Structural stability, electronic structure, and optical properties of ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{BaI}}_3$ hybrid perovskite are examined from theory as well as experiment. Solution-processed thin films of ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{BaI}}_3$ exhibited a high transparency in the wavelength range of 400–825 nm (1.5–3.1 eV for which the photon current density is highest in the solar spectrum) which essentially justifies a high band gap of 4 eV obtained by theoretical estimation. Also, the x-ray diffraction patterns of the thin films match well with the $\left\{00l\right\}$ peaks of the simulated pattern obtained from the relaxed unit cell of ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{BaI}}_3$, crystallizing in the $I4/mcm$ space group, with lattice parameters, $a=9.30$ Å, $c=13.94$ Å. Atom projected density of state and band structure calculations reveal the conduction and valence band edges to be comprised primarily of barium $d$ orbitals and iodine $p$ orbitals, respectively. The larger band gap of ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{BaI}}_3$ compared to ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$ can be attributed to the lower electronegativity coupled with the lack of $d$ orbitals in the valence band of ${\mathrm{Ba}}^{2+}$. A more detailed analysis reveals the excellent chemical and mechanical stability of ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{BaI}}_3$ against humidity, unlike its lead halide counterpart, which degrades under such conditions. We propose La to be a suitable dopant to make this compound a promising candidate for transparent conductor applications, especially for all perovskite solar cells. This claim is supported by our calculated results on charge concentration, effective mass, and vacancy formation energies.

We use an external spin as a dynamical probe of many-body localization. The probe spin is coupled to an interacting and disordered environment described by a Heisenberg spin chain in a random field. The spin-chain environment can be tuned between a thermalizing delocalized phase and nonthermalizing localized phase, both in its ground and high-energy states. We study the decoherence of the probe spin when it couples to the environment prepared in three states: the ground state, the infinite temperature state, and a high-energy Néel state. In the nonthermalizing many-body localized regime, the coherence shows scaling behavior in the disorder strength. The long-time dynamics of the probe spin shows a logarithmic dephasing in analogy with the logarithmic growth of entanglement entropy for a bipartition of a many-body localized system. In summary, we show that decoherence of the probe spin provides clear signatures of many-body localization.

The low-temperature variation of the London penetration depth $\lambda \left(T\right)$ in the candidate topological superconductor ${\mathrm{Nb}}_x{\mathrm{Bi}}_2{\mathrm{Se}}_3 \left(x=0.25\right)$ is reported for several crystals. The measurements were carried out by means of a tunnel-diode oscillator technique in both field orientations ($H_{\mathrm{rf}}\parallel c$ and $H_{\mathrm{rf}}\parallel ab$ planes). All samples exhibited power-law behavior at low temperatures ($\mathrm{\Delta }\lambda \sim T^2$) clearly indicating the presence of point nodes in the superconducting order parameter. The results presented here are consistent with a nematic odd-parity spin-triplet $E_u$ pairing state in ${\mathrm{Nb}}_x{\mathrm{Bi}}_2{\mathrm{Se}}_3$.

The formalism to construct the machine learning potentials (MLPs) is presented. We introduce the spilling factor for the simultaneous error estimation and the recursive bisection method for the reduction of the computational cost. The formalism is applied for the $\beta$-phase vanadium monohydride. The first-principles calculations based on density functional theory (DFT) are used to prepare the sample data set from which the MLP for the vanadium monohydride (VH) system is constructed. In the molecular dynamics simulation with the MLP, the time-averaged structure of $\beta \text{−}\mathrm{VH}$ is predicted correctly to be the body-centered tetragonal structure with the octahedral $\left(O\right)$ site occupation of H. The average lattice constants are in good agreement with the experimental data which are not able to be reproduced by the static DFT calculation. The $O$-site occupation of H observed in the average structure is, however, a saddle point on the potential-energy surface, and the actual hydrogen occupation is found to be the $4T$ configuration.

