The Kitaev-Heisenberg model defined on both honeycomb and triangular lattices has been studied intensively in recent years as a possible model to describe spin-orbital physics in iridium oxides. In the model, there are many phases characteristic for each lattice. However, there is no study of how the phases in the two lattices merge with each other when geometry changes from the honeycomb lattice to triangular lattice. We investigate the ground state of the Kitaev-Heisenberg model defined on the system connecting the honeycomb and triangular lattices, named a honeycomb-triangular lattice. We obtain a ground state phase diagram of this model with classical spins by using the Luttinger-Tisza method and classical Monte Carlo simulation. In addition to known phases in the honeycomb and triangular lattices, we find coexisting phases consisting of the known phases. From the exact diagonalization and density-matrix renormalization-group calculations for the model with quantum spins, we find phases similar to the classical results, implying a small effect of quantum fluctuation on the phase diagram.

Aftereffect measurements have been performed on synthetic ferrimagnets showing strong perpendicular magnetic anisotropy, namely (Co/Pt)/Ir/(Co/Pt), by measuring the magnetization of the sample as a function of time for several different applied magnetic fields. Unexpected magnetic relaxation has been observed. Indeed, for some particular applied magnetic fields, the magnetization as a function of time first plummets and then increases. This nonmonotonic aftereffect can be understood by considering the possible magnetic states and the transitions between those states. This understanding has been confirmed and detailed using magneto-optical Kerr effect (MOKE) microscopy. Indeed, the measurements show nucleation and propagation of a transient metastable magnetic configuration. Furthermore, we were able to obtain a good qualitative agreement between a simple one-dimensional model and the experimental observation. We could strongly support the hypothesis that this peculiar nonmonotonic behavior could be a very general feature that should be observed in any antiferromagnetically coupled system (synthetic ferrimagnets, synthetic antiferromagnets) with perpendicular magnetic anisotropy.

The spin polarization induced by a temperature gradient (heat current) in a magnetized two-dimensional electron gas (2DEG) with a Rashba spin-orbit interaction is considered theoretically within the linear response theory. Using the Matsubara Green's function formalism, we calculate the temperature dependence of the spin polarization for arbitrary orientation of the exchange field. The limit of a nonmagnetic 2DEG (zero exchange field) is also considered. Role of vertex corrections is investigated, and physical mechanisms of the spin polarization emergence are discussed. For a magnetic system, we identify a term in the spin polarization that stems from the Berry curvature of the corresponding electronic bands.

We report on the magnetotransport properties of a spin-polarized two-dimensional electron system (2DES) formed in ${\mathrm{LaAlO}}_3\text{(LAO)}\text{/}{\mathrm{EuTiO}}_3\text{/}{\mathrm{SrTiO}}_3$ (STO) heterostructures. We show that, at low temperature, the 2DES magnetoconductance exhibits weak antilocalization corrections related to Rashba spin-orbit scattering, in analogy with the LAO/STO 2DES. However, the characteristic spin-orbit scattering field decreases substantially for carrier density higher than $1.9\times {10}^{13}{\text{cm}}^{-2}$. We attribute this behavior to the masking effect of ferromagnetism, which sets in at the same carrier density and at a temperature below 10 K. Our work shows that, while weak antilocalization corrections to the magnetoconductance are strongly reduced by the emergence of ferromagnetism, they persist in a large part of the phase diagram of a spin-polarized oxide 2DES.

Comparing the adsorption heights of various graphene nanoribbons on Cu(111) and Au(111) surfaces to those of graphene and $\pi$-conjugated planar organic molecules, we observe that two-dimensional graphene adsorbs much further away from the surface than both one-dimensional graphene nanoribbons and $\pi$-conjugated planar molecules—which represent zero-dimensional graphene flakes. We show that this is a direct consequence of the adsorbates' dimensionality. Our results provide invaluable insights into the interplay of Pauli repulsion, pushback effect, and chemical interaction for graphenelike adsorbates of any dimensionality on metal surfaces.

Plasmon spectroscopy methods are highly sensitive to the small volumes of material due to subwavelength localization of light increasing light-matter interaction. Recent research has shown a high potential of plasmon quantum generator (spaser) or amplifier (sped) for sensing in the infrared optical region. Trinitrotoluene (TNT) molecules fingerprints are considered as an example. Basing on Lindblad equations, we implement full quantum mechanical theory of graphene plasmon generator to investigate how a small amount of absorbing atoms influences the spectrum of a graphene spaser. We analyze the optimal type of an active medium, the number of active molecules, and the pump level to achieve the highest sensitivity and show that optimized structure is sensitive to dozens of atoms. Our research is useful for the development of near- and mid-IR spectroscopy based on plasmon quantum amplifier.

