The exploration of topological phases in carbon allotropes is always an attractive topic. Here, we propose a metastable carbon allotrope, which possesses exotically helical carbon chains bridged by quadrangle rings. This unique structure with $s{p}^2\text{−}s{p}^3$ bonding networks crystallizes in a noncentrosymmetric face-centered orthogonal (fco) lattice with six atoms in a unit cell, thus, named ${\mathrm{fco}\text{−}\mathrm{C}}_6$. This carbon phase possesses the considerable stability, thus, facilitating its experimental fabrication. More importantly, ${\mathrm{fco}\text{−}\mathrm{C}}_6$ exhibits ideal Weyl semimetallic features with the minimum number of Weyl points in a time-reversal system. First-principles calculations and symmetry arguments uncover that the Weyl points are guaranteed to lie along the high-symmetry paths, and thus well separated in momentum space. We investigate the topological surface states of ${\mathrm{fco}\text{−}\mathrm{C}}_6$ projected on a semi-infinite (010) surface. There are only two extremely long Fermi arcs across the projected Fermi surface. Therefore, this paper provides an ideal candidate to detect Weyl fermions and can realize further applications in carbon allotropes.

In the transition metal dichalcogenide ${\mathrm{IrTe}}_2$, low-temperature charge-ordered phase transitions involving Ir dimers lead to the occurrence of stripe phases of different periodicities, and nearly degenerate energies. Bulk-sensitive measurements have shown that, upon cooling, ${\mathrm{IrTe}}_2$ undergoes two such first-order transitions to $(5\times 1\times 5)$ and $(8\times 1\times 8)$ reconstructed phases at $T_{c_1}\sim 280$ K and $T_{c_2}\sim 180$ K, respectively. Here, using surface sensitive probes of the electronic structure of ${\mathrm{IrTe}}_2$, we reveal the first-order phase transition at $T_{c_3}=165$ K to the $(6\times 1)$ stripes phase, previously proposed to be the surface ground state. This is achieved by combining x-ray photoemission spectroscopy and angle-resolved photoemission spectroscopy, which give access to the evolution of stripe domains and a particular surface state, the energy of which is dependent on the Ir dimer length. By performing measurements over a full thermal cycle, we also report the complete hysteresis of all these phases.

In this paper, the possibility of a superconducting state mediated by the valence fluctuations in Ce-based heavy-fermion systems under high pressure is investigated for the extended periodic Anderson model. In this extended version, an additional local Coulomb repulsion between the localized and conduction electrons is included. In the framework of the projector-based renormalization method, we derive self-consistent equations for the superconducting order parameters. Our numerical evaluation for a two-dimensional case specifies that superconductivity in the heavy-fermion systems has a $d$-wave character and is mediated by valence fluctuations. By use of some additional simplifications, a BCS-like equation is found; an effective pairing interaction then is delivered. The interaction depends strongly on momentum and becomes dominant in the valence transition regime.

We report high-pressure x-ray diffraction, x-ray absorption spectroscopy, and electrical transport measurements on $\mathrm{Cu}M{\mathrm{O}}_2$ ($M=\mathrm{Cr}$, Mn) delafossitelike oxides in an attempt to study their structural and electronic evolution with pressure. Recent studies of the similar $\mathrm{CuFe}{\mathrm{O}}_2$ delafossite has revealed a pressure-induced breaking of the unusual high axial anisotropy resulting in a structural phase transition coinciding with the metal-metal intervalence charge-transfer phenomenon. The present study revealed other possible scenarios responsible for the collapse of the high axial anisotropy and evolution of the O-Cu-O bonds in delafossitelike materials under pressure. Thus in $\mathrm{CuMn}{\mathrm{O}}_2$, the O-Cu-O dumbbells tilt with respect to the $c$ axis at $P>13$ GPa, but in contrast to $\mathrm{CuFe}{\mathrm{O}}_2$, the tilting is continuous with pressure increase, justifying a second-order phase transition within the C2/m structure. Meanwhile in $\mathrm{CuCr}{\mathrm{O}}_2$ ($R\overline{3}m$) the first-order structural phase transition to the monoclinic structure ($P{2}_1/m$) is observed at about 26 GPa characterized by the discontinuous bending of the O-Cu-O bond in contrast to the tilting in the case of $\mathrm{CuFe}{\mathrm{O}}_2$ and $\mathrm{CuMn}{\mathrm{O}}_2$. In both studied systems, we did not find clear evidence of valence transformations, similar to that observed in $\mathrm{CuFe}{\mathrm{O}}_2$.

