We describe a large-disorder renormalization group (LDRG) method for the Anderson model of localization in one dimension which decimates eigenstates based on the size of their wave functions rather than their energy. We show that our LDRG scheme flows to infinite disorder, and thus becomes asymptotically exact. We use it to obtain the disorder-averaged inverse participation ratio (IPR) and density of states (DOS) for the entire spectrum. A modified scheme is formulated for higher dimensions, which is found to be less efficient, but capable of improvement.

We performed a first-principles study of the structural, vibrational, electronic, and magnetic properties of ${\mathrm{NaMnF}}_3$ under applied isotropic pressure. We found that ${\mathrm{NaMnF}}_3$ undergoes a reconstructive phase transition at 8 GPa from the $Pnma$ distorted perovskite structure toward the $Cmcm$ postperovskite structure. This is confirmed by a sudden change of the Mn-F-Mn bondings where the crystal goes from corner-shared octahedra in the $Pnma$ phase to edge-shared octahedra in the $Cmcm$ phase. The magnetic ordering also changes from a $G$-type antiferromagnetic ordering in the $Pnma$ phase to a $C$-type antiferromagnetic ordering in the $Cmcm$ phase. Interestingly, we found that the high-spin $d$-orbital filling is kept at the phase transition which has never been observed in the known magnetic postperovskite structures. We also found a highly noncollinear magnetic ordering in the $Cmcm$ postperovskite phase that drives a large ferromagnetic canting of the spins. We discuss the validity of these results with respect to the $U$ and $J$ parameters of the $GGA+U$ exchange-correlation functional used in our study and conclude that large-spin canting is a promising property of the postperovskite fluoride compounds.

We show on the basis of electronic structure calculations that the uranium $5f$ magnetic moment in ${\mathrm{URu}}_2{\mathrm{Si}}_2$ exhibits a unique Ising behavior, which surprisingly arises from itinerant electronic states. The origin of the unusual Ising behavior is analyzed as due to the peculiar near-Fermi edge nested electronic structure of ${\mathrm{URu}}_2{\mathrm{Si}}_2$ involving its strong spin-orbit interaction. The Ising anisotropy has pertinent implications for theories applicable to explaining the hidden order phase in ${\mathrm{URu}}_2{\mathrm{Si}}_2$.

We have investigated anomalously slow magnetic domain growth in an antiferromagnetic (AF) phase of isosceles triangular lattice Ising antiferromagnets ${\mathrm{Co}}_{1-x}{\mathrm{Mg}}_x{\mathrm{Nb}}_2{\mathrm{O}}_6$ with $x=0$ and 0.004, by means of time-resolved neutron diffraction measurements. Applying the multi-profile-deconvolution analysis to the observed diffraction profiles, we have revealed that time evolutions of spin correlation lengths along the $a$ and $b$ axes, ${\xi }_a$ and ${\xi }_b$, are well described by a power-law scaling form of ${\xi }_{0\alpha }+L_{\alpha }{(t/{\tau }_{\alpha })}^n$ $\left(\alpha =a,b\right)$ with a universal growth exponent of $n=0.2$. The characteristic time scale of the domain growth, ${\tau }_{\alpha }$, was found to exhibit Arrhenius-type temperature dependence in the AF phase. A comparison between the results of the $x=0$ and 0.004 samples has revealed that the nonmagnetic substitution significantly reduces the initial correlation length, ${\xi }_{0\alpha }$, and the activation energy in the Arrhenius-type temperature dependence of ${\tau }_{\alpha }$. We have also found that magnetic domain growth in a magnetic-field-induced ferrimagnetic phase also follows the power law with the growth exponent of $n=0.2$. On the basis of these results, we have concluded that the existence of ferromagnetic Ising spin chains running along the $c$ axis and the geometrical spin frustration in the $ab$ plane are the keys to the domain growth kinetics in this system. The former and the latter govern the characteristic time scale and the growth exponent, respectively.

Superconductivity (SC) and charge-density wave (CDW) are two contrasting yet relevant collective electronic states, which have received sustained interest for decades. Here, we report that, in a layered europium bismuth sulfofluoride, ${\mathrm{EuBiS}}_2\mathrm{F}$, a CDW-like transition occurs at 280 K, below which SC emerges at 0.3 K, without any extrinsic doping. The Eu ions were found to exhibit an anomalously temperature-independent mixed valence of about +2.2, associated with the formation of a possible dynamic CDW. The mixed valence of Eu gives rise to self electron doping into the conduction bands mainly consisting of the in-plane $\mathrm{Bi}6p$ states, which in turn brings about the CDW and SC. In particular, the electronic specific-heat coefficient is enhanced by $\sim 50$ times, owing to the significant hybridizations between $\mathrm{Eu}4f$ and $\mathrm{Bi}6p$ electrons, as verified by band-structure calculations. Thus ${\mathrm{EuBiS}}_2\mathrm{F}$ manifests itself as an unprecedented material that simultaneously accommodates SC, CDW, and $f$-electron valence instability.