The electron-doping-driven collapse of the charge gap and staggered magnetization of the spin-orbit-assisted Mott insulator ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$ is explored via first-principles computational methods. In the antiferromagnetic phase, the gap and magnetization are observed to decrease slowly with increasing doping, with an abrupt collapse of both the gap and the magnetization at an electron concentration corresponding to 4.8% substitution of Sr with La, in excellent agreement with experiment. Additionally, we describe the structural effects of electron doping in ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$ via a competition between the steric effect from smaller La atoms substituted within the lattice and the dominant doping-driven deformation-potential effect. Curiously, our first-principles calculations fail to capture the low-temperature structural distortion reported in the low-gap phase of ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$, supporting the notion that this distortion arises as a secondary manifestation of an unconventional electronic order parameter in this material.

We numerically assess model wave functions for the recently proposed particle-hole-symmetric Pfaffian (“PH-Pfaffian”) topological order, a phase consistent with the recently reported thermal Hall conductance [M. Banerjee et al., Nature 559, 205 (2018)] at the ever enigmatic $\nu =5/2$ quantum Hall plateau. We find that the most natural Moore-Read-inspired trial state for the PH-Pfaffian, when projected into the lowest Landau level, exhibits a remarkable numerical similarity on accessible system sizes with the corresponding (compressible) composite Fermi liquid. Consequently, this PH-Pfaffian trial state performs reasonably well energetically in the half-filled lowest Landau level, but is likely not a good starting point for understanding the $\nu =5/2$ ground state. Our results suggest that the PH-Pfaffian model wave function either encodes anomalously weak $p$-wave pairing of composite fermions or fails to represent a gapped, incompressible phase altogether.

We demonstrate the possibility to reduce the heating of nanoelectronic systems in nonequilibrium situations by increasing the temperature of the environment. Such cooling by heating is possible for a variety of experimental conditions where the relevant transport-induced excitation processes become quenched and deexcitation processes are enhanced upon an increase of temperature. The phenomenon turns out to be robust with respect to all relevant parameters. It is especially pronounced for higher bias voltages and weak to moderate coupling. Our findings have implications for open quantum systems in general, where electron transport is coupled to mechanical (phononic) or photonic degrees of freedom. In particular, molecular junctions with rigid tunneling pathways or quantum dot circuit QED systems meet the required conditions.

Band crossings that occur on a mirror plane are compelled to form nodal loops in the momentum space in the absence of spin-orbit coupling (SOC). When other equivalent mirror planes are present, multiple such nodal loops can combine to form interesting topological structures involving crossed nodal lines. Here, based on first-principles calculations and an effective $\mathbf{k}·\mathbf{p}$ model analysis, we show that CaAuAs hosts a unique starfruitlike crossed-nodal-line structure in the bulk electronic dispersion, which is comprised of three nodal loops that cross each other at the time-reversal-invariant momentum point $A$. When the SOC is turned on, the nodal loops are gapped out, resulting in a stable Dirac semimetal state with a pair of Dirac points along the $\mathrm{\Gamma }\text{−}A$ direction in the Brillouin zone. These Dirac points are protected by the combination of time-reversal, inversion, and $C_3$ rotation symmetries. We discuss how a systematic elimination of the symmetry constraints yields a Weyl semimetal and eventually a topological insulator state.

We show that the Joule heating in Hall devices can be optimized by engineering the shape of the device. By using a geometrical method based on conformal transformations, we show the existence of two equivalence classes for Hall devices, exemplified by the Hall bar and the Corbino disk. The elements inside each class are related by means of a conformal mapping. Whereas the power dissipated by the systems of the Hall bar class formed by simply connected systems is conformally invariant and equal for each element of the class, that for systems of the Corbino disk class whose elements are doubly connected depends on their geometry and can thus be controlled by modifying the shape of the device. This general property could be used for a better performance of Hall devices.

