Transient grating (TG) spectroscopy is an important experimental technique to measure mean-free-path (MFP) spectra using observations of quasiballistic heat conduction. To obtain MFP spectra, the measurements must be interpreted within the framework of the frequency-dependent Boltzmann transport equation (BTE), but previous solutions have restricted validity due to simplifying assumptions. Here we analyze heat conduction in TG using a new analytical solution of the frequency-dependent BTE that accurately describes thermal transport from the diffusive to ballistic regimes. We demonstrate that our result enables a more accurate measurement of MFP spectra and thus will lead to an improved understanding of heat conduction in solids.

We improve the linear tetrahedron method to overcome systematic errors due to overestimations (underestimations) in integrals for convex (concave) functions, respectively. Our method is applicable to various types of calculations such as the total energy, the charge (spin) density, response functions, and the phonon frequency, in contrast with the Blöchl correction, which is applicable to only the first two. We demonstrate the ability of our method by calculating phonons in MgB${}_2$ and fcc lithium.

We investigate the magnetism of the ${\text{Co}}^{\mathrm{II}}$${}_4$(OH)${}_2$(${\text{C}}_{10}$${\text{H}}_{16}$${\text{O}}_4$)${}_3$ metal-organic framework, which displays complex inorganic chains separated from each other by distances of 1 to 2 nm and orders at 5.4 K. The zero-field magnetic structure is determined using neutron powder diffraction: it is mainly antiferromagnetic but possesses a ferromagnetic component along the c axis. This magnetic structure persists in presence of a magnetic field. Alternating current susceptibility measurements confirm the existence of a single thermally activated regime over seven decades in frequency ($E/k_B\approx 64$ K), whereas time-dependent relaxation of the magnetization after saturation in an external field leads to a two times smaller energy barrier. These experiments probe the slow dynamics of domain walls within the chains: we propose that the ac measurements are sensitive to the motion of existing domain walls within the chains, while the magnetization measurements are governed by the creation of domain walls.

We present a study of the derivatives of current-voltage $I\left(V\right)$ characteristics of point contacts (PCs) based on Ba${}_{1-x}$Na${}_x$Fe${}_2$As${}_2$ ($x=0.25$) in the normal and superconducting states. The detailed analysis of $dV/dI\left(V\right)$ data (also given in Appendix A) shows that the thermal regime, when the temperature increases with voltage at a rate of about 1.8 K/mV, is realized in the investigated PCs at least at high biases $V$ above the superconducting (SC) gap $\Delta$. In this case, the specific resistivity $\rho \left(T\right)$ in the PC core is responsible for a peculiar $dV/dI\left(V\right)$ behavior, while a pronounced asymmetry of $dV/dI\left(V\right)$ is caused by the large value of thermopower in this material. A reproducible minima detected in $dV/dI\left(V\right)$ at low biases in the range $\pm$(6–9) mV, well below the SC critical temperature $T_c$, could be connected with the manifestation of the SC gap $\Delta$. An evaluation of these Andreev-reflection-like structures on $dV/dI\left(V\right)$ points out to the preferred value of $2\Delta /k_{\mathrm{B}}{T}_c\approx 6$. The expected second gap features in $dV/dI\left(V\right)$ are hard to resolve unambiguously, likely due to impurity scattering, spatial inhomogeneity, and transition to the mentioned thermal regime as the bias further increases. Suggestions are made how to separate the spectroscopic features in $dV/dI\left(V\right)$ from those caused by the thermal regime.

