We study the combined effect of interactions and disorder on topological order in one dimension. Toward that end, we consider a generalized Kitaev chain including fermion-fermion interactions and disorder in the chemical potential. We determine the phase diagram by performing density-matrix renormalization-group calculations on the corresponding spin-1/2 chain. We find that moderate disorder or repulsive interactions individually stabilize the topological order, which remains valid for their combined effect. However, both repulsive and attractive interactions lead to a suppression of the topological phase at strong disorder.

The electronic states of potassium- (K-) intercalated zigzag-type polycyclic aromatic (PLA) hydrocarbon [polyacene PLAs] ${\mathrm{K}}_x\left(\mathrm{PLAs}\right)$ are studied for a series of the four smallest molecules: naphthalene (NN), anthracene (AN), tetracene (TN), and pentacene (PN), focusing on their 1:1 stoichiometric phases. Clear experimental differences are identified between the first group [${\mathrm{K}}_1\left(\mathrm{NN}\right)$ and ${\mathrm{K}}_1\left(\mathrm{AN}\right)$] and the second group [${\mathrm{K}}_1\left(\mathrm{TN}\right)$ and ${\mathrm{K}}_1\left(\mathrm{PN}\right)$] by magnetic, vibrational, and optical measurements. The first group is categorized as a Mott insulator with an antiferromagnetic ground state with energy of ∼10 meV, whereas the second group is classified as a band insulator via dimer formation due to the spin Peierls instability. In the latter system, the first thermally accessible triplet states are located far apart from the singlet ground states and are not detected by electron spin-resonance spectroscopy until 300 K being very different from what is observed for the hole-doped PN reported earlier. The results give a new systematic understanding on the electronic states of electron-doped PLAs sensitive to the energetic balance among on-site Coulomb repulsion, bandwidth, and the Peierls instability.

Recent high-resolution absorption spectroscopy on excited excitons in cuprous oxide [Nature (London) 514, 343 (2014)] has revealed significant deviations of their spectrum from that of the ideal hydrogen-like series. Here we show that the complex band dispersion of the crystal, which determines the kinetic energy of electrons and holes, strongly affects the exciton binding energy. Specifically, we show that the nonparabolicity of the band dispersion is the main cause of the deviation from the hydrogen series. Experimental data collected from high-resolution absorption spectroscopy in electric fields validate the assignment of the deviation to the nonparabolicity of the band dispersion.

We report the temperature-dependent complex dielectric function of pristine bulk ${\mathrm{SrTiO}}_3$ between 4.2 and 300 K within the energy range of 0.6–6.5 eV determined by spectroscopic ellipsometry. Fundamental indirect and direct band-gap energies have been extracted and are discussed with regard to existing state-of-the-art theoretical calculations. Furthermore, the dielectric function around the fundamental direct gap is analyzed by considering excitonic states. The excitonic effects, including the Coulomb enhancement of the continuum, are characterized using an extension of the Elliott's formula considering both the real and imaginary parts of the dielectric function. The Urbach tail below the indirect edge shows an unconventional temperature-dependent behavior correlated to the microstructural changes near the structural phase transition around 105 K from the low-temperature tetragonal phase to the cubic phase. The temperature-dependent characterization reveals that the fundamental indirect edge as well as the Urbach tail are affected conspicuously by the structural phase transition while the fundamental direct edge is not. Moreover, the indirect edge follows Varshni's rule only in the cubic phase and the direct edge exhibits an anomalous linear increase with increasing temperature.

The driving of charge carriers confined in a quantum well lacking the center of space inversion by an alternating electric field leads to the formation of a direct electric current. We develop a microscopic theory of such a quantum ratchet effect for quantum wells subjected to a static magnetic field. We show that the ratchet current emerges for a linearly polarized alternating electric field as well as a rotating electric field and drastically increases at the cyclotron resonance conditions. For the magnetic field tilted with respect to the quantum well normal, the ratchet current contains an additional resonance at the first subharmonic of the cyclotron resonance.

