We present complete analytical and numerical results that demonstrate the anomalous universal fluctuations of the spin Hall conductance in chiral materials such as graphene and topological insulators. We investigate both the corresponding fluctuations, the universal fractionated and the universal quantized, and also the open channel orbital number crossover between the two regimes. In particular, we show that the Wigner-Dyson symmetries do not properly describe such conductances and the preponderant role of the chiral classes on the Dirac quantum dots. The results are analytical and solve outstanding issues.

We study the thermal conductivity of the disordered two-dimensional electron gas. To this end, we analyze the heat density–heat density correlation function concentrating on the scattering processes induced by the Coulomb interaction in the subtemperature energy range. These scattering processes are at the origin of logarithmic corrections violating the Wiedemann-Franz law. Special care is devoted to the definition of the heat density in the presence of the long-range Coulomb interaction. To clarify the structure of the correlation function, we present details of a perturbative calculation. While the conservation of energy strongly constrains the general form of the heat density–heat density correlation function, the balance of various terms turns out to be rather different from that for the correlation functions of other conserved quantities such as the density-density or spin density–spin density correlation function.

The one-band Hubbard model on the pyrochlore lattice contains an extended quantum spin-liquid phase formed from the manifold of singlet dimer coverings. We demonstrate that the massive and deconfined spinon excitations of this system have fermionic statistics. Holonic quasiparticles introduced by doping are also fermions and we explain this counterintuitive but general result.

We used atomistic simulations to study the origin of the change of resistance over time in the amorphous phase of GeTe, a prototypical phase-change material (PCM). Understanding the cause of resistance drift is one of the biggest challenges to improve multilevel storage technology. For this purpose, we generated amorphous structures via classical molecular-dynamics simulations under conditions as close as possible to the experimental operating ones of such memory devices. Moreover, we used the replica-exchange technique to generate structures comparable with those obtained in the experiment after long annealing that show an increase of resistance. This framework allowed us to overcome the main limitation of previous simulations, based on density-functional theory, that suffered from being computationally too expensive therefore limited to the nanosecond time scale. We found that resistance drift is caused by consumption of Ge atom clusters in which the coordination of at least one Ge atom differs from that of the crystalline phase and by removal of stretched bonds in the amorphous network, leading to a shift of the Fermi level towards the middle of the band gap. These results show that one route to design better memory devices based on current chalcogenide alloys is to reduce the resistance drift by increasing the rigidity of the amorphous network.

The dynamics of the free-exciton capture by boron acceptors and phosphorus donors in diamond is observed in the picosecond range by time-resolved photoluminescence experiments at low temperature. The formation of boron-bound excitons is observed with a delay of 410 ps after the formation of free excitons. For phosphorus, this delay is 120 ps. This is the result of the free-exciton capture by ${\mathrm{B}}^0$ and ${\mathrm{P}}^0$ impurities. The lifetimes of boron- and phosphorus-bound excitons are measured and found to be equal to 270 and 70 ps, respectively. These values are about four orders of magnitude shorter than for the same impurities in silicon. $E_i$ being the ionization energy of dopants, these results scale well with the $E_i^4$ dependence of the Auger recombination rate expected for bound excitons in indirect band-gap semiconductors.

The (0001) surface of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ is readily exposed by cleavage between two van der Waals (vdW) bonded quintuple layers (QLs) and it is widely accepted to be the most stable surface. However, little is known of the stability of other surfaces which may play important roles in growth, kinetics, and electronics. In this work, we identify the stability of the non-vdW-exfoliated surfaces of ${\mathrm{Bi}}_{}{\mathrm{Te}}_3$ using first-principles density functional theory calculations. It is found that the energetics of the surfaces follows the order of $\left(0001\right)\to \left(01\overline{1}5\right)\to \left(11\overline{2}0\right)\to \left(01\overline{1}0\right)\to \left(01\overline{1}\overline{10}\right)\to \left(01\overline{1}\overline{4})-\mathrm{A}/\mathrm{B}\to \right(01\overline{1}\overline{1})-\mathrm{A}/\mathrm{B}\to (02\overline{2}\overline{5}$), which can be understood almost entirely in terms of the density of dangling bonds on each surface. Using a modified Wulff construction theory, we determine an evolution diagram for sample geometry as a function of the binding of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ to the substrate. As binding to the substrate increases, the sample geometry transitions from three-dimensional to two-dimensional and from hexagonal to triangular in nature.

