The thermal and lattice dynamical properties of seven silicon clathrate framework structures are investigated with ab initio density functional methods (frameworks I, II, IV, V, VII, VIII, and H). The negative thermal expansion (NTE) phenomenon is investigated by means of quasiharmonic approximation and applying it to equal time displacement correlation functions. The thermal properties of the studied clathrate frameworks, excluding the VII framework, resemble those of the crystalline silicon diamond structure. The clathrate framework VII was found to have an anomalous NTE temperature range up to 300 K and it is suitable for further studies of the mechanisms of NTE. Investigation of the displacement correlation functions revealed that in NTE, the volume derivatives of the mean square displacements and mean square relative displacements of atoms behave similarly to the vibrational entropy volume derivatives and consequently to the coefficients of thermal expansion as a function of temperature. All studied clathrate frameworks, excluding the VII framework, possess a phonon band gap or even two in the case of framework V.

High-intensity pulsed neutron scattering reveals a new set of magnetic excitations in the pinwheel valence-bond solid state of the distorted kagome lattice antiferromagnet Rb${}_2$Cu${}_3$SnF${}_{12}$. The polarization of the dominant dispersive modes ($2\text{meV}<\hslash \omega <7\text{meV}$) is determined and found consistent with a dimer series expansion with strong Dzyaloshinskii–Moriya interactions ($D/J=0.18$). A weakly dispersive mode near 5 meV and shifted “ghosts” of the main modes are attributed to the enlarged unit cell below a $T=215$ K structural transition. Continuum scattering between 8 and 10 meV might be interpreted as a remnant of the kagome spinon continuum [Nature (London) 492, 406 (2012).].

We present a model of classical hard-core dimers on the square lattice that contains an Ising nematic phase in its phase diagram. We consider a model with an attractive interaction for parallel dimers on a given plaquette of the square lattice and an attractive interaction for neighboring parallel dimers on the same row (viz column) of the lattice. By extensive Monte Carlo simulations we find that with a finite density of holes the phase diagram has, with rising temperatures, a columnar crystalline phase, an Ising nematic liquid phase and a disordered fluid phase, separated by Ising continuous phase transitions. We present strong evidence for the Ising universality class of both transitions. The Ising nematic phase can be interpreted as either an intermediate classical thermodynamic phase (possibly of a strongly correlated antiferromagnet) or as a phase of a two-dimensional quantum dimer model using the Rokhsar-Kivelson construction of exactly solvable quantum Hamiltonians.

We present results on the temperature dependence of ultrafast electron and lattice dynamics, measured with pump-probe transient reflectivity experiments, of an epitaxially grown LaCoO${}_3$ thin film under tensile strain. Probing spin-polarized transitions into the antibonding $e_g$ band provides a measure of the low-spin fraction, both as a function of temperature and time after photoexcitation. It is observed that femtosecond laser pulses destabilize the constant low-spin fraction ($\sim$63%–64%) in equilibrium into a thermally activated state, driven by a subpicosecond change in spin gap $\Delta$. From the time evolution of the low-spin fraction, it is possible to disentangle the thermal and lattice contributions to the spin state. A lattice mediated spin repulsion, identified as the governing factor determining the equilibrium spin state in thin-film ${\mathrm{LaCoO}}_3$, is observed. These results suggests that time-resolved spectroscopy is a sensitive probe of the spin state in ${\mathrm{LaCoO}}_3$ thin films, with the potential to bring forward quantitative insight into the complicated interplay between structure and spin state in LaCoO${}_3$.

We use scaling and renormalization-group techniques to analyze the leading nonanalyticities in a Fermi liquid. We show that a physically motivated scaling hypothesis reproduces the results known from perturbation theory for the density of states, the density-of-states fluctuations, the specific heat, the spin susceptibility, and the nematic magnetic susceptibility. We also discuss the absence of nonanalytic terms in the density susceptibility. We then use a recent effective field theory for clean electron systems to derive the scaling hypothesis by means of renormalization-group techniques. This shows that the exponents (although not the prefactors) of the nonanalyticities that were previously derived by means of perturbative techniques are indeed exact, and can be understood as the leading corrections to scaling at the stable Fermi-liquid fixed point.