The electronic structure and magnetic properties of the $2H-{\mathrm{NbS}}_2$ compound intercalated by Cr, Mn, and Fe, respectively, have been investigated by means of the Korringa-Kohn-Rostoker method. Magnetic torque calculations result in an easy plane magnetocrystalline anisotropy (MCA) of ${\mathrm{Cr}}_{1/3}{\mathrm{NbS}}_2$ for the ground state ($T=0$ K). Relativistic disorder local moment calculations show that the corresponding anisotropy constant decreases monotonously with rising temperature in line with experiment. Going from the Cr and Mn to the Fe intercalated system, the modification of the electronic structure results in a change of the easy axis from in-plane to out-of-plane. It is shown that for ${\mathrm{Cr}}_{1/3}{\mathrm{NbS}}_2$ and ${\mathrm{Mn}}_{1/3}{\mathrm{NbS}}_2$, the in-plane MCA and Dzyaloshinskii-Moriya interactions give rise to a helimagnetic structure along the hexagonal $c$ axis, following the experimental observations. The negative exchange interactions in the ${\mathrm{Fe}}_{1/3}{\mathrm{NbS}}_2$ compound result in a noncollinear frustrated magnetic structure if the MCA is not taken into account. It is shown, however, that a strong MCA along the hexagonal $c$ axis leads to a magnetic ordering referred to as an ordering of the third kind, which was observed experimentally.

The detailed optical properties have been determined for the iron-based materials $A{\mathrm{Fe}}_2{\mathrm{As}}_2$, where $A=\mathrm{Ca}$, Sr, and Ba, for light polarized in the iron-arsenic (a-b) planes over a wide frequency range, above and below the magnetic and structural transitions at $T_N=138$, 195, and 172 K, respectively. The real and imaginary parts of the complex conductivity are fit simultaneously using two Drude terms in combination with a series of oscillators. Above $T_N$, the free-carrier response consists of a weak, narrow Drude term, and a strong, broad Drude term, both of which show only a weak temperature dependence. Below $T_N$ there is a slight decrease of the plasma frequency but a dramatic drop in the scattering rate for the narrow Drude term, and for the broad Drude term there is a significant decrease in the plasma frequency, while the decrease in the scattering rate, albeit significant, is not as severe. The small values observed for the scattering rates for the narrow Drude term for $T\ll T_N$ may be related to the Dirac conelike dispersion of the electronic bands. Below $T_N$ new features emerge in the optical conductivity that are associated with the reconstruction Fermi surface and the gapping of bands at ${\mathrm{\Delta }}_1\simeq 45\text{–}80$ meV, and ${\mathrm{\Delta }}_2\simeq 110\text{–}210$ meV. The reduction in the spectral weight associated with the free carriers is captured by the gap structure; specifically, the spectral weight from the narrow Drude term appears to be transferred into the low-energy gap feature, while the missing weight from the broad term shifts to the high-energy gap.

The transport properties of an electronic Wigner solid (WS) on the surface of superfluid helium-4 are investigated in a geometry that allows WS systems of significantly different sizes to be defined. We probe and compare transport phenomena attributed to the electronic order, such as Bragg-Cherenkov scattering and the WS sliding transition, in the cases of a long homogeneous WS, a small WS island, and a long inhomogeneous WS. We find no significant WS size effects on the transport properties, in contrast to predictions of theoretical works, which indicates an absence of long-range order in the WS systems under consideration. For the inhomogeneous WS, consisting of two distinct WSs connected in series, a complex interplay of individual WS transport properties is observed, as revealed in the observation of two separate Bragg-Cherenkov plateaus and WS sliding transitions. A simple model is proposed that provides a qualitative explanation of this behavior.

Transport properties of highly mobile 2D electrons are studied in symmetric GaAs quantum wells placed in titled magnetic fields. Quantum positive magnetoresistance (QPMR) is observed in magnetic fields perpendicular to the 2D layer. Application of in-plane magnetic field produces a dramatic decrease of the QPMR. This decrease correlates strongly with the reduction of the amplitude of Shubnikov–de Haas resistance oscillations due to modification of the electron spectrum via enhanced Zeeman splitting. Surprisingly no quantization of the spectrum is detected when the Zeeman energy exceeds half of the cyclotron energy suggesting an abrupt transformation of the electron dynamics. Observed angular evolution of QPMR implies strong mixing between spin subbands. Theoretical estimations indicate that in the presence of spin-orbital interaction the elastic impurity scattering provides significant contribution to the spin mixing in GaAs quantum wells at high filling factors.