We study the influence of ripple waves on the band structures of strained armchair graphene nanoribbons. We argue that the Zeeman pseudospin (p-spin) splitting energy induced by ripple waves might not be neglected for smaller widths of armchair graphene nanoribbons (GNRs). We show that the p-spin splitting energy breaks the symmetry of degeneracy due to the ripple-induced Zeeman effect in GNRs, originating from electromechanical coupling. We estimate the p-spin lifetime in strained armchair GNRs caused by in-plane phonon modes for possible applications in straintronics and quantum information processing. By considering higher order terms in the strain tensor expansion, we also demonstrate that highly asymmetric band structures of GNRs induce asymmetric phonon-mediated p-spin relaxation. Such asymmetric p-spin relaxation is not possible for unstrained armchair and zigzag GNRs. In particular, we report that the p-spin transition rate decreases like $B_0^5$ (as a function of p-magnetic fields), $L^{-9}$ (as a function of GNR width) and ${\tau }_e^{-1}$, where ${\tau }_e$ is the externally applied tensile edge stress.

We investigate ultrathin highly ordered layers of tin(II)-phthalocyanine (SnPc) on top of a monolayer (ML) of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) on Ag(111). The films are analyzed structurally by means of scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED) as well as optically using differential reflectance spectroscopy (DRS). We find that the first ML of SnPc is entirely rearranged upon bilayer (BL) formation, yielding a commensurate registry in higher-order coincidence with the underlying PTCDA lattice. SnPc layers adsorbed on top self-assemble in further BLs. Within each BL the molecules are arranged pairwise, i.e., stacked as physical dimers, providing a characteristic absorption spectrum with strongly redshifted components compared to SnPc monomers. This altered spectral envelope mainly originates from strong orbital overlap of stacked molecules within each BL. In contrast, adjacent BLs show only weak orbital overlap, which is responsible for an additional redshift of the low-energy transition band. Our results demonstrate that a simple modification of the metal substrate surface, e.g., by a PTCDA wetting layer, has beneficial effects on structural ordering of SnPc multilayers adsorbed on top. The impact on the optical absorption spectrum manifests in a narrow and intense absorption peak in the near-infrared spectral region which is significantly less pronounced if the PTCDA layer is omitted.

The Weyl semimetal NbP was found to exhibit topological Fermi arcs and exotic magnetotransport properties. Here, we report on magnetic quantum-oscillation measurements on NbP and construct the three-dimensional Fermi surface with the help of band-structure calculations. We reveal a pair of spin-orbit-split electron pockets at the Fermi energy and a similar pair of hole pockets, all of which are strongly anisotropic. The Weyl points that are located in the $k_z\approx \pi /c$ plane are found to exist 5 meV above the Fermi energy. Therefore, we predict that the chiral anomaly effect can be realized in NbP by electron doping to drive the Fermi energy to the Weyl points.

We investigate the transport properties of different realizations of one-dimensional quantum wires coupled to a number of external electrodes in terms of the full counting statistics. Focusing on the setups in which edge states of Majorana type are realizable, we analyze the effects of the exchange of charge carriers with the environment. We find a very strong suppression of the nonlocal current correlations, which decay exponentially with the coupling strength and the wire length. Local properties such as transport currents and current correlations measured at individual contacts between the wire and external electrodes turn out to develop an asymmetry when they are energy resolved. We provide a number of exact analytical solutions and discuss experimental implications of our findings.