We calculate the dynamical structure factor $S(q,\omega )$ of a weakly interacting helical edge state in the presence of a magnetic field $B$. The latter opens a gap of width $2B$ in the single-particle spectrum, which becomes strongly nonlinear near the Dirac point. For chemical potentials $\left|\mu \right|>B$, the system then behaves as a nonlinear helical Luttinger liquid, and a mobile-impurity analysis reveals power-law singularities in $S(q,\omega )$ which depend on the interaction strength as well as on the spin texture of the edge states. For $\left|\mu \right|<B$, the low-energy excitations are gapped, and we determine $S(q,\omega )$ by using an analogy to exciton physics.

Magnetic-electronic studies of mixed-valence Fe${}_2$OBO${}_3$ have shown that ionic charge order (CO) is disrupted at ∼16 GPa. The pertinent minority-spin carrier exhibits persistent intersite electron exchange Fe${}^{2+}$ $\iff$ Fe${}^{3+}$ to well beyond this pressure. Temperature-dependent electrical transport measurements over an extended pressure range presented here demonstrate that the electronic structure remains gapped to well beyond 16 GPa. Extrapolation of data to higher pressure suggests that metallization will only prevail at $P>50$ GPa. Both the persistent gapped electronic state across the CO instability and signature of carrier confinement to Fe-Fe dimers in the Fe${}^{2+}$ $\iff$ Fe${}^{3+}$ electron exchange are rationalized as crossover from a Wigner crystal (site centered) insulator to a dimer Mott (bond centered type) insulator—“Wigner-Mott transition” at ∼16 GPa. The dimer insulating state is a consequence of modulation of the relevant hopping parameter $t$ in quasi-low-dimensional features in the structure (ribbons and chains). Complementary structural studies suggest that the $a$ axis is appreciably more compressible than other crystallographic directions of the original monoclinic unit cell. Therefore, such a modulation in $t$ may arise from Peierls type distortions along the $a$ axis or else stems from intrinsic modulation in the $c$ axis direction of the unit cell. This is aided by a monoclinic ($P$2${}_1/c$) → orthorhombic (Pmcn) structural adjustment that is concurrent across the electronic transition. Pressure tuning of relative values of on-site $U$/$t$ and intersite $V$/$t$ Coulomb interaction parameters of the quasi-low-dimensional features evolve the system from site-centered to dimer-centered electron localization.

We have studied the optical response and dynamical behavior of photocarriers in BiFeO${}_3$ thin films by means of transient absorption (TA) and photocurrent (PC) measurements. PC and absorption spectroscopy indicate that BiFeO${}_3$ thin films have an indirect band gap energy of ∼2.4 eV. The TA and PC decay dynamics have fast (∼1 ns) and slow (∼100 ns) components that are attributed to the localization of free carriers to shallow trap states and the recombination of trapped carriers, respectively. The long decay time of the PC is caused by the thermal activation of trapped carriers into the conduction band. Long-lived trapped photocarriers can be linked to the ferroelectricity and give rise to unique photoinduced phenomena in BiFeO${}_3$.

The local physical properties of polycrystalline semiconducting films drive their performances in a wide variety of optoelectronic devices but are still not completely elucidated. These properties are investigated and correlated on the same region of polycrystalline CdTe films by combining electron backscattered diffraction, μ-Laue x-ray experiments using synchrotron radiation, electron beam-induced current, and cathodoluminescence. The local band bending is revealed at random grain boundaries: its characteristics vary from one grain to another, depending on the nature of grain boundaries and the doping level in the nearby grains, in agreement with the theoretical approach of the double Schottky potential barriers. In contrast, no local band bending occurs at Σ3 growth twins since these extended defects have no dangling bonds in their center. Additionally, the density of unpaired dislocations and the components of the strain and stress tensors are found to be highly nonuniform from one grain to another and within the grains themselves. This reveals that grain-to-grain interactions (i.e., collective effects) occur during the Volmer-Weber-type growth. These findings emphasize the critical importance of grain boundary design engineering. They also highlight how polycrystalline semiconducting films work locally and show the complexity of the local physical processes governing their macroscopic performances in optoelectronic devices.

