The lattice dynamics of layered Ba${}_{1-x}$Na${}_x$Ti${}_2$Sb${}_2$O pnictides ($0\le x\le 0.3$) is studied experimentally by means of Raman scattering and theoretically within the density functional theory. Both symmetry allowed Raman-active modes are observed in the spectra and assigned to the lattice eigenmodes: $z$-axis motion of Sb ions ($A$${}_{1g}$-symmetry mode) and in-$xy$-plane displacement of Sb ($E$${}_g$-symmetry vibration). Calculated and experimental frequencies of these modes are found to be in good agreement. The $E$${}_g$ mode exhibits a nonmonotonous dependence of its frequency upon doping, which is found to correlate with the variation of Sb-Ti distances and strength of the interaction between Ti${}_2$Sb${}_2$O slabs.

In this paper, we present systematic measurements of the temperature and magnetic field dependencies of the thermodynamic and transport properties of the Yb-based heavy fermion YbPtBi for temperatures down to 0.02 K with magnetic fields up to 140 kOe to address the possible existence of a field-tuned quantum critical point. Measurements of magnetic-field- and temperature-dependent resistivity, specific heat, thermal expansion, Hall effect, and thermoelectric power indicate that the AFM order can be suppressed by an applied magnetic field of $H_c\sim 4$ kOe. In the $H-T$ phase diagram of YbPtBi, three regimes of its low-temperature states emerge: (I) AFM state, characterized by a spin density wave-like feature, which can be suppressed to $T=0$ by the relatively small magnetic field of $H_c\sim 4$ kOe; (II) field-induced anomalous state in which the electrical resistivity follows $\Delta \rho \left(T\right)\propto T^{1.5}$ between $H_c$ and $\sim$8 kOe; and (III) Fermi liquid (FL) state in which $\Delta \rho \left(T\right)\propto T^2$ for $H\ge 8$ kOe. Regions I and II are separated at $T=0$ by what appears to be a quantum critical point. Whereas region III appears to be a FL associated with the hybridized 4$f$ states of Yb, region II may be a manifestation of a spin liquid state.

For a set of ten crystalline materials (oxides and semiconductors), we compute the electronic band structures using the Tran-Blaha [Phys. Rev. Lett. 102, 226401 (2009)] (TB09) functional. The band widths and gaps are compared with those from the local-density approximation (LDA) functional, many-body perturbation theory (MBPT), and experiments. At the density-functional theory (DFT) level, TB09 leads to band gaps in much better agreement with experiments than LDA. However, we observe that it globally underestimates, often strongly, the valence (and conduction) band widths (more than LDA). MBPT corrections are calculated starting from both LDA and TB09 eigenenergies and wave functions. They lead to a much better agreement with experimental data for band widths. The band gaps obtained starting from TB09 are close to those from quasiparticle self-consistent $GW$ calculations, at a much reduced cost. Finally, we explore the possibility to tune one of the semiempirical parameters of the TB09 functional in order to obtain simultaneously better band gaps and widths. We find that these requirements are conflicting.

Measurements in clean two-dimensional electron systems confined to wide GaAs quantum wells in which two electric subbands are occupied reveal an unexpected rotation of the orientation of the stripe phase observed at a half-filled Landau level. Remarkably, the reorientation is sensitive to the spin of the half-filled Landau level and the symmetry of the charge distribution in the quantum well.

In this paper, we report on a long, ms range, hole spin relaxation time in InGaAs/GaAs quantum wells probed by cyclotron resonance spectroscopy in pulsed magnetic fields. In our experiments, we found strong hysteresis in the spectral weights of cyclotron resonance absorption lines when rapidly changing magnetic field is used for the experiment. The hysteresis vanishes when a much slower changing magnetic field is used. We attribute this behavior to a long energy relaxation time between the two lowest spin-split hole Landau levels, i.e., a long hole spin relaxation time. We also present transition frequencies calculated using a $4\times 4$ Luttinger Hamiltonian, which confirm our findings.