We study models of interacting fermions in one dimension to investigate the crossover from integrability to nonintegrability, i.e., quantum chaos, as a function of system size. Using exact diagonalization of finite-sized systems, we study this crossover by obtaining the energy level statistics and Drude weight associated with transport. Our results reinforce the idea that for system size $L\to \infty$ nonintegrability sets in for an arbitrarily small integrability-breaking perturbation. The crossover value of the perturbation scales as a power law $\sim L^{-\eta }$ when the integrable system is gapless. The exponent $\eta \approx 3$ appears to be robust to microscopic details and the precise form of the perturbation. We conjecture that the exponent in the power law is characteristic of the random matrix ensemble describing the nonintegrable system. For systems with a gap, the crossover scaling appears to be faster than a power law.

We propose $B$-site ${\mathrm{Bi}}^{5+}$-doped ferroelectric perovskite materials as suitable candidates for the bulk photovoltaic effect and related solar applications. The low-lying $6s$ empty states of the electronegative Bi atom produce empty bands in the energy gap of the parent materials, effectively lowering the band gap by 1–2 eV, depending on the composition of the ferroelectric end member and the concentration of ${\mathrm{Bi}}^{5+}$ in the solid solution. The polarization decreases but survives upon doping, which enables the “shift-current” mechanism for photocurrent generation, while the decreased band gap allows absorption of much of the visible spectrum. The magnitude of shift-current response is calculated for $0.75{\mathrm{Pb}}_2{\mathrm{InNbO}}_6\text{−}0.25{\mathrm{Ba}}_2{\mathrm{InBiO}}_6$ (PIN-BIB) and $0.75{\mathrm{Pb}}_2{\mathrm{ScNbO}}_6\text{−}0.25{\mathrm{Ba}}_2{\mathrm{ScNbO}}_6$ (PSN-BSB) and is predicted to exceed the visible-light bulk photovoltaic response of all previously reported materials, including ${\mathrm{BiFeO}}_3$. Furthermore, the existence of their intermediate bands and multiple band gaps, combined with Fermi-level tuning by $A$-site co-doping, also allows for their potential application in traditional $p-n$ junction-based solar cells as broad-spectrum photoabsorbers.

Intermediate-band (IB) photovoltaic materials are designed to absorb a wider range of the solar spectrum by dividing the gap with a set of bands that plays a mediating role in two-step absorption processes. Thus far, the conventional model of IB absorbers has been focused on a single half-filled IB. We show that a multiple-IB picture with filled and empty bands provides a more convincing explanation of the experimental findings in some cases, and it should be considered as a possible scenario in understanding and engineering IB solar cells. Toward that end, we report the formation of two sets of IBs in a Ti-substituted ${\mathrm{CuGaS}}_2$ compound. Using hybrid functional calculations within the density functional theory framework, we show that the lower IBs are occupied and mainly derive from the Ti $3d_{x^2-y^2}$ states, while the higher ones originate from the Ti $3d_{z^2}$ states and are unoccupied. The positions of the IBs within the gap are found to depend weakly on the dopant concentration and the relative distances of the dopant ions. In addition, for a more pertinent comparison to real materials, we present a practical way to combine and analyze the densities of states resulting from the energetically most probable microscopic structures. This multi-intermediate-band picture provides a platform to comprehend unexplained features of the recent experimental study on the material [X. Lv et al., Solar Energy 103, 480 (2014)].

Topological insulators represent a paradigm shift in surface physics. The most extensively studied ${\mathrm{Bi}}_2{\mathrm{Se}}_3$-type topological insulators exhibit layered structures, wherein neighboring layers are weakly bonded by van der Waals interactions. Using first-principles density-functional theory calculations, we investigate the impact of the stacking sequence on the energetics and band structure properties of three polymorphs of ${\mathrm{Bi}}_2{\mathrm{Se}}_3,{\mathrm{Bi}}_2{\mathrm{Te}}_3$, and ${\mathrm{Sb}}_2{\mathrm{Te}}_3$. Considering their ultrathin films up to 6 nm as a function of its layer thickness, the overall dispersion of the band structure is found to be insensitive to the stacking sequence, while the band gap is highly sensitive, which may also affect the critical thickness for the onset of the topologically nontrivial phase. Our calculations are consistent with both experimental and theoretical results, where available. We further investigate tribological layer slippage, where we find a relatively low energy barrier between two of the considered structures. Both the stacking-dependent band gap and low slippage energy barriers suggest that polymorphic stacking modification may offer an alternative route for controlling the properties of this new state of matter.

