We studied the plateau-plateau transitions that characterize the electrical transport in the quantum Hall regime in a high mobility bilayer graphene flake encapsulated by hexagonal boron nitride at magnetic fields up to 9 T and temperatures above 300 mK. We measured independently the exponent $\kappa$ of the temperature-induced transition broadening, the critical exponent $\gamma$ of the localization length, and the exponent $p$ ruling the temperature scaling of the coherence length, finding consistency with the relation $\gamma =p/2\kappa$. The observed value of $\kappa =0.30(0.28,0.32)$ deviates from that of the quantum Hall critical point. The obtained $\gamma =1.25(0.96,1.54)$ questions the validity of a pure Anderson transition, and reveals percolation as the underlying driving mechanism.

Although hydrogen is the simplest of all molecular species, its nuclear quantum effects dominate the structure and thermodynamical properties of the condensed phases. With a recently developed nonempirical quantum molecular dynamics simulation method, we present intuitive understandings of real-time dynamics of each $\mathit{para}$-hydrogen$\left(p\text{−}{\mathrm{H}}_2\right)$ molecule in the liquid phase including its H-H bond vibrations, molecular orientations, and librational motions. The short- and long-living memories of angular dynamics we found suggest that $p\text{−}{\mathrm{H}}_2$ should be described as a librating nonspherical diatomic molecule and can be characterized by two typical dynamics: kinetic motions inside a solvation shell and diffusive dynamics out of the shell. The real-time trajectory reveals that structural rearrangements of a $p\text{−}{\mathrm{H}}_2$ molecule such as breakout from a solvation shell significantly correlate with its intramolecular structure and fluctuations. The H-H bond power spectra completely fitted by two functions also indicate that liquid $p\text{−}{\mathrm{H}}_2$ has two typical structures. The condensed-phase effects on shifts and broadening of H-H bond stretching frequencies, intrinsic librational dynamics, and molecular orientational distributions are computationally demonstrated and physically rationalized.

We study the effect of starting point and self-consistency within $GW$ on the band-edge positions of semiconductors and insulators. Compared to calculations based on a semilocal starting point, the use of a hybrid-functional starting point shows a larger quasiparticle correction for both band-edge states. When the self-consistent treatment is employed, the band-gap opening is found to result mostly from a shift of the valence-band edge. Within the non-self-consistent methods, we analyse the performance of empirical and nonempirical schemes in which the starting point is optimally tuned. We further assess the accuracy of the band-edge positions through the calculation of ionization potentials of surfaces. The ionization potentials for most systems are reasonably well described by one-shot calculations. However, in the case of ${\mathrm{TiO}}_2$, we find that the use of self-consistency is critical to obtain a good agreement with experiment.

We generalize Laughlin's flux insertion argument, originally discussed in the context of the quantum Hall effect, to topological phases protected by non-on-site unitary symmetries, in particular by parity symmetry or parity symmetry combined with an on-site unitary symmetry. As a model, we discuss fermionic or bosonic systems in two spatial dimensions with CP symmetry, which are, by the CPT theorem, related to time-reversal symmetric topological insulators (e.g., the quantum spin Hall effect). In particular, we develop the stability/instability (or “gappability”/“ingappablity”) criteria for nonchiral conformal field theories with parity symmetry that may emerge as an edge state of a symmetry-protected topological phase. A necessary ingredient, as it turns out, is to consider the edge conformal field theories on unoriented surfaces, such as the Klein bottle, which arises naturally from enforcing parity symmetry by a projection operation.

We show that geometric frustration and strong correlation in the triangular-lattice Hubbard model lead a rich phase structure of $\sqrt{3}\times \sqrt{3}$ spin-charge textured electronic states over a wide region of electron doping $0\le x\le 0.40$. In addition to the ${120}^{\circ }$ Néel ordered insulator at half filling, we found a spin charge ordered insulator at $x=1/3$ with collinear antiferromagnetic (AF) order on the underlying unfrustrated honeycomb lattice. Separating the two insulating phases is a Lifshitz transition between a noncollinear AF ordered metal and one with coexisting charge order. We obtain the phase diagram and the evolution of the Fermi surface (FS). Remarkably, the correlated ground states near $x=1/3$ emerge as doping the “1/3 AF insulator” by excess carriers, leading to electron and hole FS pockets with important implications for the cobaltate superconducting state.