The same bulk two-dimensional topological phase can have multiple distinct, fully chiral edge phases. We show that this can occur in the integer quantum Hall states at $\nu =8$ and 12, with experimentally testable consequences. We show that this can occur in Abelian fractional quantum Hall states as well, with the simplest examples being at $\nu =\frac{8}{7},\frac{12}{11},\frac{8}{15},\frac{16}{5}$. We give a general criterion for the existence of multiple distinct chiral edge phases for the same bulk phase and discuss experimental consequences. Edge phases correspond to lattices while bulk phases correspond to genera of lattices. Since there are typically multiple lattices in a genus, the bulk-edge correspondence is typically one-to-many; there are usually many stable fully chiral edge phases corresponding to the same bulk. We explain these correspondences using the theory of integral quadratic forms. We show that fermionic systems can have edge phases with only bosonic low-energy excitations and discuss a fermionic generalization of the relation between bulk topological spins and the central charge. The latter follows from our demonstration that every fermionic topological phase can be represented as a bosonic topological phase, together with some number of filled Landau levels. Our analysis shows that every Abelian topological phase can be decomposed into a tensor product of theories associated with prime numbers $p$ in which every quasiparticle has a topological spin that is a $p^n$th root of unity for some $n$. It also leads to a simple demonstration that all Abelian topological phases can be represented by $\mathrm{U}{\left(1\right)}^N$ Chern-Simons theory parameterized by a $K$ matrix.

We have studied the electron hopping in a two-CdSe quantum dot (QD) system linked by an azobenzene-derived light-switching molecule. This system can be considered as a prototype of a QD supercrystal. Following the computational strategies given in our recent work [I.-H. Chu et al., J. Phys. Chem. C 115, 21409 (2011)], we have investigated the effects of molecular attachment, molecular isomer (trans and cis), and QD size on the electron hopping rate using Marcus theory. Our results indicate that molecular attachment has a large impact on the system for both isomers. In the most energetically favorable attachment, the cis isomer provides significantly greater coupling between the two QDs and hence the electron hopping rate is greater compared to the trans isomer. As a result, the carrier mobility of the QD array in the low carrier density, weak external electric-field regime is several orders of magnitude higher in the $\mathit{cis}$ compared to the $\mathit{trans}$ configuration. This demonstration of mobility modulation using QDs and azobenzene could lead to an alternative type of switching device.

The effect of long-distance coherent tunneling (LDCT) on the charge and heat currents in serially coupled triple quantum dots (TQDs) connected to electrodes is illustrated by using a combination of the extended Hurbbard and Anderson models. The charge and heat currents are calculated with a closed-form Landauer expression for the transmission coefficient suitable for the Coulomb blockade regime. The physical parameters including bias-dependent quantum dot energy levels, electron Coulomb interactions, and electron hopping strengths are calculated in the framework of effective mass theory for semiconductor TQDs. We demonstrate that the effect of LDCT on the charge and heat currents can be robust. In addition, it is shown that prominent heat rectification behavior can exist in the TQD system with asymmetrical energy levels.

We propose a spin-orbit qubit in a nanowire quantum dot on the surface of a multiferroic insulator with a cycloidal spiral magnetic order. The spiral exchange field from the multiferroic insulator causes an inhomogeneous Zeeman-like interaction on the electron spin in the quantum dot, producing a spin-orbit qubit. The absence of an external magnetic field benefits the integration of such a spin-orbit qubit into high-quality superconducting resonators. By exploiting the Rashba spin-orbit coupling in the quantum dot via a gate voltage, one can obtain an effective spin-photon coupling with an efficient on-off switching. This makes the proposed device controllable and promising for hybrid quantum circuits.

Indium on silicon (111) exhibits a Peierls-like phase transition from a $(4\times 1)$ reconstructed high-temperature phase to a $(8\times 2)$ reconstructed ground state. A controversial debate is going on what kind of phase transition it is: first- or second-order. We employed high-resolution low-energy electron diffraction under slow thermal heating and cooling cycles to follow the phase transition. A robust hysteresis of diffraction spot intensities with a width of 8.6 K has been observed, which is independent of the heating and cooling rate. This hysteresis directly proves the existence of an energy barrier at the phase transition temperature and thus the first-order behavior of this phase transition.

Atomically precise tailoring of graphene can enable unusual transport pathways and new nanometer-scale functional devices. Here we describe a recipe for the controlled production of highly regular “5-5-8” line defects in graphene by means of simultaneous electron irradiation and Joule heating by applied electric current. High-resolution transmission electron microscopy reveals individual steps of the growth process. Extending earlier theoretical work suggesting valley-discriminating capabilities of a graphene 5-5-8 line defect, we perform first-principles calculations of transport and find a strong energy dependence of valley polarization of the charge carriers across the defect. These findings inspire us to propose a compact electrostatically gated “valley valve” device, a critical component for valleytronics.