We present our experimental and theoretical study on Co diffusion in the first few atomic layers of Cu(001). While the diffusion of Co atoms in Cu(001) is usually expected to be intense at a temperature $T\ge 800$ K, in the vicinity of the surface, it is already activated at a considerably lower temperature $T\sim 650$ K whereas the diffusion in the deep bulk region is still inhibited. This intense near-surface diffusion provides an accumulation of Co atoms in the first six atomic layers upon a single stage Co deposition. The details of the near-surface diffusion are studied by analyzing the distribution of the location depth of the buried single Co atoms. The depth of location of single Co atoms is deduced from scanning tunneling microscopy (STM) data. The subsurface Co atoms induce a perturbation of the electron density of states of the surface which is observable as apparent ringlike ripples in the STM images of the atomically flat Cu surface. The depth of location of the buried Co atoms is determined from the diameter of those rings. The largest amount of Co atoms is found to be in the third layer, emphasizing that the near-surface diffusion should be described by diffusion parameters different from those for bulk diffusion. A model that describes the embedding process of Co atoms into Cu(001) layers is developed. The model assumes Co atoms to diffuse via the vacancy and ring-exchange mechanisms. The energy barriers for the interlayer Co diffusion via those mechanisms are calculated using the nudged elastic band method. The model satisfactorily explains the experimental results. Our study reveals that the energy barriers for Co diffusion in the first five atomic layers of Cu(001) are lower than those in the bulk. This defines the region in which Co diffusion should be considered as a near-surface one.

It is proposed and demonstrated that the $Q$ factor of optical resonators can be significantly enhanced by introducing an extra anisotropic cladding. We study the optical resonances of all-dielectric core-shell nanoresonators and show that radially anisotropic claddings can be employed to squeeze more energy into the core area, leading to stronger light confinement and thus significant $Q$-factor enhancement. We further demonstrate that the required homogenous claddings of unusual anisotropy parameters can be realized through all-dielectric multilayered isotropic structures. It is expected that the mechanism we have revealed not only offers extra flexibilities of resonance manipulations for conventional dielectric structures, but also may shed new light onto investigations into unconventional nanostructures consisting of two-dimensional materials that are intrinsically highly anisotropic.

The local density approximation (LDA) constructed through quantum Monte Carlo calculations of the homogeneous electron gas (HEG) is the most common approximation to the exchange-correlation functional in density functional theory. We introduce an alternative set of LDAs constructed from slablike systems of one, two, and three electrons that resemble the HEG within a finite region, and illustrate the concept in one dimension. Comparing with the exact densities and Kohn-Sham potentials for various test systems, we find that the LDAs give a good account of the self-interaction correction, but are less reliable when correlation is stronger or currents flow.

We show a tight-binding model for double perovskite monolayers grown in the {001} crystallographic direction can exhibit higher Chern numbers for the lowest band of the model dispersion in the absence of an applied magnetic field, including $\pm 2,\pm 4,\pm 6$, and $\pm 8$, for model parameters and terms relevant to ${\mathrm{Sr}}_2{\mathrm{CrWO}}_6$ (SCWO). We further show it is possible to tune between these different topological phases by applying a weak in-plane magnetic field. These results may be important for experimentally realizing exotic band topologies—and even quantum anomalous Hall insulators with higher Chern numbers—based on this physics.

We study the quantum phase transition of $U\left(1\right)$ charged Dirac fermions Yukawa coupled to the Kekulé valence-bond-solid order parameter with $Z_3$ symmetry of the honeycomb lattice. The symmetry allows for the presence of the term in the action which is cubic in the Kekulé order parameter and which is expected to prevent the quantum phase transition in question from being continuous. The Gross-Neveu-Yukawa theory for the transition is investigated using a perturbative renormalization group and within the $\epsilon$ expansion close to four space-time dimensions. For a vanishing $U\left(1\right)$ charge we show that quantum fluctuations may render the phase transition continuous only sufficiently far away from 3+1 dimensions, where the validity of the conclusions based on the leading-order $\epsilon$ expansion appears questionable. In the presence of a fluctuating gauge field, on the other hand, we find quantum critical behavior even at weak coupling to appear close to 3+1 dimensions, that is, within the domain of validity of the perturbation theory. We also determine the renormalization-group scaling of the cubic coupling at higher-loop orders and for a large number of Dirac fermions for vanishing charge.