We propose a two-dimensional Hong-Ou-Mandel (HOM) type interference experiment for massless Dirac fermions in graphene and 3D topological insulators. Since massless Dirac fermions exhibit linear dispersion, similar to photons in vacuum, they can be used to obtain the HOM interference intensity pattern as a function of the delay time between two massless Dirac fermions. We show that while the Coulomb interaction leads to a significant change in the angle dependence of the tunneling of two identical massless Dirac fermions incident from opposite sides of a potential barrier, it does not affect the HOM interference pattern. We apply our formalism to develop a massless Dirac fermion beam splitter (BS) for controlling the transmission and reflection coefficients. We calculate the resulting time-resolved correlation function for two identical massless Dirac fermions scattering off the BS.

Nitrogen impurities help to stabilize the negatively-charged-state of ${\mathrm{NV}}^{-}$ in diamond, whereas magnetic fluctuations from nitrogen spins lead to decoherence of ${\mathrm{NV}}^{-}$ qubits. It is not known what donor concentration optimizes these conflicting requirements. Here we used 10-MeV ${}^{15}{\mathrm{N}}^{3+}$ ion implantation to create ${\mathrm{NV}}^{-}$ in ultrapure diamond. Optically detected magnetic resonance of single centers revealed a high creation yield of $40\pm 3\%$ from ${}^{15}{\mathrm{N}}^{3+}$ ions and an additional yield of $56\pm 3\%$ from ${}^{14}\mathrm{N}$ impurities. High-temperature anneal was used to reduce residual defects, and charge stable ${\mathrm{NV}}^{-}$, even in a dilute ${}^{14}\mathrm{N}$ impurity concentration of 0.06 ppb were created with long coherence times.

We theoretically predict a nonequilibrium phase transition in quantum spin systems induced by a laser, which provides a purely quantum-mechanical way of coherently controlling magnetization. Namely, when a circularly polarized laser is applied to a spin system, the magnetic component of a laser is shown to induce a magnetization normal to the plane of polarization, leading to an ultrafast phase transition. We first demonstrate this phenomenon numerically for an $S=1$ antiferromagnetic Heisenberg spin chain, where a new state emerges with magnetization perpendicular to the polarization plane of the laser in place of the topologically ordered Haldane state. We then elucidate its physical mechanism by mapping the system to an effective static model. The theory also indicates that the phenomenon should occur in general quantum spin systems with a magnetic anisotropy. The required laser frequency is in the terahertz range, with the required intensity being within a prospective experimental feasibility.

We propose a model for a superconductor where both spin-singlet and chiral triplet pairing amplitudes can coexist. By solving the Bogoliubov–de Gennes equations with a general pair potential that accounts for both spin states we study experimental signatures of normal metal and superconductor hybrids. The interplay between the spin-singlet and triplet correlations manifests in the appearance of two effective gaps. When the amplitude of the spin-triplet component is stronger than that of the spin singlet, a topological phase transition into a nontrivial regime occurs. As a result, the normal metal–superconductor conductance evolves from a conventional gap profile onto an unconventional zero-bias peak. Additionally, in the topologically nontrivial phase, Andreev bound states formed at Josephson junctions present zero-energy modes; the number of those modes depends on the relative chirality of the junction. Finally, we present results for the current-phase relation and the temperature dependence of the Josephson critical current within both topological phases for several system parameters.

The lead free relaxor ${\mathrm{Na}}_{1/2}{\mathrm{Bi}}_{1/2}{\mathrm{TiO}}_3$ (NBT) undergoes a structural cubic-to-tetragonal transition near 800 K which is caused by the cooperative rotations of ${\mathrm{O}}_6$ octahedra. These rotations are also accompanied by the displacements of the cations and the formation of the polar nanodomains (PNDs) that are responsible for the characteristic dielectric dispersion of relaxor ferroelectrics. Because of their intrinsic properties, spontaneous polarization, and lack of inversion symmetry, these PNDs are also piezoelectric and can mediate an interaction between polarization and strain or couple the optic and acoustic phonons. Because PNDs introduce a local tetragonal symmetry, the phonon coupling they mediate is found to be anisotropic. In this paper we present inelastic neutron scattering results on coupled transverse acoustic (TA) and transverse optic (TO) phonons in the [110] and [001] directions and across the cubic-tetragonal phase transition at $T_C\sim 800$ K. The phonon spectra are analyzed using a mode coupling model. In the [110] direction, as in other relaxors and some ferroelectric perovskites, a precipitous drop of the TO phonon into the TA branch or “waterfall” is observed at a certain $q_{\mathrm{wf}}\sim 0.14$ r.l.u. In the [001] direction, the highly overdamped line shape can be fitted with closely positioned bare mode energies which are largely overlapping along the dispersion curves. Two competing lattice coupling mechanism are proposed to explain these observations.