We build an effective impedance for two-dimensional (2D) photonic crystals (PCs) comprising a rectangular lattice of dielectric cylinders with the incident electric field polarized along the axis of the cylinders. In particular, we discuss the feasibility of constructing an effective impedance for the case where the Bloch wave vector is far away from the center of Brillouin zone, where the optical response of the PC is necessarily anisotropic, and hence the effective description becomes inevitably angle dependent. We employ the scattering theory and treat the 2D system as a stack of 1D arrays. We consider only the zero-order interlayer diffraction, and all the higher order diffraction terms of interlayer scattering are ignored. This approximation works well when the higher order diffraction terms are all evanescent waves and the interlayer distance is far enough for them to decay out. Scattering theory enables the calculation of transmission and reflection coefficients of a finite-sized slab, and we extract the effective parameters such as the effective impedance $\left(Z_{\mathrm{e}}\right)$ and the effective refractive index $\left(n_{\mathrm{e}}\right)$ using a parameter retrieval method. We note that $n_{\mathrm{e}}$ is uniquely defined only in a very limited region of the reciprocal space. $(n_{\mathrm{e}}{k}_{0}a\ll 1$, where $k_0$ is the wave vector inside the vacuum and $a$ is thickness of the slab for retrieval), but $Z_{\mathrm{e}}$ is uniquely defined and has a well-defined meaning inside a much larger domain in the reciprocal space. For a lossless system, the effective impedance $Z_{\mathrm{e}}$ is purely real for the pass band and purely imaginary in the band gaps. Using the sign of the imaginary part of $Z_{\mathrm{e}}$, we can classify the band gaps into two groups, and this classification explains why there is usually no surface state on the boundary of typical fully gapped PCs composed of a lattice of dielectric cylinders. This effective medium approach also allows us to predict the dispersion of surface states even when the surface wave vectors are well beyond the zone center.

We study the competition of spin- and charge-density waves and their quantum multicritical behavior for the semimetal-insulator transitions of low-dimensional Dirac fermions. Employing the effective Gross-Neveu-Yukawa theory with two order parameters as a model for graphene and a growing number of other two-dimensional Dirac materials allows us to describe the physics near the multicritical point at which the semimetallic and the spin- and charge-density-wave phases meet. With the help of a functional renormalization group approach, we are able to reveal a complex structure of fixed points, the stability properties of which decisively depend on the number of Dirac fermions $N_f$. We give estimates for the critical exponents and observe crucial quantitative corrections as compared to the previous first-order $\epsilon$ expansion. For small $N_f$, the universal behavior near the multicritical point is determined by the chiral Heisenberg universality class supplemented by a decoupled, purely bosonic, Ising sector. At large $N_f$, a novel fixed point with nontrivial couplings between all sectors becomes stable. At intermediate $N_f$, including the graphene case $\left(N_f=2\right)$, no stable and physically admissible fixed point exists. Graphene's phase diagram in the vicinity of the intersection between the semimetal, antiferromagnetic, and staggered density phases should consequently be governed by a triple point exhibiting first-order transitions.

Heterostructures utilizing topological insulators exhibit a remarkable spin-torque efficiency. However, the exact origin of the strong torque, in particular whether it stems from the spin-momentum locking of the topological surface states or rather from spin-Hall physics of the topological-insulator bulk, remains unclear. Here, we explore a mechanism of spin-torque generation purely based on the topological surface states. We consider topological-insulator-based bilayers involving ferromagnetic metal (TI/FM) and magnetically doped topological insulators (TI/mdTI), respectively. By ascribing the key theoretical differences between the two setups to location and number of active surface states, we describe both setups within the same framework of spin diffusion of the nonequilibrium spin density of the topological surface states. For the TI/FM bilayer, we find large spin-torque efficiencies of roughly equal magnitude for both in-plane and out-of-plane spin torques. For the TI/mdTI bilayer, we elucidate the dominance of the spin-transfer-like torque. However, we cannot explain the orders of magnitude enhancement reported. Nevertheless, our model gives an intuitive picture of spin-torque generation in topological-insulator-based bilayers and provides theoretical constraints on spin-torque generation due to topological surface states.

A multifaceted approach is used to identify the Fe centers associated with $\mathrm{KNb}{\mathrm{O}}_3:\mathrm{Fe}$, determine the location of energy levels, and conclude which centers play a vital role in the photorefractive effect; with such an understanding, the physical parameters may be modified to provide mature materials for dynamic holographic applications. A correlated study is performed on as-grown and reduced $\mathrm{KNb}{\mathrm{O}}_3:\mathrm{Fe}$ crystals, where a uniform reduction is achieved through a modified electroreduction process. This investigation identifies which Fe centers are reduced and which are unaffected, allowing the existence of both charge donors and acceptors as required for photorefraction, resulting in significant improvements of the photorefractive properties. Available charge transitions as a function of photon energy are identified and associated with conditions necessary for major improvements in beam-coupling efficiencies and response times. The understanding of the dynamics and defect structure is supported by photorefractive beam-coupling data. A revised explanation of the photorefractive dynamics is given as this understanding of the defects in $\mathrm{KNb}{\mathrm{O}}_3:\mathrm{Fe}$ is no longer described by the commonly used standard model. This new fundamental understanding that enables the development of improved materials for dynamic holographic applications may also be transferred to other materials classes for disparate applications.