Spin-orbit (SO) coupling is the crucial parameter to drive topological-insulating phases in electronic band models. In particular, the generic emergence of SO coupling involves the Rashba term which fully breaks the SU(2) spin symmetry. As soon as interactions are taken into account, however, many theoretical studies have to content themselves with the analysis of a simplified U(1)-conserving SO term without Rashba coupling. We intend to fill this gap by studying the Kane-Mele-Hubbard (KMH) model in the presence of Rashba SO coupling and present the first systematic analysis of the effect of Rashba SO coupling in a correlated two-dimensional topological insulator. We apply the variational cluster approach (VCA) to determine the interacting phase diagram by computing local density of states, magnetization, single particle spectral function, and edge states. Preceded by a detailed VCA analysis of the KMH model in the presence of U(1)-conserving SO coupling, we find that the additional Rashba SO coupling drives new electronic phases such as a metallic regime and a weak topological-semiconductor phase which persist in the presence of interactions.

${\mathrm{KTaO}}_3$ is an incipient ferroelectric material with an extrapolated transition temperature below 0 K. It contains a small number of “unavoidable defects” which are randomly distributed. Some of these defects are polar and their interaction leads to macroscopic coherent polar structures at low temperatures. In this article it is shown that freezing of local defect dipoles coincides with elastic stiffening and damping of ultrasonic waves in ${\mathrm{KTaO}}_3$. The elastic freezing anomalies are accompanied by stepwise increases of piezoelectricity, forming a thermal polar staircase below ca. 120 K and a gigantic enhancement below 50 K. A small spontaneous polarization also emerges below this temperature, gradually increasing to a value of $0.045 \mu \mathrm{C} \mathrm{cm}{}^{-2}$ at 5 K with increasing coherency of defect dipoles. The orientation of this spontaneous polarization depends on a weak strain-induced anisotropy of the macroscopic sample. Defect-induced ferroelectricity, as demonstrated for ${\mathrm{KTaO}}_3$, may be a possible way forward to develop functional device materials based on the switching of coherently interacting defects.

We observe a strong negative magnetoresistance at nonquantizing magnetic fields in a high-mobility two-dimensional electron gas. This strong negative magnetoresistance consists of a narrow peak around zero magnetic field and a huge magnetoresistance at larger fields. The peak shows parabolic magnetic field dependence and is attributed to the interplay of smooth disorder and rare strong scatterers. We identify the rare strong scatterers as macroscopic defects in the material and determine their density from the peak curvature.

Metals interacting via short-range antiferromagnetic fluctuations are unstable to sign-changing superconductivity at low temperatures. For the cuprates, this leading instability leads to the well-known $d$-wave superconducting state. However, there is also a secondary instability to an incommensurate charge-density wave, with a predominantly $d$-wave form factor, arising from the same antiferromagnetic fluctuations. Recent experiments in the pseudogap regime of the hole-doped cuprates have found strong evidence for such a charge-density wave order and, in particular, the predicted $d$-wave form factor. However, the observed wave vector of the charge order differs from the leading instability in Hartree-Fock theory, and is that of a subleading instability. In this paper, we examine the feedback of superconducting fluctuations on these different charge-density wave states, and find that over at least a small temperature window, they prefer the experimentally observed wave vector.

A flop of electric polarization from $P\parallel c \left(P_c\right)$ to $P\parallel a \left(P_a\right)$ is observed in ${\mathrm{MnTiO}}_3$ as a spin-flop transition is triggered by a $c$-axis magnetic field, $H_{\parallel c}=7$ T. The critical magnetic field $H_{\parallel c}$ for $P_a$ is significantly reduced in ${\mathrm{Mn}}_{1-x}{\mathrm{Ni}}_x{\mathrm{TiO}}_3 \left(x=0.33\right)$. $P_a$ and $P_c$ have been observed with both $H_{\parallel c}$ and $H_{\parallel a}$. Neutron diffraction measurements revealed similar magnetic arrangements for the two compositions where the ordered spins couple antiferromagnetically with their nearest intra- and interplanar neighbors. In the $x=0.33$ system, the uniaxial and planar anisotropies of $\mathrm{Mn}{}^{2+}$ and $\mathrm{Ni}{}^{2+}$ compete and give rise to a spin reorientation transition at $T_R$. A magnetic field, $H_{\parallel c}$, aligns the spins along $c$ for $T_R<T<T_N$. The rotation of the collinear spins away from the $c$ axis for $T<T_R$ alters the magnetic point symmetry and gives rise to a new magnetoelectric (ME) susceptibility tensor form. Such linear ME response provides a satisfactory explanation for the behavior of the field-induced electric polarization in both compositions. As the Ni content increases to $x=0.5$ and 0.68, the ME effect disappears as a new magnetic phase emerges.