The behavior of phase coherence is studied in two-dimensional hole gas through the integer quantum Hall plateau-to-plateau transition. From the plateau transition as a function of temperature, scaling properties of multiple transitions are analyzed. Our results are in good agreement with the assumption of the zero-point fluctuations of the coherent holes, and support the intrinsic saturation of the coherence time at low temperature limit. The critical exponent $p$ can also be determined under the scheme of the zero-point fluctuations. The similarity and difference in experimental observations between quantum Griffiths singularity and plateau transition is discussed. The spin-orbit coupling effect's influence on the plateau transition is explored by comparing the results from different transitions.

We report the magnetoresistance of a charge-density wave (CDW) in $o-{\mathrm{TaS}}_3$ whiskers at 4.2 K under a magnetic field up to 5.2 T. An anisotropic negative magnetoresistance was found in the nonlinear regime of current-voltage characteristics. The angle dependence of the magnetoresistance, studied by rotating the magnetic field upon the $c$ axis, exhibited a twofold symmetry. The magnetoresistance amplitude exhibited maxima when the field was parallel to the $a$ axis, whereas it vanished to the $b$ axis. The observed anisotropy may come from a difference in interchain coupling of adjacent CDWs along the $a$ and $b$ axes. Comparison of the anisotropy to the scanning tunneling microscope image of CDWs allows us to provide a simple picture to explain the magnetoresistance in terms of delocalization of quantum interference of CDWs extending over the $b-c$ plane.

We investigate the interplay between long-range and direct photon-assisted transport in a triple quantum dot chain where local ac voltages are applied to the outer dots. We propose the phase difference between the two ac voltages as an external parameter, which can be easily tuned to manipulate the current characteristics. For gate voltages in phase opposition we find quantum destructive interferences analogous to the interferences in closed-loop undriven triple dots. As the voltages oscillate in phase, interferences between multiple paths give rise to dark states. Those totally cancel the current, and could be experimentally resolved.

We consider a superconducting microwave cavity capacitively coupled to both a quantum conductor and its electronic reservoirs. We analyze in detail how the measurements of the cavity microwave field, which are related to the electronic charge susceptibility, can be used to extract information on the transport properties of the quantum conductor. We show that the asymmetry of the capacitive couplings between the electronic reservoirs and the cavity plays a crucial role in relating optical measurements to transport properties. For asymmetric capacitive couplings, photonic measurements can be used to probe the finite low-frequency admittance of the quantum conductor, the real part of which is related to the differential conductance. In particular, when the quantum dot is far from resonance, the charge susceptibility is directly proportional to the admittance for a large range of frequencies and voltages. However, when the quantum conductor is near resonance, such a relation generally holds only at low frequency and for equal tunnel coupling or low voltage. Beyond this low-energy near-equilibrium regime, the charge susceptibility and thus the optical transmission offer new insights into the quantum conductors since the optical observables are not directly connected to transport quantities. For symmetric lead capacitive couplings, we show that the optical measurements can be used to reveal the Korringa-Shiba relation, connecting the reactive to the dissipative part of the susceptibility, at low frequency and low bias.

We show that a Weyl or Dirac semimetal thin film with a strong in-plane magnetic field becomes a two-dimensional Fermi liquid with interesting properties. The Fermi surface in this system is strongly anisotropic, which originates from a combination of chiral bulk channels and Fermi arcs. The area enclosed by the Fermi surface depends strongly on the in-plane magnetic field component parallel to the Weyl/Dirac node splitting, which leads to unusual behavior in quantum oscillations when the magnetic field is tilted out of the plane. We estimate the oscillation frequencies and the regimes where such effects could be seen in ${\mathrm{Cd}}_3{\mathrm{As}}_2, {\mathrm{Na}}_3\mathrm{Bi}$, and TaAs.