Spin and lattice dynamics of ${\mathrm{CaMn}}_7{\mathrm{O}}_{12}$ ceramics were investigated using infrared, THz, and inelastic neutron scattering (INS) spectroscopies in the temperature range 2 to 590 K, and, at low temperatures, in applied magnetic fields of up to 12 T. On cooling, we observed phonon splitting accompanying the structural phase transition at $T_{\mathrm{C}}=450$ K as well as the onset of the incommensurately modulated structure at 250 K. In the two antiferromagnetic phases below $T_{\mathrm{N}1}=90$ K and $T_{\mathrm{N}2}=48$ K, several infrared-active excitations emerge in the meV range; their frequencies correspond to the maxima in the magnon density of states obtained by INS. At the magnetic phase transitions, these modes display strong anomalies and, for some of them, a transfer of dielectric strength from the higher-frequency phonons is observed. We propose that these modes are electromagnons. Remarkably, at least two of these modes remain active also in the paramagnetic phase; for this reason, we call them paraelectromagnons. In accordance with this observation, quasielastic neutron scattering revealed short-range magnetic correlations persisting within temperatures up to 500 K above $T_{\mathrm{N}1}$.

Ferromagnetic domain patterns and three-dimensional domain-wall configurations in thin CoCrPt films with perpendicular magnetic anisotropy were studied in detail by combining magnetic force microscopy and polarized neutron reflectometry with micromagnetic simulations. With the first method, lateral dimension of domains with alternative magnetization directions normal to the surface and separated by domain walls in 20-nm-thick CoCrPt films were determined in good agreement with micromagnetic simulations. Quantitative analysis of data on reflectometry shows that domain walls consist of a Bloch wall in the center of the thin film, which is gradually transformed into a pair of Néel caps at the surfaces. The width and in-depth thickness of the Bloch wall element, transition region, and Néel caps are found consistent with micromagnetic calculations. A complex structure of domain walls serves to compromise a competition between exchange interactions, keeping spins parallel, magnetic anisotropy orienting magnetization normal to the surface, and demagnetizing fields, promoting in-plane magnetization. It is shown that the result of such competition strongly depends on the film thickness, and in the thinner CoCrPt film (10 nm thick), simple Bloch walls separate domains. Their lateral dimensions estimated from neutron scattering experiments agree with micromagnetic simulations.

We present a transfer matrix approach that combines the Blonder-Tinkham-Klapwijk formalism and self-consistent solutions to the Bogoliubov–de Gennes equations and use it to study the tunneling conductance and spin transport in ferromagnet $\left(\mathrm{F}\right)$-superconductor $\left(\mathrm{S}\right)$ trilayers $\left({\mathrm{F}}_1{\mathrm{F}}_2\mathrm{S}\right)$ as functions of bias voltage. The self-consistency ensures that the spin and charge conservation laws are properly satisfied. We consider forward and angularly averaged conductances over a broad range of the strength of the exchange fields and $\mathrm{F}$ thicknesses, as the relative in-plane magnetization angle, $\varphi$, between the two ferromagnets varies. The $\varphi$ dependence of the self-consistent conductance curves in the trilayers can differ substantially from that obtained via a non-self-consistent approach. The zero-bias forward conductance peak exhibits, as $\varphi$ varies, resonance effects intricately associated with particular combinations of the geometrical and material parameters. We find, when the magnetizations are noncollinear, signatures of the anomalous Andreev reflections in the subgap regions of the angularly averaged conductances. When ${\mathrm{F}}_1$ is half metallic, the angularly averaged subgap conductance chiefly arises from anomalous Andreev reflection. The in-plane components of the spin current are strongly bias dependent, while the out-of-plane spin current component is only weakly dependent upon voltage. The components of the spin current aligned with the local exchange field of one of the F layers are conserved in that layer and in the S region, while they oscillate in the other layer. We compute the spin transfer torques, in connection with the oscillatory behavior of spin currents, and verify that the spin continuity equation is strictly obeyed in our method.

The paper by G. R. Berdiyorov, M. V. Milosevic, and F. M. Peeters [Phys. Rev. B 81, 144511 (2010)] studies theoretically the dynamic properties of a superconducting loop. The authors allege that their consideration of an asymmetric loop relates to our experimental results in this field. We try to show that their theoretical results can not be used for interpreting the experimental results obtained by measuring of the quantum periodicity in the critical current, dc voltage, and resistance of superconducting loops.