Silicon nanocrystals (SiNCs) have been under active investigation in the last decades and have been considered as a promising candidate for many optoelectronic applications including highly efficient solar cells. Some of the fundamental properties of interest in these nanostructures is the temperature dependence of their optical absorption onset and how this is controlled by different passivation regimes. In the present work we employ first-principles calculations in conjunction with the special displacement method to study the temperature dependence of the band gap renormalization of freestanding hydrogen-terminated, and oxidized SiNCs, as well as matrix-embedded SiNCs in amorphous silica, and we obtain good agreement with experimental photoluminescence data. We also provide strong evidence that the electron-phonon interplay at the surface of the nanocrystal is suppressed by oxidation and the surrounding amorphous matrix. For the matrix-embedded SiNCs, we show a high correlation between the temperature dependence of the band gap and the Si-Si strained bonds. This result emphasizes the immanent relationship of electron-phonon coupling and thermal structural distortions. We also demonstrate that, apart from quantum confinement, Si-Si strained bonds are the major cause of zero-phonon quasidirect transitions in matrix-embedded SiNCs. As a final point, we clarify that, unlike optical absorption in bulk Si, phonon-assisted electronic transitions play a secondary role in SiNCs.

Metal-insulator transition was microscopically investigated by orbital-resolved nuclear magnetic resonance (OR-NMR) spectroscopy in a single crystal of vanadium dioxide ${\mathrm{VO}}_2$. Observations of the anisotropic ${}^{51}\mathrm{V}$ Knight shift and the nuclear quadrupole frequency allow us to evaluate orbital-dependent spin susceptibility and $d$ orbital occupations. The result is consistent with the degenerated $t_{2g}$ orbitals in a correlated metallic phase and the $d$ orbital ordering in a nonmagnetic insulating phase. The predominant orbital pointing along the chain facilitates a spin-singlet formation triggering metal-insulator transition. The asymmetry of magnetic and electric hyperfine tensors suggests the $d$ orbital reformation favored by a low-symmetry crystal field, forming a localized molecular orbital. The result highlights the cooperative electron correlation and electron-phonon coupling in Mott transition with orbital degrees of freedom.

$\mathit{n}$-type Ge/SiGe quantum wells have been suggested as a promising platform for the realization of a Si-compatible THz laser. Focusing on this material system, we have developed a numerical model to describe the intersubband carrier dynamics which restores the equilibrium after pulsed optical excitation in asymmetric coupled Ge/SiGe quantum wells. We take into account inelastic and elastic scattering processes and investigate different quantum-well geometries, doping densities, and excitation regimes. In this configuration space, we disentangle the effect on the overall dynamics of each scattering channel and provide intersubband relaxation times, finding larger values with respect to III-V based materials, thanks to the weaker electron-phonon coupling with respect to III-V compounds. Finally, the model is used to study and optimize the population inversion between the first- and second-excited subband levels and to assess its dependence on the lattice temperature, providing a sound theoretical framework to guide forthcoming experiments.

Thermal transport properties bear a pivotal role in influencing the performance of phase change memory (PCM) devices, in which the PCM operation involves fast and reversible phase change between amorphous and crystalline phases. In this paper, we present a systematic experimental and theoretical study on the thermal conductivity of GeTe at high temperatures involving fast change from amorphous to crystalline phase upon heating. Modulated photothermal radiometry (MPTR) is used to experimentally determine thermal conductivity of GeTe at high temperatures in both amorphous and crystalline phases. Thermal boundary resistances are accurately taken into account for experimental consideration. To develop a concrete understanding of the underlying physical mechanism, rigorous and in-depth theoretical exercises are carried out. For this, first-principles density functional methods and linearized Boltzmann transport equations (LBTE) are employed using both direct and relaxation time based approach (RTA) and compared with that of the phenomenological Slack model. The amorphous phase experimental data has been described using the minimal thermal conductivity model with sufficient precision. The theoretical estimation involving direct solution and RTA method are found to retrieve well the trend of the experimental thermal conductivity for crystalline GeTe at high temperatures despite being slightly overestimated and underestimated, respectively, compared to the experimental data. A rough estimate of vacancy contribution has been found to modify the direct solution in such a way that it agrees excellently with the experiment. Umklapp scattering has been determined as the significant phonon-phonon scattering process. Umklapp scattering parameter has been identified for GeTe for the whole temperature range which can uniquely determine and compare Umklapp scattering processes for different materials.