We investigate the angular dependence of the spin torque generated when applying a temperature difference across a spin valve. Our study shows the presence of a nontrivial fixed point in this angular dependence. This fixed point opens the possibility for a temperature gradient to stabilize radio frequency oscillations without the need for an external magnetic field. This so-called “wavy” behavior can already be found upon applying a voltage difference across a spin valve but we find that this effect is much more pronounced with a temperature difference. We find that a spin asymmetry of the Seebeck coefficient of the order of $20\mu {\mathrm{V}\mathrm{K}}^{-1}$ should be large enough for a temperature gradient of a few degrees to trigger the radio-frequency oscillations. Our semiclassical theory is fully parametrized with experimentally measured(able) parameters and allows one to quantitatively predict the amplitude of the torque.

We report results from a systematic electron-transport study in a broad temperature range on $12 {\mathrm{CaB}}_6$ single crystals. None of the crystals were intentionally doped. The different carrier densities observed presumably arise from slight variations in the Ca:B stoichiometry. In these crystals, the variation of the electrical resistivity and of the Hall effect with temperature can be consistently accounted for by the model we propose, in which B-antisite defects (B atom replacing Ca atom) are “amphoteric.” The magnetotransport measurements reveal that most of the samples we have studied are close to a metal-insulator transition at low temperatures. The magnetoresistance changes smoothly from negative—for weakly metallic samples—to positive values—for samples in a localized regime.

We present an ab initio study of the hybridization of localized surface plasmons in a metal nanoparticle dimer. The atomic structure, which is often neglected in theoretical studies of quantum nanoplasmonics, has a strong impact on the optical absorption properties when subnanometric gaps between the nanoparticles are considered. We demonstrate that this influences the hybridization of optical resonances of the dimer, and leads to significantly smaller electric field enhancements as compared to the standard jellium model. In addition, we show that the corrugation of the metal surface at a microscopic scale becomes as important as other well-known quantum corrections to the plasmonic response, implying that the atomic structure has to be taken into account to obtain quantitative predictions for realistic nanoplasmonic devices.

Temperature and electric field effects on rhombohedral $\left(R\right)$ ternary ${\mathrm{Pb}(\mathrm{In}}_{1/2}{\mathrm{Nb}}_{1/2}){\mathrm{O}}_3\text{−}{\mathrm{Pb}(\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}{)\mathrm{O}}_3\text{−}{\mathrm{PbTiO}}_3$ (PIN-PMN-PT) ferroelectric single crystals were comprehensively studied by x-ray diffraction. We have focused on how the individual phase transitions as well as the phase transition sequences depend on thermal and electrical history. Electric field-temperature phase diagrams have been constructed under [001] field-cooling and field-heating conditions. As happens to the $R$ phase of binary PMN-PT crystals [H. Cao, J. F. Li, D. Viehland, and G. Y. Xu, Phys. Rev. B 73, 184110 (2006)], the $R$ phase of the zero-field-cooled (ZFC) state is replaced by a monoclinic A $(M$) phase in the field-cooled (FC) diagram. In particular, reciprocal-space mesh scans demonstrated that the $M$ phase was stable for crystals poled along the [001] crystallographic direction rather than the initial $R$ phase of the ZFC state. Furthermore, an $E$-field-induced phase transformational sequence of $R\to M$→ tetragonal $\left(T\right)$ was observed at constant temperature, revealing a gradual increase in the $c$ lattice parameter. These findings demonstrate that the ternary PIN-PMN-PT crystals exhibit common phase transition features with binary PMN-PT and ${\mathrm{Pb}(\mathrm{Zn}}_{1/3}{\mathrm{Nb}}_{2/3}{)\mathrm{O}}_3\text{−}{\mathrm{PbTiO}}_3$ (PZN-PT) ones for compositions in the low PT side of the morphotropic phase boundary.