Whereas the concept of topological band structures was developed originally for insulators with a bulk band gap, it has become increasingly clear that the prime consequences of a nontrivial topology—spin-momentum locking of surface states—can also be encountered in gapless systems. We show that point-group symmetries allow for helical semimetals, i.e., semimetals with Dirac-like topological surface states, to exist. The presence of this state, however, critically depends on the presence of crystal inversion symmetry. Using the paradigmatic example of mercury chalcogenides HgX (X = Te, Se, S), we show that an infinitesimally small broken inversion symmetry (BIS) renders the helical semimetallic state unstable. The BIS is also very important in the fully gapped topological insulating regime, renormalizing the surface Dirac cones in an anisotropic manner. As a consequence, the handedness of the Dirac cones can be flipped by a biaxial stress field.

In topologically trivial materials, the existence of surface states within the bulk band gap depends on the surface condition. A surface state exists if the system is Shockley inverted, and vice versa; furthermore, this surface condition in a superlattice can be tuned from one case to the other by varying the terminating atomic plane within a superlattice period. By contrast, we demonstrate here based on first-principles calculations for ${\text{Bi}}_2$${\text{Se}}_3$ cleaved at different atomic planes within a quintuple layer that topological surface states always span the bulk band gap in accordance with the topologically nontrivial nature of ${\text{Bi}}_2$${\text{Se}}_3$. However, the number of surface bands, the band dispersion relations, and the degree of spin polarization are strongly dependent on the cleavage plane. Multiple Dirac cones can exist at the zone center and/or zone boundary, and a massive but topologically nontrivial Dirac cone is observed for a Se-terminated surface. The results confirm the robustness of the topological nature of the system, but the details of the topological surface states themselves can vary substantially within the broad topological constraint. The persistent existence of topological states within the gap is discussed in terms of the Shockley criterion.

We report an implementation of the constrained random phase approximation (cRPA) method within the projector augmented-wave framework. It allows for the calculation of the screened interaction in the same Wannier orbitals as our recent DFT+$U$ and DFT+DMFT implementations. We present calculations of the dynamical Coulomb screened interaction in uranium dioxide and $\alpha$ and $\gamma$ cerium on Wannier functions. We show that a self-consistent calculation of the static screened interaction in DFT+$U$ together with a consistent Wannier basis is mandatory for $\gamma$ cerium and uranium dioxide. We emphasize that a static approximation for the screened interaction in $\alpha$ cerium is too drastic.

We report the electric transport and thermodynamic properties of the skutterudite-related ${\mathrm{La}}_3{\mathrm{Co}}_4{\mathrm{Sn}}_{13}$ and ${\mathrm{La}}_3{\mathrm{Rh}}_4{\mathrm{Sn}}_{13}$ superconductors. Applying an external pressure to ${\mathrm{La}}_3{\mathrm{Rh}}_4{\mathrm{Sn}}_{13}$, the resistive superconducting critical temperature $T_c$ decreases, while the critical temperature of ${\mathrm{La}}_3{\mathrm{Co}}_4{\mathrm{Sn}}_{13}$ is enhanced with increasing pressure. The positive pressure coefficient $d{T}_c/dP$ correlates with a subtle structural transition in ${\mathrm{La}}_3{\mathrm{Co}}_4{\mathrm{Sn}}_{13}$ and is discussed in the context of lattice instabilities. Specific-heat data show that both compounds are typical BCS superconductors. However, ${\mathrm{La}}_3{\mathrm{Rh}}_4{\mathrm{Sn}}_{13}$ also exhibits a second superconducting phase at higher temperatures, which is characteristic of inhomogeneous superconductors. We calculate the specific heat for an inhomogeneous superconducting phase, which agrees well with experimental $C\left(T\right)$ data for ${\mathrm{La}}_3{\mathrm{Rh}}_4{\mathrm{Sn}}_{13}$. We also found that an applied pressure reduces this second superconducting phase.