The linear thermal expansion coefficients of two-dimensional honeycomb structures graphene, $h$-BN, $h$-MoS${}_2$, and $h$-MoSe${}_2$ are systematically studied by using first-principles based quasiharmonic approximation. This approach is first tested on diamond crystal and excellent agreement with the available experimental data is achieved. Our simulations show that the linear thermal expansion coefficients of graphene and $h$-BN are more negative than that of their multilayered counterparts graphite and white graphite. In addition, there is a remarkable distinction between the coefficients of these two materials in particular at low temperatures. Contrary to graphene and $h$-BN, lattice thermal expansion coefficient of MoS${}_2$ and MoSe${}_2$ are always positive, and the values are comparable with those predicted for diamond.

We consider, in the framework of the central spin $s=1/2$ model, driven dynamics of two electrons in a double quantum dot subject to hyperfine interaction with nuclear spins and spin-orbit coupling. The nuclear subsystem dynamically evolves in response to Landau-Zener singlet-triplet transitions of the electronic subsystem controlled by external gate voltages. Without noise and spin-orbit coupling, subsequent Landau-Zener transitions die out after about ${10}^4$ sweeps, the system self-quenches, and nuclear spins reach one of the numerous glassy dark states. We present an analytical model that captures this phenomenon. We also account for the multi-nuclear-specie content of the dots and numerically determine the evolution of around ${10}^7$ nuclear spins in up to $2\times {10}^5$ Landau-Zener transitions. Without spin-orbit coupling, self-quenching is robust and sets in for arbitrary ratios of the nuclear spin precession times and the waiting time between Landau-Zener sweeps as well as under moderate noise. In the presence of spin-orbit coupling of a moderate magnitude, and when the waiting time is in resonance with the precession time of one of the nuclear species, the dynamical evolution of nuclear polarization results in stroboscopic screening of spin-orbit coupling. However, small deviations from the resonance or strong spin-orbit coupling destroy this screening. We suggest that the success of the feedback loop technique for building nuclear gradients is based on the effect of spin-orbit coupling.

Based on ab initio calculations we discuss the Kondo effect due to a Co adatom on a graphene zigzag nanoribbon. A Co atom located at a hollow site behaves as a spin $\mathcal{S}=1/2$ impurity with $d_{xz}$ and $d_{yz}$ orbitals contributing to the magnetic moment. Dynamical correlations are analyzed with the use of complementary approximations: the mean field slave boson approach, noncrossing approximation, and equation-of-motion method. The impact of interplay between spin and orbital degrees of freedom together with the effect of peculiarities of electronic and magnetic structure of the nanoribbon on many-body resonances is examined.

We report on the transition from two-dimensional to one-dimensional (1D) behavior observed in submonolayer deposition on a tubular substrate. Using kinetic Monte Carlo simulations we show that the island density increases when the tube diameter decreases, approaching the 1D behavior. This increase is confirmed by experiments of FePt nanoparticle deposition on carbon nanotubes. Kinetic Monte Carlo simulations also indicate that the consequences of confinement sharply disappear when the perimeter of the tube is slightly larger than the distance between islands.

We investigated the localized electronic properties of nanoporous gold films by using an ultrahigh vacuum scanning tunneling microscope at low temperature (4.2 K). Second derivative scanning tunneling spectroscopy shows the plasmon peaks of the nanoporous gold films, which are excited by inelastic tunneling electrons. We propose that the nanorod model is appropriate for nanoporous gold studies at the nanometer scale. These results are supported by a three-dimensional electron tomography analysis and theoretical calculations of nanoporous gold with an ellipsoidal shape.

Thermal conductivity measurements have been performed on the superconducting ferromagnet UCoGe down to very low temperature and under magnetic field. In addition to the electronic quasiparticle thermal conductivity, additional contributions to the thermal transport are detected: they are sensitive to the amplitude and direction of the magnetic field and at low temperature, they display a strong anisotropy with the heat current direction. We identify these contributions as arising from magnetic fluctuations. Detection of such fluctuations on the thermal transport in 3D weak ferromagnets is very rare if not unique and pledges for a strongly itinerant character of the magnetism of UCoGe.