We combined synchrotron-based infrared and Raman spectroscopies, diamond anvil cell techniques, and first principles calculations to unveil pressure-induced distortions in quasi-one-dimensional Cu(pyz)$\left({\mathrm{NO}}_3\right){}_2$. The crossover at 0.7 GPa is local in nature whereas the transition at 5 GPa lowers symmetry from $Pmna$ to $P{222}_1$ and is predicted to slightly increase magnetic dimensionality. Comparison with prior magnetoinfrared results reveals the striking role of out-of-plane bending of the pyrazine ligand, a finding that we discuss in terms of the possibility of using pressure to bias the magnetic quantum critical transition in this classic $S=1/2$ antiferromagnet.

We perform theoretical studies on the newly discovered superconductor FeS. Using density functional theory, we obtain the electronic structure of FeS. Using maximally localized Wannier functions, we obtain an effective tight-binding model with five Fe $3d$ orbitals. Based on this model, we investigate, by the unbiased singular-mode functional renormalization group, the superconductivity derived from correlation effects. We find the Hund's rule coupling plays a key role in triggering superconductivity, and the pairing symmetry is $d_{x^2-y^2}$ wave (viewed in the unfolded zone). The gap function is nodal/nodeless on the hole/electron Fermi pockets, similarly to the case in LaOFeP. The nodal gap is in agreement with experiments.

We present a theory for the inverse spin Hall effect in a thin film of topological insulator (TI) ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, connected to a reservoir with applied spin bias, in the ballistic regime. In the case that either the spin polarization of the spin bias is along the longitudinal direction, or the hybridization gap $\mathrm{\Delta }$ of the surface states vanishes, the spin Hall angle ${\mathrm{\Theta }}_{\mathrm{sh}}$ tends to infinity, indicating that the spin bias is perfectly converted into a measurable transverse charge current, essentially without generating a longitudinal spin current in the TI. In other cases, with increasing the Fermi energy $E_{\mathrm{F}}$ from the bottom of the conduction band of surface states, ${\mathrm{\Theta }}_{\mathrm{sh}}$ grows continuously from zero and exhibits an interesting linear dependence on $E_{\mathrm{F}}/\mathrm{\Delta }$ for $E_{\mathrm{F}}\gg \mathrm{\Delta }$. We also find that the inverse Edelstein effect occurs, when the in-plane transverse component of the spin polarization vector is nonzero. The spin-to-charge conversion becomes complete, when the spin polarization vector is along the transverse direction, or the hybridization gap $\mathrm{\Delta }$ vanishes.

Irradiation with 4 MeV protons was used to systematically introduce defects in single crystals of the iron-arsenide superconductor ${\mathrm{BaFe}}_2\left({\mathrm{As}}_{1-x}{\mathrm{P}}_x\right){}_2, x=0.33$. The effect of disorder on the low-temperature behavior of the London penetration depth $\lambda \left(T\right)$ and transition temperature $T_c$ was investigated. In nearly optimally doped samples with $T_c\sim 29$ K, signatures of a superconducting gap with nodes were observed. Contrary to previous reports on electron-irradiated crystals, we do not see a disorder-driven lifting of accidental nodes, and we observe that proton-induced defects are weaker pair breakers than electron-induced defects. We attribute our findings to anisotropic electron scattering caused by proton irradiation defects.

We study the nature of excitons bound to $I_1$ basal plane stacking faults in ensembles of ultrathin GaN nanowires by continuous-wave and time-resolved photoluminescence spectroscopy. These ultrathin nanowires, obtained by the thermal decomposition of spontaneously formed GaN nanowire ensembles, are tapered and have tip diameters down to 6 nm. With decreasing nanowire diameter, we observe a strong blueshift of the transition originating from the radiative decay of stacking fault-bound excitons. Moreover, the radiative lifetime of this transition in the ultrathin nanowires is independent of temperature up to 60 K and significantly longer than that of the corresponding transition in as-grown nanowires. These findings reveal a zero-dimensional character of the confined exciton state and thus demonstrate that $I_1$ stacking faults in ultrathin nanowires act as genuine quantum dots.