In order to clarify the mechanism associated with pressure/magnetic-field-induced giant ferroelectric polarization in ${\mathrm{TbMnO}}_3$, this work investigated changes in magnetic ordering brought about by variations in temperature, magnetic field, and pressure. This was accomplished by means of neutron diffraction analyses under high pressures and high magnetic fields, employing a single crystal. The incommensurate magnetic ordering of a cycloid structure was found to be stable below the reported critical pressure of 4.5 GPa. In contrast, a commensurate $E$-type spin ordering of Mn spins and a noncollinear configuration of Tb spins with $\mathit{k}=(0,\frac{1}{2},0)$ appeared above 4.5 GPa. The application of a magnetic field along the $a$ axis $\left(H_{\parallel a}\right)$ under pressure induces a $\mathit{k}=(0,0,0)$ antiferromagnetic structure in the case of Tb spins above $H_{\parallel a}$, enhancing the ferroelectric polarization, while the $E$-type ordering of Mn spins is stable even above the critical field. From the present experimental findings, we conclude that the $E$-type ordering of Mn spins induces giant ferroelectric polarization through an exchange striction mechanism. The $H_{\parallel a}$-induced polarization enhancement can be understood by considering that the polarization, reduced by the polar ordering of Tb moments in a zero field, can be recovered through a field-induced change to nonpolar $\mathit{k}=(0,0,0)$ ordering at $H_{\parallel a}\sim 2$ T.

We present a combined angle-resolved photoemission spectroscopy (ARPES) and first-principles calculations study of the electronic structure of ${\mathrm{LaAgSb}}_2$ in the entire first Brillouin zone. We observe a Dirac-cone-like structure in the vicinity of the Fermi level formed by the crossing of two linear energy bands, as well as the nested segments of a Fermi surface pocket emerging from the cone. Our ARPES results show the close relationship of the Dirac cone to the charge-density-wave ordering, providing consistent explanations for exotic behaviors in this material.

We report on the real-space observation of the two-dimensional distorted triangular lattice of physisorbed oxygen $\left({\mathrm{O}}_2\right)$ molecules on an Ag(111) surface by low-temperature scanning tunneling microscopy. The physisorbed state of the ${\mathrm{O}}_2$ monolayers was confirmed by measuring their thermal stability, which showed good agreement with previous thermal desorption spectroscopy. The distortion of the observed lattice was reproduced quantitatively by considering the intermolecular exchange interaction in Monte Carlo calculations, indicating a critical role of antiferromagnetic ordering of ${\mathrm{O}}_2$ spins. In tunneling spectra, the Kondo resonance was not observed on the ${\mathrm{O}}_2$ layer at 4.7 K unlike the case of physisorbed ${\mathrm{O}}_2$ on Ag(110). These results indicate that an intrinsic $S=1$ spin of the ${\mathrm{O}}_2$ molecules was preserved to form a two-dimensional antiferromagnetic quantum spin system on the surface.

We investigate the spin and spin-orbital textures and electronic structures of topologically protected surface states at side surfaces of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ by using slab models within density-functional theory. This is motivated by recent experiments on nanowires, nanoribbons, and nanoplates of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ with side surfaces. In particular, two representative surfaces normal to the (111) surface, such as $\left(1\overline{1}0\right)$ and $\left(11\overline{2}\right)$ surfaces, are examined, in the presence of time-reversal symmetry and inversion symmetry. The $\left(1\overline{1}0\right)$ surface lying in the mirror plane has twofold $\left({\mathrm{C}}_2\right)$ rotational symmetry, whereas the $\left(11\overline{2}\right)$ surface has only mirror symmetry. For the $\left(1\overline{1}0\right)$ surface, we find that a Dirac cone with strongly anisotropic Fermi velocity is formed at $\mathrm{\Gamma }$ with the Dirac point at the Fermi level, and that the spin texture reveals features of Rashba-type combined with Dresselhaus-type spin-orbit coupling. For the $\left(11\overline{2}\right)$ surface, a Dirac cone is found at either $\mathrm{\Gamma }$ or the $Y$ point (along the mirror symmetry axis) below the Fermi level. In this case, the spin texture of the surface states strikingly differs from that of the (111) and $\left(1\overline{1}0\right)$ surfaces: (i) the in-plane spin polarization dominantly aligned perpendicular to the [111] direction or the mirror symmetry axis, (ii) the Dresselhaus-type spin texture, and (iii) significant out-of-plane spin polarization away from the mirror symmetry axis. Our findings distinctively differ from the previous works based on the effective bulk model Hamiltonian. Our calculated spin and spin-orbital textures and band structures can be observed by spin-resolved angle-resolved photoemission spectroscopy.