The high pressure structural phase diagram of ${\mathrm{EuTiO}}_3$ is revisited by means of single crystal x-ray diffraction at pressures below $30\text{GPa}$. We report a transition at about $3\text{GPa}$ with a surprisingly small lattice distortion. At $23\text{GPa}$ we have detected a yet unknown transition into a new orthorhombic phase (space group $Ibam$). Combining our observations on structures obtained on single crystals and polycrystalline samples with complementary methods, such as electrical resistivity and nuclear forward scattering of synchrotron radiation measured on polycrystalline samples, we address issues relevant to chemistry and disorder.

In the last decade, significant progress has been made in the many-body Hamiltonian formulation of liquid dynamics theory. Earlier analysis of experimental data for a wide variety of elemental liquids had provided the reliable but qualitative description of the atomic motion: vibrations in a representative $3N$-dimensional potential energy valley, plus transits, in which atoms cross the intersections between these valleys. Recent comparisons of first-principles theory with experiment for several elemental liquids at melt revealed a highly accurate and versatile theory. In the present work, we report on an extensive quench study of the entire condensed-matter structure-energy distribution for a metallic Na MD system. With these results, all that was learned from experimental data is confirmed, refined in detail, and made more accurate. We show the entire structure-energy distribution, composed of widespread symmetrics and higher lying randoms, we show the increasing dominance of the randoms as $N$ increases, until the symmetrics vanish completely, and we show the random distribution continue its spectacular narrowing as $N$ continues to increase. This behavior certifies our early assignment of the random distribution to the liquid phase and our prediction of macroscopic uniformity of the random structures. Procedures are discussed to identify and calibrate a single random structure to represent the liquid, and the role of the structure energy in liquid thermodynamics is described. A comparison with other liquid dynamics theories is observed in the Introduction, and the relation to the equation-of-state program is noted in the Conclusion.

The dynamics of the phonon scattering caused by the guest-host interactions in fully filled skutterudites $\left(\mathrm{Yb},\mathrm{La},\mathrm{Ba}\right)\mathrm{F}{\mathrm{e}}_4\mathrm{S}{\mathrm{b}}_{12}$ are studied by ab initio molecular dynamics. The characteristics of filler vibrations are found to be the origin of ultralow lifetimes for the filler-dominant modes. Under the phonon-phonon interaction diagram, the large amplitudes of the filler vibrations and the coupling between filler and framework phonon modes greatly increase the strength of phonon scatterings. The lattice thermal conductivities for the three skutterudites, however, are all overestimated comparing with the experimental results. The introduction of the resonant scattering is thus necessary to account for the deviations. Furthermore, by applying the wavelet-based analysis to the resonant scattering process, the time-frequency power spectrum shows clearly that the time-varying localized motions of the fillers periodically absorb the heat-carrying lattice phonons and emit them. This process is the origin of the resonant scattering mechanism in skutterudites, and strongly interferes with the propagation of phonons near the frequency range of the fillers.

We present an experimental investigation of the magnetic structure in a tetramer system ${\mathrm{SeCuO}}_3$ using neutron powder-diffraction and nuclear resonance techniques. We establish a commensurate antiferromagnetic ordering with a propagation vector $\mathbf{k}=(0,0,1)$. The order parameter follows a critical behavior near $T_N=8\mathrm{K}$ with a critical exponent $\beta =0.32$ in agreement with a three-dimensional universality class. Evidence is presented that a singlet state starts to form on tetramers at temperatures as high as 200 K, and its signature is preserved within the ordered state through a strong renormalization of the ordered magnetic moment on two nonequivalent copper sites $m_{\mathrm{Cu}1}\approx 0.35{\mu }_{\mathrm{B}}$ and $m_{\mathrm{Cu}2}<0.8{\mu }_{\mathrm{B}}$ at 1.5 K.

Indium-doped SnTe has been of interest because the system can exhibit both topological surface states and bulk superconductivity. While the enhancement of the superconducting transition temperature is established, the character of the electronic states induced by indium doping remains poorly understood. We report a study of magnetotransport in a series of ${\mathrm{Sn}}_{1-x}{\mathrm{In}}_x\mathrm{Te}$ single crystals with $0.1\le x\le 0.45$. From measurements of the Hall effect, we find that the dominant carrier type changes from holelike to electronlike at $x\sim 0.25$; one would expect electronlike carriers if the In ions have a valence of $+3$. For single crystals with $x=0.45$, corresponding to the highest superconducting transition temperature, pronounced Shubnikov–de Haas oscillations are observed in the normal state. In measurements of magnetoresistance, we find evidence for weak antilocalization (WAL). We attribute both the quantum oscillations and the WAL to bulk Dirac-like hole pockets, previously observed in photoemission studies, which coexist with the dominant electronlike carriers.