We present an optical spectroscopy study on P-doped ${\mathrm{CaFe}}_2{\mathrm{As}}_2$ which experiences a structural phase transition from tetragonal to collapsed tetragonal (cT) phase near 75 K. The measurement reveals a sudden reduction of low-frequency spectral weight and the emergence of a feature near $3200 \mathrm{cm}{}^{-1}$ (0.4 eV) in optical conductivity across the transition, indicating an abrupt reconstruction of band structure. The appearance of the feature is related to the interband transition arising from the sinking of hole bands near the $\Gamma$ point below Fermi level in the cT phase, as expected from the density function theory calculations in combination with the dynamical mean field theory. However, the reduction of Drude spectral weight is at variance with those calculations. The measurement also indicates an absence of the abnormal spectral weight transfer at high energy (near 0.5–0.7 eV) in the cT phase, suggesting a suppression of the electron correlation effect.

We theoretically study the magnetic field orientation dependence of longitudinal and transverse flux line lattice form factors in uniaxial superconductors with anisotropy ratio corresponding to ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$. We discuss influences of the anisotropy ratio of coherence length and differences between the $s$-wave and the $d_{x^2-y^2}$-wave pairings. The calculations are performed by two methods, the Eilenberger theory and the London theory comparatively, and we study the cutoff function of the extended London theory, which will be helpful in the analysis of the small angle neutron scattering in the vortex states.

We report on the determination of micromagnetic parameters of epilayers of the ferromagnetic semiconductor (Ga,Mn)As, which has an easy axis in the sample plane, and (Ga,Mn)(As,P), which has an easy axis perpendicular to the sample plane. We use an optical analog of ferromagnetic resonance where the laser-pulse-induced precession of magnetization is measured directly in the time domain. By the analysis of a single set of pump-and-probe magneto-optical data, we determined the magnetic anisotropy fields, the spin stiffness, and the Gilbert damping constant in these two materials. We show that incorporation of 10% of phosphorus in (Ga,Mn)As with 6% of manganese leads not only to the expected sign change of the perpendicular-to-plane anisotropy field but also to an increase of the Gilbert damping and to a reduction of the spin stiffness. The observed changes in the micromagnetic parameters upon incorporating P in (Ga,Mn)As are consistent with the reduced hole density, conductivity, and Curie temperature of the (Ga,Mn)(As,P) material. We also show that the apparent magnetization precession damping is stronger for the $n=1$ spin wave resonance mode than for the $n=0$ uniform magnetization precession mode.

We report on the formation of one-dimensional Au/Ag core-shell nanostructures in superfluid helium nanodroplets $\left({\mathrm{He}}_N\right)$ and their subsequent surface deposition under soft landing conditions. In vortex charged ${\mathrm{He}}_N$, dopant atoms and clusters prefer to agglomerate along vortex cores, which serve as personal cryo-templates for each nanowire. A sequential pickup scheme enables the fabrication of Au/Ag structures with either Au as core and Ag as shell or vice versa. The core-shell structure is in both cases shown by energy-dispersive x-ray spectroscopy (EDX) in a scanning transmission electron microscope (STEM). The inherent spatial resolution and element sensitivity of this method allows a direct observation of the two different phases. High resolution STEM studies elucidate morphological details of the Au/Ag nanostructures and, together with the EDX analysis, indicate that the nanowires are formed in a multicenter aggregation process.

We propose classification schemes for characterizing two-dimensional topological phases with nontrivial weak indices. Here, weak implies that the Chern number in the corresponding phase is trivial, while the system shows edge states along specific boundaries. As concrete examples, we analyze different versions of the so-called Wilson-Dirac model with (i) anisotropic Wilson terms, (ii) next-nearest-neighbor hopping terms, and (iii) a superlattice generalization of the model, here in the tight-binding implementation. For types (i) and (ii) a graphic classification of strong properties is successfully generalized for classifying weak properties. As for type (iii), weak properties are attributed to quantized Berry phase $\pi$ along a Wilson loop.