We report specific heat measurements on a high-quality single crystal of the heavy-fermion compound ${\mathrm{CePt}}_2{\mathrm{In}}_7$ in magnetic fields up to 27 T. The zero-field specific heat data above the Néel temperature, $T_N$, suggest a moderately enhanced value of the electronic specific heat coefficient $\gamma =180\mathrm{mJ}/{\mathrm{K}}^2\mathrm{mol}$. For $T<T_N$, the data at zero applied magnetic field are consistent with the existence of an anisotropic spin-density wave opening a gap on almost the entire Fermi surface, suggesting extreme two-dimensional electronic and magnetic structures for ${\mathrm{CePt}}_2{\mathrm{In}}_7$. $T_N$ is monotonically suppressed by magnetic field applied along the $c$ axis. When a field is applied parallel to the $a$ axis, $T_N$ first increases at low field up to about 10 T and then decreases monotonically at a higher field. The magnetic phase diagram based on specific heat measurements suggests that a field-induced quantum critical point is likely to occur slightly below 60 T for both principal orientations of the magnetic field.

We present an experimental study of the double-quantum-well (DQW) system made of two-dimensional layers with inverted energy band spectrum: HgTe. The magnetotransport reveals a considerably larger overlap of the conduction and valence subbands than in known HgTe single quantum wells (QW), which may be regulated here by an applied gate voltage $V_g$. This large overlap manifests itself in a much higher critical field $B_c$ separating the range above it with a plain behavior of the Hall magnetoresistance ${\rho }_{xy}\left(B\right)$, where the quantum peculiarities shift linearly with $V_g$, and the range below with a complicated behavior. In the latter case, specific structures in ${\rho }_{xy}\left(B\right)$ are formed like a double-$N$-shaped ${\rho }_{xy}\left(B\right)$, reentrant sign-alternating quantum Hall effect with transitions into a zero-filling-factor state, etc., which are clearly manifested here due to better magnetic quantization at high fields, as compared to the features seen earlier in a single HgTe QW. The coexisting electrons and holes were found in the whole investigated range of positive and negative $V_g$ as revealed (i) from fits to the low-field $N$-shaped ${\rho }_{xy}\left(B\right)$, (ii) from the Fourier analysis of oscillations in ${\rho }_{xx}\left(B\right)$, and (iii) from a specific behavior of ${\rho }_{xy}\left(B\right)$ at high positive $V_g$. A peculiar feature here is that the found electron density $n$ remains almost constant in the whole range of investigated $V_g$ while the hole density $p$ drops down from the value a factor of 6 larger than $n$ at extreme negative $V_g$ to almost zero at extreme positive $V_g$ passing through the charge-neutrality point. We show that this difference between $n$ and $p$ stems from an order of magnitude larger density of states for holes in the lateral valence subband maxima than for electrons in the conduction subband minimum. We analyze our observations on the basis of a calculated picture of magnetic levels in a DQW and suggest that their specificity is due to (i) a nonmonotonic course of the valence subband magnetic levels and an oscillating behavior of the valence subband top versus field due to lateral maxima in the energy spectrum, (ii) a reduced gap between the lowest-electron and the highest-hole magnetic levels where the electron- and hole-type localized states are superposed, and (iii) a possible formation of the interlayer electron-hole excitons.

We use molecular dynamics phonon wave packet (WP) simulations to study acoustic propagation and attenuation in amorphous silica $\left(a\text{−}\mathrm{Si}{\mathrm{O}}_2\right)$ at frequencies near the boson peak (BP) position and compare them with the results of equilibrium molecular dynamics (EMD) simulations. The sound attenuation coefficients obtained from WP simulations are generally consistent with those from EMD predictions and have reasonable agreement with the existing experimental data. Near the BP position, we found the frequency-dependent sound attenuation coefficients for longitudinal and transverse modes both follow the Rayleigh-scattering fourth power law. Above the BP frequency, however, the propagating phonon is essentially attenuated in $a\text{−}\mathrm{Si}{\mathrm{O}}_2$ within a few nanometers, and the accurate determination of the sound attenuation coefficients by the WP simulation becomes challenging. The modeling results provide a reference for future experimental investigations of sound attenuation in $a\text{−}\mathrm{Si}{\mathrm{O}}_2$ thin film using narrow-band coherent phonons.