We investigate the steady-state electronic transport through a suspended dimer molecule coupled to leads. When strongly coupled to a vibrational mode, the electron transport is enhanced at the phonon resonant frequency and higher order resonances. The temperature and bias determine the nature of the phonon-assisted resonances, with clear absorption and emission peaks. The strong coupling also induces a Frank-Condon-like blockade, suppressing the current between the resonances. We compare an analytical polaron transformation method to two exact numerical methods: the Hierarchy equations of motion and an exact diagonalization in the Fock basis. In the steady state, our two numerical results are an exact match and qualitatively reflect the main features of the polaron treatment. Our results indicate the possibility of a new type of molecular transistor or sensor where the current can be extremely sensitive to small changes in the energies of the electronic states in the dimer.

We theoretically investigate interaction effects in transport phenomena in bilayer graphene (BLG). For the minimal conductivity in pristine BLG, we find that the conductivity assumes a constant interaction-limited value for $T\to 0$, with the first correction being $\propto \sqrt{T}$. We furthermore study the Coulomb drag resistivity between two BLGs in the whole range from deep within the Fermi-liquid regime all the way to the charge neutrality (CN) point. While in the Fermi-liquid regime drag behaves very similarly to drag in a standard two-dimensional electron gas, in the vicinity of CN we find that the drag resistivity crucially depends on the ratio between inelastic and elastic scattering, as recently found in the case of monolayer graphene.

Temperature-dependent photoluminescence spectra of a range of semiconductor nanocrystals are presented which reveal an unanticipated thermodynamic relationship between the core excitonic states and the surface of the nanocrystal. These results challenge the widely held view of the surface emission arising from an energetically broad distribution of midgap defect states at the surface. We show that a simpler model based on semiclassical electron transfer and invoking only a single surface state uniquely describes all observables.

The effect of strain on the phonon modes of monolayer and few-layer MoS${}_2$ has been investigated by observing the strain-induced shifts of the Raman-active modes. Uniaxial strain was applied to a sample of thin-layer MoS${}_2$ sandwiched between two layers of optically transparent polymer. The resulting band shifts of the $E_{2g}^1$ ($\sim$$385.3{\mathrm{cm}}^{-1}$) and $A_{1g}$ ($\sim$$402.4{\mathrm{cm}}^{-1}$) Raman modes were found to be small but observable. First-principles plane-wave calculations based on density functional perturbation theory were used to determine the Grüneisen parameters for the $E_{1g}$, $E_{2g}^1$, $A_{1g}$, and $A_{2u}$ modes and predict the experimentally observed band shifts for the monolayer material. The polymer–MoS${}_2$ interface is found to remain intact through several strain cycles. As an emerging 2D material with potential in future nanoelectronics, these results have important consequences for the incorporation of thin-layer MoS${}_2$ into devices.

We study the coherent excitations of a composite fermion operator $f_{i\sigma }{(-1)}^{n_{i\overline{\sigma }}}$, where $f_{i\sigma }$ is the fermion operator for interacting electrons and $n_{i\overline{\sigma }}$ is the number operator of electrons with the opposite spin. In the two-impurity Anderson model, we show that the excitation of this composite fermion has a finite spectral weight near the Fermi energy in the regime dominated by intersite spin exchange coupling where the Kondo fixed point is prevented. From scattering off this coherent composite fermion mode, the excitation of the regular fermion $f_{i\sigma }$ develops a pseudogap and its self-energy is singular. Conversely, when the regular fermion develops Kondo resonance in the Kondo resonance regime, the excitation of the composite fermion develops a pseudogap instead. We argue that the composite fermion could develop a Fermi surface but “hidden” from charge excitations in lattice generalizations.

High-quality epitaxial thin films of $J_{\mathrm{eff}}$ $=$ 1/2 Mott insulator Sr${}_2$IrO${}_4$ with increasing in-plane tensile strain have been grown on top of SrTiO${}_3$(001) substrates. Increasing the in-plane tensile strain up to ∼0.3$\%$ was observed to drop the $c$/$a$ tetragonality by 1.2$\%$. X-ray absorption spectroscopy detected a strong reduction of the linear dichroism upon increasing in-plane tensile strain towards a reduced anisotropy in the local electronic structure. While the most relaxed thin film shows a consistent dependence with previously reported single crystal bulk measurements, electrical transport reveals a charge gap reduction from 200 meV down to 50 meV for the thinnest and most epitaxy-distorted film. We argue that the reduced tetragonality plays a major role in the change of the electronic structure, which is reflected in the change of the transport properties. Our work opens the possibility for exploiting epitaxial strain as a tool for both structural and functional manipulation of spin-orbit Mott systems.