The Fano line shape arises from the interference of two excitation pathways to reach a continuum. Its generality has resulted in a tremendous success in explaining the line shapes of many one-dimensional spectroscopies—absorption, emission, scattering, conductance, photofragmentation—applied to very varied systems—atoms, molecules, semiconductors, and metals. Unraveling a spectroscopy into a second dimension reveals the relationship between states in addition to decongesting the spectra. Femtosecond-resolved two-dimensional electronic spectroscopy (2DES) is a four-wave mixing technique that measures the time evolution of the populations and coherences of excited states. It has been applied extensively to the dynamics of photosynthetic units, and more recently to materials with extended band structures. In this paper, we solve the full time-dependent third-order response, measured in 2DES, of a Fano model and give the system parameters that become accessible.

Recently, an unusual metallic state with a substantially nonuniform distribution of the charge and magnetic density in ${\mathrm{CoO}}_2$ planes was found experimentally in the ${\mathrm{Na}}_x{\mathrm{CoO}}_2$ compound with $x>0.6$. We have investigated the origin of such an electron disproportionation in the lamellar sodium cobaltates by calculating the ion states as a function of the strength of the electron correlations in the $d$(Co) shells within the GGA+$U$ approximation for a system with a realistic crystal structure. It was found that the nonuniformity of spin and charge densities are induced by an ordering of the sodium cations and enhanced correlations. Two important magnetic states of cobalt lattice competing with each other at realistic values of the correlation parameter were found—low-spin hexagons lattice (LS) and higher-spin kagome lattice (HS-KSL). In the heterogeneous metallic HS-KSL phase, magnetic Co ions form a kagome structure. In LS phase, the kagome pattern is decomposed into hexagons and the Co ions possess the minimal values of their spin. Coexistence of these states could explain the emergence of the disproportionation with the peculiar kagome structure experimentally revealed in previous studies of the cobaltates.

We study the spin current injected by spin pumping into single layer graphene and the two-dimensional electron gas (2DEG) with ferromagnetic contacts using scattering theory. The spin currents pumped into graphene are very distinct from that into the 2DEG since they are affected by Klein tunneling.

A great deal of attention has been paid to the topological phases engineered by photonics over the past few years. Here, we propose a topological quantum phase transition to a quantum anomalous Hall (QAH) phase induced by off-resonant circularly polarized light in a two-dimensional system that is initially in a quantum spin Hall phase or a trivial insulator phase. This provides an alternative method to realize the QAH effect, other than magnetic doping. The circularly polarized light effectively creates a Zeeman exchange field and a renormalized Dirac mass, which are tunable by varying the intensity of the light and drive the quantum phase transition. Both the transverse and longitudinal Hall conductivities are studied, and the former is consistent with the topological phase transition when the Fermi level lies in the band gap. A highly controllable spin-polarized longitudinal electrical current can be generated when the Fermi level is in the conduction band, which may be useful for designing topological spintronics.

We investigate the $1/f$ noise properties of epitaxial graphene devices at low temperatures as a function of temperature, current, and magnetic flux density. At low currents, an exponential decay of the $1/f$ noise power spectral density with increasing temperature is observed that indicates mesoscopic conductance fluctuations as the origin of $1/f$ noise at temperatures below 50 K. At higher currents, deviations from the typical quadratic current dependence and the exponential temperature dependence occur as a result of nonequilibrium conditions due to current heating. By applying the Kubakaddi theory [S. S. Kubakaddi, Phys. Rev. B 79, 075417 (2009)], a model describing the $1/f$ noise power spectral density of nonequilibrium mesoscopic conductance fluctuations in epitaxial graphene is developed and used to determine the energy loss rate per carrier. In the regime of Shubnikov–de Haas oscillations, a strong increase of $1/f$ noise is observed, which we attribute to an additional conductance fluctuation mechanism due to localized states in quantizing magnetic fields. When the device enters the regime of quantized Hall resistance, the $1/f$ noise vanishes. It reappears if the current is increased and quantum Hall breakdown sets in.