We propose a fully ab initio method, the opposing crystal potential (OCP), to calculate the crystal-field parameters of transition-metal impurities in insulator hosts. Through constrained density functional calculations, OCP obtains the constraining Lagrange multipliers, which act as a cancellation potential against the crystal field and lead to spherical $d$-electron distribution. The method is applied to several insulators doped with ${\mathrm{Mn}}^{4+}$ and ${\mathrm{Mn}}^{2+}$ ions and shown to be in good agreement with experiment.

We present ab initio $GW$ plus cumulant-expansion calculations for an organic compound ${\left(\mathrm{TMTSF}\right)}_2{\mathrm{PF}}_6$ and a transition-metal oxide ${\mathrm{SrVO}}_3$. These materials exhibit characteristic low-energy band structures around the Fermi level, which bring about interesting low-energy properties; the low-energy bands near the Fermi level are isolated from the other bands, and, in the isolated bands, unusually low-energy plasmon excitations occur. To study the effect of this low-energy-plasmon fluctuation on the electronic structure, we calculate spectral functions and photoemission spectra using the ab initio cumulant expansion of the Green's function based on the $GW$ self-energy. We found that the low-energy plasmon fluctuation leads to an appreciable renormalization of the low-energy bands and a transfer of the spectral weight into the incoherent part, thus resulting in an agreement with experimental photoemission data.

We present a theoretical calculation of the absorption spectrum of cuprous oxide $\left({\mathrm{Cu}}_2\mathrm{O}\right)$ based on the general theory developed by Y. Toyozawa. An inclusion not only of acoustic phonons but also of optical phonons and of specific properties of the excitons in ${\mathrm{Cu}}_2\mathrm{O}$ like the central-cell corrections for the $1S$ exciton allows us to calculate the experimentally observed linewidths in experiments by T. Kazimierczuk et al. [T. Kazimierczuk, D. Fröhlich, S. Scheel, H. Stolz, and M. Bayer, Nature (London) 514, 343 (2014)] within the same order of magnitude, which demonstrates a clear improvement in comparison to earlier work on this topic. We also discuss a variety of further effects, which explain the still observable discrepancy between theory and experiment but can hardly be included in theoretical calculations.

The hyperfine interaction with nuclear spins (or, Overhauser noise) has long been viewed as a leading source of decoherence in individual quantum dot spin qubits. Here we show that in a coupled multiqubit system consisting of as few as four spins, interactions with nuclear spins can have the opposite effect where they instead preserve the collective quantum state of the system. This noise-induced state preservation can be realized in a linear spin qubit array using current technological capabilities. Our proposal requires no control over the Overhauser fields in the array; only experimental control over the average interqubit coupling between nearest neighbors is needed, and this is readily achieved by tuning gate voltages. Our results illustrate how the role of the environment can transform from harmful to helpful in the progression from single-qubit to multiqubit quantum systems.

The spin-momentum-locked nature of the robust surface states of three-dimensional topological insulators (3D TIs) makes them promising candidates for spintronics applications. Surface potentials which respect time-reversal symmetry can exist at the surface between a 3D TI and the trivial vacuum. These potentials can distort the spin texture of the surface states while retaining their gapless nature. In this work, the effect of all such surface potentials on the spin textures is studied. Since a tunnel magnetoresistance signal carries the information of the spin texture, it is proposed that spin-polarized tunneling of electrons to a 3D TI surface can be used to uniquely identify the surface potentials and quantitatively characterize them.