The trigonal compound ${\mathrm{EuMg}}_2{\mathrm{Bi}}_2$ has recently been discussed in terms of its topological band properties. These are intertwined with its magnetic properties. Here detailed studies of the magnetic, thermal, and electronic-transport properties of ${\mathrm{EuMg}}_2{\mathrm{Bi}}_2$ single crystals are presented. The ${\mathrm{Eu}}^{+2}$ spins 7/2 in ${\mathrm{EuMg}}_2{\mathrm{Bi}}_2$ exhibit an antiferromagnetic (AFM) transition at a temperature $T_{\mathrm{N}}=6.7$ K, as previously reported. By analyzing the anisotropic magnetic susceptibility $\chi$ data below $T_{\mathrm{N}}$ in terms of molecular-field theory (MFT), the AFM structure is inferred to be a $c$-axis helix, where the ordered moments in the hexagonal $ab$-plane layers are aligned ferromagnetically in the $ab$ plane with a turn angle between the moments in adjacent moment planes along the $c$ axis of $\approx {120}^{\circ }$. An alternate but less likely magnetic structure is a planar structure with nearest-neighbor Eu spins aligned at $\approx {120}^{\circ }$ with respect to each other, where these ordered-moment layers are stacked along the $c$ axis. The magnetic heat capacity exhibits a $\lambda$ anomaly at $T_{\mathrm{N}}$ with evidence of dynamic short-range magnetic fluctuations both above and below $T_{\mathrm{N}}$. The high-$T$ limit of the magnetic entropy is close to the theoretical value for spins 7/2. The in-plane electrical resistivity $\rho \left(T\right)$ data indicate metallic character with a mild and disorder-sensitive upturn below $T_{\min }=23$ K. An anomalous rapid drop in $\rho \left(T\right)$ on cooling below $T_{\mathrm{N}}$ as found in zero field is replaced by a two-step decrease in magnetic fields. The $\rho \left(T\right)$ measurements also reveal an additional transition below $T_{\mathrm{N}}$ in applied fields of unknown origin that is not observed in the other measurements and may be associated with an incommensurate to commensurate AFM transition. The dependence of $T_{\mathrm{N}}$ on the $c$-axis magnetic field $H_{\perp }$ was derived from the field-dependent $\chi \left(T\right)$, $C_{\mathrm{p}}\left(T\right)$, and $\rho \left(T\right)$ measurements. This $T_{\mathrm{N}}\left(H_{\perp }\right)$ was found to be consistent with the prediction of MFT for a $c$-axis helix with $S=7/2$ and was used to generate a phase diagram in the $H_{\perp }\text{−}T$ plane.

A theory of phonon-mediated superconductivity in strong-coupling amorphous materials is developed based on an effective description of structural disorder and its effect on the vibrational spectrum. The theory accounts for the diffusivelike transport of vibrational excitations due to disorder-induced scattering within the Eliashberg theory of strong-coupling superconductivity. The theory provides a good analytical description of the Eliashberg function ${\alpha }^{2}F\left(\omega \right)$ in comparison with experiments and allows one to disentangle the effects of transverse and longitudinal excitations on the Eliashberg function. In particular, it shows that the transverse excitations play a crucial role in driving an increase or excess in the Eliashberg function at low energy, which is related to the boson peak phenomenon in vibrational spectra of glasses. This low-energy excess, on the one hand, drives an enhancement of the electron-phonon coupling but at the same time reduces the characteristic energy scale ${\omega }_{\log }$ in the Allen-Dynes formula. As a consequence, the nonmonotonicity of $T_c$ as a function of alloying (disorder) in Pb-based systems can be rationalized. The case of Al-based systems, where disorder increases $T_c$ from the start, is also analyzed. General material-design principles for enhancing $T_c$ in amorphous superconductors are presented.

Spin transport at metallic interfaces is an essential ingredient of various spintronic device concepts, such as giant magnetoresistance, spin-transfer torque, and spin pumping. Spin-orbit coupling plays an important role in many such devices. In particular, spin current is partially absorbed at the interface due to spin-orbit coupling. We develop a general magnetoelectronic circuit theory and generalize the concept of spin-mixing conductance, accounting for various mechanisms responsible for spin-flip scattering. For the special case when exchange interactions dominate, we give a simple expression for the spin-mixing conductance in terms of the contributions responsible for spin relaxation (i.e., spin memory loss), spin torque, and spin precession. The spin memory loss parameter $\delta$ is related to spin-flip transmission and reflection probabilities. There is no straightforward relation between spin torque and spin memory loss. We calculate the spin-flip scattering rates for N|N, F|N, and F|F interfaces using the Landauer-Büttiker method within the linear muffin-tin orbital method and determine the values of $\delta$ using circuit theory.