We propose and simulate the relativistic Doppler reflection of terahertz (THz) radiation normally incident on a plasma front in a semiconductor medium, resulting in a significant frequency upshift for the reflected radiation. The plasma front is generated by linear interband excitation of the semiconductor by a counterpropagating femtosecond optical pulse. High-resistivity silicon is identified as an ideal medium for experiments, as it possesses a desirable optical penetration depth, upshift factor, and low THz absorption and dispersion. The depletion of the optical pump pulse results in a spatiotemporal plasma profile, which leads to results that go beyond the existing analytic theory. We employ one-dimensional finite-difference time-domain simulations to predict the reflected THz pulses vs a range of realistic experimental parameters. The results indicate that a significant frequency upshift can be expected for both conventional and ultrabroadband THz pulses, and that this technique may be suitable to provide higher-bandwidth THz radiation extending into the midinfrared.

The incorporation of hydrogen shallow donors gives rise to ${\text{Mn}}^{2+}$ fine and hyperfine lines in the electron paramagnetic resonance of ZnO. This cannot be explained by recharging of isolated Mn atoms. First-principles density functional calculations reveal that transition metals readily form complexes with acceptors by a charge-transfer process or by bond formation. This mechanism has implications on both complex partners. Acceptors are neutralized and uncommon charge states of the transition metals are stabilized. Implications for dilute magnetic ZnO are discussed.

In this paper we present scanning tunneling microscopy density-of-states measurements and electronic structure calculations of the topological insulator Bi${}_2$Se${}_3$. The measurements show significant background states in addition to the expected Dirac cone. Density-functional calculations using a slab model and analysis of the partial density of states show that the background is consistent with bulklike states with small amplitudes at the surface. The topological surface states coexist with bulklike states in the valence band, appearing as a shoulder in the projected band structure. These results strongly support the picture suggested by recent scattering experiments of the quantum interference of topological and bulklike surface states.

We investigate the interplay of quantum interference effects and electronic-vibrational coupling in electron transport through single-molecule junctions, employing a nonequilibrium Green's function approach. Our findings show that inelastic processes lead, in general, to a quenching of quantum interference effects. This quenching is more pronounced for increasing bias voltages and levels of vibrational excitation. As a result of this vibrationally induced decoherence, vibrational signatures in the transport characteristics of a molecular contact may strongly deviate from a simple Franck-Condon picture. This includes signatures in both the resonant and the nonresonant transport regimes. Moreover, it is shown that local cooling by electron-hole pair creation processes can influence the transport characteristics profoundly, giving rise to a significant temperature dependence of the electrical current.

The dc magnetization $M$($T$) studies on monoclinic compositions of ($1-x$)BiFeO${}_3$-$x$PbTiO${}_3$ (BF-$x$PT), $x$ $=$ 0.25 and 0.27, reveal another anomaly at spin-reorientation phase transition ($T_{\mathrm{OPT}}$) below the Néel transition temperature ($T_N$). From a Rietveld refinement of the magnetic structure using neutron powder diffraction data, it is shown that the anomaly at $T_{\mathrm{OPT}}$ is due to a spin-reorientation transition from a long-range magnetically ordered phase (${\mathit{G}}_{\mathbf{y}}$, ${\mathit{F}}_{\mathit{x}z}$) stable at $T_{\mathrm{OPT}}<T<T_N$ to another long-range ordered phase (${\mathit{G}}_{\mathit{xz}}$, ${\mathit{F}}_{\mathbf{y}}$) stable below T${}_{\mathrm{OPT}}$, wherein the ferromagnetic component of the noncollinear magnetic structure undergoes a spin flop. The spin-reorientation transition is not linked with any structural phase transition as confirmed by x-ray diffraction studies. Further, this transition is not accompanied with any magnetoelastic coupling. Unlike the Néel transition, the spin-reorientation transition is purely of magnetic origin. The spin-reorientation transition in BF-$x$PT is similar to the Morin transition in hematite and differs from the spin-reorientation transition in orthoferrites that is driven by the coupling of two magnetic sublattices.