We report a thermoelectric investigation of the stripe and superconducting phases of the cuprate ${\mathrm{La}}_{2-x}{\mathrm{Ba}}_x{\mathrm{CuO}}_4$ near $x=1/8$. We vary the doping and the magnetic field to identify features in the field-symmetric Nernst effect consistent with the reports of time-reversal symmetry breaking above the superconducting $T_{\mathrm{c}}$. Crucially, we further detect a field-invariant peak at the stripe charge order temperature, $T_{\mathrm{CO}}$. Our observations suggest the onset of a nontrivial charge ordered phase at $T_{\mathrm{CO}}$, and the subsequent presence of spontaneously generated vortices before the emergence of bulk superconductivity.

A conformal pinning array is created by taking a conformal transformation of a uniform hexagonal lattice to create a structure in which the sixfold ordering of the original lattice is preserved but which has a spatial gradient in the pinning site density. With a series of conformal arrays it is possible to create asymmetric substrates, and it was previously shown that when an ac drive is applied parallel to the asymmetry direction, a pronounced ratchet effect occurs with a net dc flow of vortices in the same direction as the ac drive. Here we show that when the ac drive is applied perpendicular to the substrate asymmetry direction, it is possible to realize a transverse ratchet effect where a net dc flow of vortices is generated perpendicular to the ac drive. The conformal transverse ratchet effect is distinct from previous versions of transverse ratchets in that it occurs due to the generation of non-Gaussian transverse vortex velocity fluctuations by the plastic motion of vortices, so that the system behaves as a noise correlation ratchet. The transverse ratchet effect is much more pronounced in the conformal arrays than in random gradient arrays and is absent in square gradient arrays due the different nature of the vortex flow in each geometry. We show that a series of reversals can occur in the transverse ratchet effect due to changes in the vortex flow across the pinning gradient as a function of vortex filling, pinning strength, and ac amplitude. We also consider the case where a dc drive applied perpendicular to the substrate asymmetry direction generates a net flow of vortices perpendicular to the dc drive, producing what is known as a geometric or drift ratchet that again arises due to non-Gaussian dynamically generated fluctuations. The drift ratchet is more efficient than the ac driven ratchet and also exhibits a series of reversals for varied parameters. Our results should be general to a wide class of systems undergoing nonequilibrium dynamics on conformal substrates, such as colloidal particles on optical traps.

We report how the superconducting phase forms in pseudo-single-crystal ${\mathrm{K}}_x\mathrm{F}{\mathrm{e}}_{2-y}\mathrm{S}{\mathrm{e}}_2$. In situ scanning electron microscopy (SEM) observation reveals that, as an order-disorder transition occurs, on cooling, most of the high-temperature iron-vacancy-disordered phase gradually changes into the iron-vacancy-ordered phase, whereas a small quantity of the high-temperature phase retains its structure and aggregates to the stripes with more iron concentration but less potassium concentration compared to the iron-vacancy-ordered phase. The stripes that are generally recognized as the superconducting phase are actually formed as a remnant of the high-temperature phase with a compositional change after an “imperfect” order-disorder transition. It should be emphasized that the phase separation in pseudo-single-crystal ${\mathrm{K}}_x\mathrm{F}{\mathrm{e}}_{2-y}\mathrm{S}{\mathrm{e}}_2$ is caused by the iron-vacancy order-disorder transition. The shrinkage of the high-temperature phase and the expansion of the newly created iron-vacancy-ordered phase during the phase separation rule out the mechanism of spinodal decomposition proposed in an early report [Z. Wang et al., Phys. Rev. B 91, 064513 (2015)]. Since the formation of the superconducting phase relies on the occurrence of the iron-vacancy order-disorder transition, it is impossible to synthesize a pure superconducting phase by a conventional solid state reaction or melt growth. By focused ion beam scanning electron microscopy, we further demonstrate that the superconducting phase forms a contiguous three-dimensional architecture composed of parallelepipeds that have a coherent orientation relationship with the iron-vacancy-ordered phase.