We investigate the emission properties of a single semiconductor quantum dot deterministically coupled to a confined optical mode in the weak-coupling regime. A strong spectral pulling, broadening, and narrowing of the cavity mode emission is evidenced when changing the spectral detuning between the emitter and the cavity. These features are theoretically accounted for by considering the phonon assisted emission of the quantum dot transition. These observations highlight a regime of cavity quantum electrodynamics scarcely investigated so far, involving spectrally broad emitters.

Recently, there is a considerable study on gapped symmetric phases of bosons that do not break any symmetry. Even without symmetry breaking, the bosons can still be in many exotic new states of matter, such as symmetry-protected topological (SPT) phases, which are short-range entangled and symmetry-enriched topological (SET) phases, which are long-range entangled. It is well known that noninteracting fermionic topological insulators are SPT states protected by time-reversal symmetry and U(1) fermion number conservation symmetry. In this paper, we construct three-dimensional exotic phases of bosons with time-reversal symmetry and boson number conservation U(1) symmetry by means of fermionic projective construction. We first construct an algebraic bosonic insulator, which is a symmetric bosonic state with an emergent U(1) gapless gauge field. We then obtain many gapped bosonic states that do not break the time-reversal symmetry and boson number conservation via proper dyon condensations. We identify the constructed states by calculating the allowed electric and magnetic charges of their excitations, as well as the statistics and the symmetric transformation properties of those excitations. This allows us to show that our constructed states can be trivial SPT states (i.e., trivial Mott insulators of bosons with symmetry), nontrivial SPT states (i.e., bosonic topological insulators), and SET states (i.e., fractional bosonic topological insulators). In nontrivial SPT states, the elementary monopole (carrying zero electric charge but unit magnetic charge) and elementary dyon (carrying both unit electric charge and unit magnetic charge) are fermionic and bosonic, respectively. In SET states, intrinsic excitations may carry fractional charge.

The functional renormalization group (fRG) approach has the property that, in general, the flow equation for the two-particle vertex generates $\mathcal{O}\left(N^4\right)$ independent variables, where $N$ is the number of interacting states (e.g., sites of a real-space discretization). In order to include the flow equation for the two-particle vertex, one needs to make further approximations if $N$ becomes too large. We present such an approximation scheme, called the coupled-ladder approximation, for the special case of an onsite interaction. Like the generic third-order-truncated fRG, the coupled-ladder approximation is exact to second order and is closely related to a simultaneous treatment of the random phase approximation in all channels, i.e., summing up parquet-type diagrams. The scheme is applied to a one-dimensional model describing a quantum point contact.

Using density functional theory with a hybrid functional, we calculate the ionization energies and electron affinities of a series of delafossite compounds ($AM$O${}_2$: $A=\text{Cu}$, Ag; $M=\text{B}$, Al, Ga, In, Sc). The alignments of the valence band maximum and the conduction band minimum, which directly relate to the ionization energies and electron affinities, were obtained by calculations of supercell slab models constructed in a nonpolar orientation. Our calculations reveal that the ionization energy decreases with an increasing atomic number of group-III elements, and thus suggest an improved $p$-type doping propensity for heavier compounds. For keeping both a low ionization energy and a band gap of sufficient size, CuScO${}_2$ is superior to the Cu-based group-III delafossites. By analyzing the electronic structures, we demonstrate that the compositional trend of the ionization energies and electron affinities is the result of a combined effect of $d$-band broadening due to Cu(Ag)-Cu(Ag) coupling and a repositioning of the $d$-band center.

We investigate adiabatic quantum pumping of chiral Majorana states in a system composed of two Mach-Zehnder type interferometers coupled via a quantum point contact. The pumped current is generated by periodic modulation of the phases accumulated by traveling around each interferometer. Using scattering matrix formalism we show that the pumped current reveals a definite signature of the chiral nature of the Majorana states involved in transport in this geometry. Furthermore, by tuning the coupling between the two interferometers the pump can operate in a regime where finite pumped current and zero conductance are expected.