragonal magnetic phase occurs. The pressure and temperature evolution of the structural and magnetic order parameters suggests that they have distinct origins.

We report the magnetization, the electrical resistivity, specific heat measurements, and band structure calculations of the layered superconductor ${\mathrm{SnTaS}}_2$. The experiments are performed on single crystals grown by the chemical vapor transport method. The resistivity and magnetic susceptibility indicate that ${\mathrm{SnTaS}}_2$ is a type-II superconductor with the transition temperature $T_c=3$ K. The upper critical field $\left(H_{c2}\right)$ shows large anisotropy for the magnetic field parallel to the $ab$ plane $\left(H//ab\right)$ and the $c$ axis $\left(H//c\right)$, and the temperature dependence of $H_{c2}$ for $H//ab$ shows an obvious unconventional upward feature at low temperature. The band structure of ${\mathrm{SnTaS}}_2$ shows several band crossings near the Fermi level, which form three nodal lines in the $k_z=0$ plane resulting in drumheadlike surface states when spin-orbit coupling is not considered. These results indicate that ${\mathrm{SnTaS}}_2$ is a superconductor with a possible topological nodal line semimetal character.