We report high-resolution inelastic x-ray scattering measurements of the acoustic phonons of ${\mathrm{URu}}_2{\mathrm{Si}}_2$. At all temperatures, the longitudinal acoustic phonon linewidths are anomalously broad at small wave vectors revealing a previously unknown anharmonicity. The phonon modes do not change significantly upon cooling into the hidden order phase. Our data suggest that the increase in thermal conductivity in the hidden order phase cannot be driven by a change in phonon dispersions or lifetimes. Hence, the phonon contribution to the thermal conductivity is likely much less significant compared to that of the magnetic excitations in the low temperature phase.

We present the results of high-pressure transport measurements on the anion-mixed molecular conductors (DIETSe)${}_{2}M{\mathrm{Br}}_2{\mathrm{Cl}}_2$ [DIETSe = diiodo(ethylenedithio)tetraselenafulvalene; $M=\mathrm{Fe}$, Ga]. They undergo a metal-insulator (M-I) transition below 9 K at ambient pressure, which is suppressed by applying pressure, indicating a spin-density-wave (SDW) transition caused by a nesting instability of the quasi-one-dimensional (Q1D) Fermi surface, as observed in the parent compounds (DIETSe)${}_{2}M{\mathrm{Cl}}_4$ ($M=\mathrm{Fe}$, Ga). In the metallic state, the existence of the Q1D Fermi surface is confirmed by observing the Lebed resonance. The critical pressures of the SDW, $P_{\mathrm{c}}$, of the $M{\mathrm{Br}}_2{\mathrm{Cl}}_2$ ($M=\mathrm{Fe}$, Ga) salts are significantly lower than those of the the $M{\mathrm{Cl}}_4$ ($M$ = Fe, Ga) salts, suggesting chemical pressure effects. Above $P_{\mathrm{c}}$, field-induced SDW transitions appear, as evidenced by kink structures in the magnetoresistance (MR) in both salts. The ${\mathrm{FeBr}}_2{\mathrm{Cl}}_2$ salt also shows antiferromagnetic (AF) ordering of $d$ spins at 4 K, below which significant spin-charge coupling is observed. A large positive MR change up to 150% appears above the spin-flop field at high pressure. At low pressure, in particular below $P_{\mathrm{c}}$, a dip or kink structure appears in MR at the spin-flop field, which shows unconventionally large hysteresis at low temperature $(T<1$ K). The hysteresis region clearly decreases with increasing pressure towards $P_{\mathrm{c}}$, strongly indicating that the coexisting SDW plays an important role in the enhancement of magnetic hysteresis besides the random exchange interaction.

By using quasiparticle self-consistent GW calculations, we reexamined the electronic structure of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ and ${\mathrm{SrRuO}}_3$. Our calculations show that the correlation effects beyond the conventional local density approximation and generalized gradient approximation are reasonably well captured by the QSGW self-energy without any ad hoc parameter or any ambiguity related to the double-counting and the downfolding issues. While spectral weight transfer to the lower and upper Hubbard band is not observed, the noticeable bandwidth reduction and effective-mass enhancement are obtained. Important features in the electronic structures that have been debated over the last decades such as the photoemission spectra at around $-3$ eV in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ and the half-metallicity for ${\mathrm{SrRuO}}_3$ are discussed in the light of our QSGW results and in comparison with the previous studies. The promising aspects of QSGW are highlighted as a first-principles calculation method to describe the moderately correlated $4d$ transition-metal oxides along with the limitations of QSGW.