It is well established that some kinds of lattice deformations in graphene monolayer, which change electron hopping in the sublattice and affect in-plane motion of electrons, may induce out-of-plane pseudomagnetic fields as large as 100 T. Here, we demonstrate that stacking misorientation in graphene bilayers mimics the effect of huge in-plane pseudomagnetic fields greater than 1000 T on the interlayer hopping of electrons. As well as addressing the similarity between the effect of in-plane pseudomagnetic fields and the twisting on the electronic band structure of the Bernal graphene bilayer, we point out that the in-plane magnetic fields (or twisting) could modify the low-energy pseudospin texture of the graphene bilayer (the pseudospin winding number is reduced from 2 to 1), thereby changing the chiralities of quasiparticles from those of spin 1 to those of spin 1/2. Our results illustrate the possibility of controllably manipulating electronic properties of Bernal graphene bilayer by introducing the in-plane magnetic field or twisting.

We demonstrate the possibility to induce and control peculiar spin textures in a helical Luttinger liquid, by means of a time-dependent magnetic scatterer. The presence of a perturbation that breaks the time-reversal symmetry opens a gap in the spectrum, inducing single-particle backscattering and a peculiar spin response. We show that in the weak backscattering regime asymmetric spin textures emerge at the left and right sides of the scatterer, whose spatial oscillations are controlled by the ratio between the magnetization frequency and the Fermi energy and by the electron interaction. This peculiar spin response marks a strong difference between helical and nonhelical liquids, which are expected to produce symmetric spin textures even in the ac regime.

Topological insulators and topological superconductors display various topological phases that are characterized by different Chern numbers or by gapless edge states. In this work we show that various quantum information methods such as the von Neumann entropy, entanglement spectrum, fidelity, and fidelity spectrum may be used to detect and distinguish topological phases and their transitions. As an example we consider a two-dimensional $p$-wave superconductor, with Rashba spin-orbit coupling and a Zeeman term. The nature of the phases and their changes are clarified by the eigenvectors of the $k$-space reduced density matrix. We show that in the topologically nontrivial phases the highest weight eigenvector is fully aligned with the triplet pairing state. A signature of the various phase transitions between two points on the parameter space is encoded in the $k$-space fidelity operator.

There is considerable interest in uncovering the physics of iron-based superconductivity from the alkaline iron selenides, a materials class containing an insulating phase (245 phase) and a superconducting (SC) phase. Due to the microstructural complexity of these superconductors, the role of the 245 phase in the development of the superconductivity has been a puzzle. Here we demonstrate a comprehensive high-pressure study on the insulating samples with pure 245 phase and biphasic SC samples. We find that the insulating behavior can be completely suppressed by pressure in the insulating samples and also identify an intermediate metallic (M′) state. The Mott insulating (MI) state of the 245 phase and the M′ state coexist over a significant range of pressure up to ∼10 GPa, the same pressure at which the superconductivity of the SC samples vanishes. Our results reveal the M′ state as a pathway that connects the insulating and SC phases of the alkaline iron selenides and indicate that the coexistence and interplay between the MI and M′ states is a necessary condition for superconductivity. Finally, we interpret the M′ state in terms of an orbital selectivity of the correlated $3d$ electrons.

Using ab initio simulations within the generalized gradient approximation, we calculate x-ray absorption near edge spectra (XANES) at the iron K edge throughout the high-pressure phase diagram and up to extreme density and temperature conditions that are representative of the Earth's inner core (up to 3 Mbar and 8000 K). We show that XANES spectra near the Fe K edge exhibit clear signatures for the different high-temperature, high-pressure phases of iron. This suggests that XANES spectroscopy might be used to resolve ongoing controversies regarding both the high-pressure melting curve of iron and the nature of the solid phases undergoing melting up to several Mbar. In contrast to diffraction measurements, it also offers a severe constraint for density functional theory predictions of the transport properties of iron by providing direct information on the electronic structure of iron at these extreme conditions.