We study a short-range resonating valence bond (RVB) wave function with diagonal links on the square lattice that permits sign-problem free wave function Monte Carlo studies. Special attention is given to entanglement properties, in particular, the study of minimum entropy states (MES) according to the method of Zhang et al. [Phys. Rev. B 85, 235151 (2012)]. We provide evidence that the MES associated with the RVB wave functions can be lifted from an associated quantum dimer picture of these wave functions, where MES states are certain linear combinations of eigenstates of a 't Hooft “magnetic loop”-type operator. From this identification, we calculate a value consistent with $ln\left(2\right)$ for the topological entanglement entropy directly for the RVB states via wave function Monte Carlo. This corroborates the ${\mathbb{Z}}_2$ nature of the RVB states. We furthermore define and elaborate on the concept of a “pre-Kasteleyn” orientation that may be useful for the study of lattices with nonplanar topology in general.

The Su-Schrieffer-Heeger (SSH) model, containing dimerized hopping and a constant onsite energy, has become a paradigmatic model for one-dimensional topological phases, soliton excitations, and fractionalized charge in the presence of chiral symmetry. Motivated by the recent developments in engineering artificial lattices, we study an alternative model where hopping is constant but the onsite energy is dimerized. We find that it has a nonsymmorphic chiral symmetry and supports topologically distinct phases described by a ${\mathbb{Z}}_2$ invariant $\nu$. In the case of multimode ribbon we also find topological phases protected by hidden symmetries and we uncover the corresponding ${\mathbb{Z}}_2$ invariants ${\nu }_n$. We show that, in contrast to the SSH case, zero-energy states do not necessarily appear at the boundary between topologically distinct phases, but instead these systems support a new kind of bulk-boundary correspondence: The energy of the topological domain wall states typically scales to zero as $1/w$, where $w$ is the width of the domain wall separating phases with different topology. Moreover, under specific circumstances we also find a faster scaling $e^{-w/\xi }$, where $\xi$ is an intrinsic length scale. We show that the spectral flow of these states and the charge of the domain walls are different than in the case of the SSH model.

Electron-phonon coupling, diagonal in a real space formulation, leads to a polaron paradigm of smoothly varying properties. However, fundamental changes, namely the singular behavior of polarons, occur if off-diagonal pairing is involved into consideration. The study of polaron transformations and related properties of matter is of particular interest for realistic models, since competition between diagonal and off-diagonal electron-phonon contributions in the presence of other strong interactions can result in unconventional behavior of the system. Here we consider a two-dimensional multiband $pd$ model of cuprate superconductors with electron-phonon interaction and analyze the features of the systems that are caused by the competition of diagonal and off-diagonal electron-phonon contributions in the limit of strong electron correlations. Using the polaronic version of the generalized tight-binding method, we describe the evolution of the band structure, Fermi surface, density of states at Fermi level, and phonon spectral function in the space of electron-phonon parameters ranging from weak to strong coupling strength of the adiabatic limit. On the phase diagram of polaron properties we reveal two quantum phase transitions and show how electron-phonon interaction gives rise to Fermi surface transformation (i) from hole pockets to true Fermi arcs and (ii) from hole to electron type of conductivity. We also demonstrate the emergence of new states in the phonon spectral function of the polaron and discuss their origin.

By using a numerically exact diagonalization technique and a block-extended version of the finite-temperature Lanczos method, we study thermodynamic properties of an $S=1/2$ Heisenberg model on the triangular lattice with an antiferromagnetic nearest-neighbor interaction $J$ and a four-spin ring-exchange interaction $J_{\mathrm{c}}$. Calculations are performed on small clusters under the periodic-boundary conditions. In contrast to the purely triangular case with $J_{\mathrm{c}}=0$, the specific heat exhibits a characteristic double-peak structure for $J_{\mathrm{c}}/J\gtrsim 0.04$. From the calculations of the entropy and the uniform magnetic susceptibility, it is shown that nonmagnetic excitations exist below the magnetic excitation for $J_{\mathrm{c}}/J\gtrsim 0.04$.

We study the band dispersion of graphene with randomly distributed structural defects using two complementary methods, exact diagonalization of the tight-binding Hamiltonian and implementing a self-consistent $T$ matrix approximation. We identify three distinct types of impurities resulting in qualitatively different spectra in the vicinity of the Dirac point. First, resonant impurities, such as vacancies or 585 defects, lead to stretching of the spectrum at the Dirac point with a finite density of localized states. This type of spectrum has been observed in epitaxial graphene by photoemission spectroscopy and discussed extensively in the literature. Second, nonresonant (weak) impurities, such as paired vacancies or Stone-Wales defects, do not stretch the spectrum but provide a line broadening that increases with energy. Finally, disorder that breaks sublattice symmetry, such as vacancies placed in only one sublattice, open a gap around the Dirac point and create an impurity band in the middle of this gap. We find good agreement between the results of the two methods and also with the experimentally measured spectra.