Using the transfer matrix in the angular-momentum space we investigate the impact of trigonal warping on magnetotransport and scaling properties of a ballistic bilayer graphene in the Corbino geometry. Although the conductivity at the charge-neutrality point and zero magnetic field exhibits a one-parameter scaling, the shot-noise characteristics, quantified by the Fano factor $\mathcal{F}$ and the third charge-transfer cumulant $\mathcal{R}$, remain pseudodiffusive. This shows that the pseudodiffusive transport regime in bilayer graphene is not related to the universal value of the conductivity but can be identified by higher charge-transfer cumulants. For Corbino disks with larger radii ratios, the conductivity is suppressed by the trigonal warping, mainly because the symmetry reduction amplifies backscattering for normal modes corresponding to angular-momentum eigenvalues $\pm 2\hslash$. Weak magnetic fields enhance the conductivity, reaching the maximal value near the crossover field $B_L=\frac{4}{3}\sqrt{3}(\hslash /e)t^{\prime }{t}_{\perp }{\left[t_0^{2}a(R_{\mathrm{o}}-R_{\mathrm{i}})\right]}^{-1}$, where $t_0$ ($t_{\perp }$) is the nearest-neighbor intralayer (interlayer) hopping integral, $t^{\prime }$ is the skew-interlayer hopping integral, and $R_{\mathrm{o}}$ ($R_{\mathrm{i}}$) is the outer (inner) disk radius. For magnetic fields $B\gtrsim B_L$ we observe quasiperiodic conductance oscillations characterized by the decreasing mean value $\langle \sigma \rangle -{\sigma }_0\propto B_L/B$, where ${\sigma }_0=(8/\pi )e^2/h$. The conductivity, as well as higher charge-transfer cumulants, show beating patterns with an envelope period proportional to $\sqrt{B/B_L}$. This constitutes a qualitative difference between the high-field ($B\gg B_L$) magnetotransport in the $t^{\prime }=0$ case [earlier discussed in Rut and Rycerz, J. Phys.: Condens. Matter 26, 485301 (2014)] and in the $t^{\prime }\ne 0$ case, providing a finite-system analog of the Lifshitz transition.

We present first-principles calculations of elastic properties of multilayered two-dimensional crystals, such as graphene, $h$-BN, and $2H\text{−}{\mathrm{MoS}}_2$, which show that their Poisson's ratios along the out-of-plane direction are negative, near zero, and positive, respectively, spanning all possibilities for the sign of the ratios. While the in-plane Poisson's ratios are all positive regardless of their disparate electronic and structural properties, the characteristic interlayer interactions as well as layer stacking structures are shown to determine the sign of their out-of-plane ratios. A thorough investigation of elastic properties as a function of the number of layers for each system is also provided, highlighting their intertwined nature between elastic and electronic properties.

A Cooper pair splitter consists of two quantum dots side-coupled to a conventional superconductor. Usually, the quantum dots are assumed to have a large charging energy compared to the superconducting gap, in order to suppress processes other than the coherent splitting of Cooper pairs. In this work, in contrast, we investigate the limit in which the charging energy is smaller than the superconducting gap. This allows us, in particular, to study the effect of a Zeeman field comparable to the charging energy. We find analytically that in this parameter regime the superconductor mediates an interdot tunneling term with a spin symmetry determined by the Zeeman field. Together with electrostatically tunable quantum dots, we show that this makes it possible to engineer a spin triplet state shared between the quantum dots. Compared to previous works, we thus extend the capabilities of the Cooper pair splitter to create entangled nonlocal electron pairs.

By using a combination of heteroepitaxial growth, structure refinement based on synchrotron x-ray diffraction, and first-principles calculations, we show that the symmetry-protected Dirac line nodes in the topological semimetallic perovskite ${\mathrm{SrIrO}}_3$ can be lifted simply by applying epitaxial constraints. In particular, the Dirac gap opens without breaking the $Pbnm$ mirror symmetry. In virtue of a symmetry-breaking analysis, we demonstrate that the original symmetry protection is related to the $n$-glide operation, which can be selectively broken by different heteroepitaxial structures. This symmetry protection renders the nodal line a nonsymmorphic Dirac semimetallic state. The results highlight the vital role of crystal symmetry in spin-orbit-coupled correlated oxides and provide a foundation for experimental realization of topological insulators in iridate-based heterostructures.