We propose the use of recurrent neural networks for classifying phases of matter based on the dynamics of experimentally accessible observables. We demonstrate this approach by training recurrent networks on the magnetization traces of two distinct models of one-dimensional disordered and interacting spin chains. The obtained phase diagram for a well-studied model of the many-body localization transition shows excellent agreement with previously known results obtained from time-independent entanglement spectra. For a periodically driven model featuring an inherently dynamical time-crystalline phase, the phase diagram that our network traces coincides with an order parameter for its expected phases.

Multishock compression experiments on hydrogen-helium (${\mathrm{H}}_2$-He) mixtures have been performed via a two-stage light-gas gun for providing equations of state (EOS) covering a wide pressure range. The initial gaseous ${\mathrm{H}}_2$-He sample was precompressed to 20–30 MPa and cooled down to around 90 K to gain high initial sample density. Up to three shock compressions were clearly observed from the time-resolved light radiance of the shocked sample recorded by a multichannel optical pyrometer. The measured EOS data of the ${\mathrm{H}}_2$-He mixture reached an unexplored range of pressure up to 60 GPa, which is well in the molecular-to-atomic transition regime. The wide-range experimental data are used to validate the state-of-the-art first-principles simulation methods. It is found that the density functional theory molecular dynamics (DFT-MD) simulations underestimate the dissociation of hydrogen and therefore predict the ${\mathrm{H}}_2$-He EOS to be stiffer than the experimental data in the molecular-to-atomic transition regime. A careful analysis of the pair correlation functions and comparison with the results of pure hydrogen revealed that DFT-MD might overestimate the effects of helium on the bond of the hydrogen molecule when hydrogen is mixed with helium. Finally, the current measurements validate a linear-mixing ab initio EOS [Astrophys. J. Suppl. S. 215, 21 (2014)] widely used in astrophysics.

We synthesized magnetically frustrated ${\mathrm{DyMn}}_2{\mathrm{O}}_5$ nanoparticles in pores of mesoporous silica, with particle sizes ranging from 7 to 20 nm, and investigated their magnetostructural correlation. We found that the lattice constants of the ${\mathrm{DyMn}}_2{\mathrm{O}}_5$ nanoparticles deviated from those of the bulk crystal below $\sim 12$ nm and their crystallographic structures at the unit cell level were distorted. The size dependences of the blocking temperature and coercive field drastically change at $\sim 12$ nm. In addition, the Weiss temperature depends strongly on particle size, and its sign changes at $\sim 12$ nm. It is considered that such features can be realized owing to the distortion caused by the ligand atoms at the surface. The orbital structures of the magnetic sites are easily modified due to the distortion of the ligand ions at the surface, so that the correlation between the crystal structure and magnetic properties can be enhanced. Moreover, magnetization of the nanoparticle results in quasi-superparamagnetic behavior. Monte Carlo calculation of the nanoparticles indicates that such a feature is realized due to the quasi-free spins induced at the surface by magnetic frustration.

Topological superconductors are characterized by topological invariants that describe the number and nature of their robust boundary modes. These invariants must also have observable consequences in the bulk of the system, akin to the quantized bulk Hall conductivity in the quantum Hall effect, but such consequences are made elusive by the spontaneous breaking of U(1) symmetry in the superconductor. Here we focus on 2+1-dimensional spinless $p$-wave superconductors and show that emergent gravity serves as a natural bulk probe for their topological invariant. This emergent gravity is due to the same attractive interaction between fermions that leads to superconductivity, and is therefore built into topological superconductors. The bulk response of a topological superconductor to the emergent gravitational field is encoded in a gravitational Chern-Simons term, and is related to the existence of robust boundary modes via energy-momentum conservation, or gravitational anomaly inflow. The gravitational Chern-Simons term implies a universal relation between variations in the superconducting order parameter and the energy-momentum currents and densities that they induce. The spontaneous breaking of U(1) symmetry in the superconductor leads to additional bulk responses, encoded in a gravitational pseudo Chern-Simons term. Although not of topological nature, these carry surprising similarities to the topological responses of the gravitational Chern-Simons term. We show how these two types of responses can be disentangled.