We study the energy and the static spin structure factor of the ground state of the spin-$1/2$ quantum Heisenberg antiferromagnetic model on the kagome lattice. By the iterative application of a few Lanczos steps on accurate projected fermionic wave functions and the Green’s function Monte Carlo technique, we find that a gapless (algebraic) $U\left(1\right)$ Dirac spin liquid is competitive with previously proposed gapped (topological) ${\mathbb{Z}}_2$ spin liquids. By performing a finite-size extrapolation of the ground-state energy, we obtain an energy per site $E/J=-0.4365\left(2\right)$, which is equal, within three error bars, to the estimates given by the density-matrix renormalization group (DMRG). Our estimate is obtained for a translationally invariant system, and, therefore, does not suffer from boundary effects, like in DMRG. Moreover, on finite toric clusters at the pure variational level, our energies are lower compared to those from DMRG calculations.

Crystallization of amorphous germanium (a-Ge) by laser or electron beam heating is a remarkably complex process that involves several distinct modes of crystal growth and the development of intricate microstructural patterns on the nanosecond to ten microsecond time scales. Here we use dynamic transmission electron microscopy (DTEM) to study the fast, complex crystallization dynamics with 10 nm spatial and 15 ns temporal resolution. We have obtained time-resolved real-space images of nanosecond laser-induced crystallization in a-Ge with unprecedentedly high spatial resolution. Direct visualization of the crystallization front allows for time-resolved snapshots of the initiation and roughening of the dendrites on submicrosecond time scales. This growth is followed by a rapid transition to a ledgelike growth mechanism that produces a layered microstructure on a time scale of several microseconds. This study provides insights into the mechanisms governing this complex crystallization process and is a dramatic demonstration of the power of DTEM for studying time-dependent material processes far from equilibrium.

Conventional ordering transitions, described by the Landau paradigm, are characterized by the symmetries broken at the critical point. Within the constrained manifold occurring at low temperatures in certain frustrated systems, unconventional transitions are possible that defy this type of description. While the critical point exists only in the limit where defects in the constraint are vanishingly rare, unconventional criticality can be observed throughout a broad region of the phase diagram. This work presents a formalism for incorporating the effects of such defects within the framework of scaling theory and the renormalization group, leading to universal results for the critical behavior. The theory is applied to two transitions occurring within a model of spin ice, and the results are confirmed using Monte Carlo simulations. Relevance to experiments, particularly in the spin-ice compounds, is discussed, along with implications for simulations of related transitions, such as the cubic dimer model and the $\mathrm{O}\left(3\right)$ sigma model with “hedgehog” suppression.

We present a detailed theory on electron energy filtering by the nonplanar potential introduced by dispersed nanoparticles or impurities in bulk materials for enhancement of the thermoelectric power factor. When electrons with energies below a certain cut-off energy are prevented from participating in conduction through the material, the Seebeck coefficient and thus the thermoelectric power factor can be drastically enhanced. Instead of using planar heterostructures which require elaborate epitaxial techniques, we study embedded nanoparticles or impurities so that the conservation of lateral momentum limiting electron transport at heterointerfaces is no longer a limiting factor. Based on the Boltzmann transport equations under the relaxation time approximation, the optimal cut-off energy level that maximizes the power factor is calculated to be a few $k_{B}T$ above the Fermi level, and is a function of the scattering parameter, Fermi level, and temperature. The maximized power factor enhancement is quantified as a function of those parameters. The electronic thermal conductivity and Lorenz number are also shown to be suppressed by the electron filtering to further enhance the thermoelectric figure of merit. We find that the power factor of PbTe at 300 K could be enhanced by more than 120$\%$ when the cut-off energy level is 0.2 eV or higher and the carrier density higher than $5\times {10}^{19}$ cm${}^{-3}$. Finally we propose the use of distributed resonant scatterings to partially realize the nonplanar electron filtering in bulk materials.