The electronic conductivity anisotropy of $\mathrm{T}{\mathrm{i}}_3\mathrm{Si}{\mathrm{C}}_2$ is directly evidenced from data collected on (i) a thin film epitaxially grown on a $\left(11\overline{2}0\right)$-oriented SiC single crystal and (ii) a single crystal. Density functional theory calculations, including the linear-response approach and coupled to a Bloch-Grüneisen model, show that the electron-phonon interactions are mainly responsible for the observed anisotropy. Detailed analysis of the electron-phonon coupling constants allows for the rationalization of these scattering processes in terms of the $\mathrm{T}{\mathrm{i}}_3\mathrm{Si}{\mathrm{C}}_2$ nanostructure, giving insights into the possibility of modifying the electron-phonon interaction in this system by substitution effects.

We investigate the effect of charge carrier interaction with surface optical phonons on the band properties of monolayer black phosphorus induced by polar substrates. We develop an analytical method based on the Lee-Low-Pines theory to calculate the spectrum of Fröhlich type continuum Hamiltonian in the long-wavelength limit. We examine the modification of a band gap and renormalization of effective masses due to the substrate-related polaronic effect. Our results show that an energy gap in supported monolayer black phosphorus is enlarged depending on a particular substrate and the interlayer distance $z$. Among the substrate considered, the largest gap broadening at $z=2.5\AA$ is observed for the ${\mathrm{Al}}_2{\mathrm{O}}_3$ substrate, which is found to be $\sim 50$ meV. Carrier-phonon coupling also renormalizes the effective masses which is more pronounced along the zigzag direction. Anisotropy of the effective masses becomes stronger by the influence of the polaronic effect corresponding to direction-dependent carrier-phonon coupling. We conclude that substrate phonons have a non-negligible effect on the static band properties of monolayer black phosphorus, which may be further exploited in its experimental and theoretical studies.

We present a theory based on first-principles calculations explaining (i) why the tunability of spin polarizations of photoelectrons from ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ (111) depends on the band index and Bloch wave vector of the surface state and (ii) why such tunability is absent in the case of isosymmetric Au (111). The results provide not only an explanation for the recent, puzzling experimental observations but also a guide toward making highly-tunable spin-polarized electron sources from topological insulators.

Magnetoelectric coupling phenomena in ${\mathrm{EuTiO}}_3$ are of considerable fundamental interest and are also understood to be key to reported multiferroic behavior in strained films, which exhibit distinctly different properties to the bulk. Here, the magnetoelastic coupling of ${\mathrm{EuTiO}}_3$ is investigated by resonant ultrasound spectroscopy with in situ applied magnetic field and stress as a function of temperature ranging from temperatures above the structural transition temperature $T{}_s$ to below the antiferromagnetic ordering temperature $T{}_n$. One single crystal and two polycrystalline samples are investigated and compared to each other. Both paramagnetic and diamagnetic transducer carriers are used, allowing an examination of the effect of both stress and magnetic field on the behavior of the sample. The properties are reported in constant field/variable temperature and in constant temperature/variable field mode where substantial differences between both data sets are observed. In addition, elastic and magnetic poling at high fields and stresses at low temperature has been performed in order to trace the history dependence of the elastic constants. Four different temperature regions are identified, characterized by unusual elastic responses. The low-temperature phase diagram has been explored and found to exhibit rich complexity. The data evidence a considerable relaxation of elastic constants at high temperatures, but with little effect from magnetic field alone above 20 K, in addition to the known low-temperature coupling.

It has been shown recently that a layer of the antiferromagnetic insulator (AFI) NiO can be used to transport spin current between a ferromagnet (FM) and a nonmagnetic metal (NM). In the experiments one uses the microwave-driven ferromagnetic resonance in a FM layer to produce a spin pumped spin current that flows through an AFI layer and reaches a NM layer where it is converted into a charge current by means of the inverse spin Hall effect. Here we present a theory for the spin transport in an AFI that relies on the spin current carried by the diffusion of thermal antiferromagnetic magnons. The theory explains quite well the measured dependence of the voltage in the NM layer on the thickness of the NiO layer.