Given the unique optical properties of LiF, it is often used as an observation window in high-temperature and -pressure experiments; hence, estimates of its transmission properties are necessary to interpret observations. Since direct measurements of the thermal conductivity of LiF at the appropriate conditions are difficult, we resort to molecular simulation methods. Using an empirical potential validated against ab initio phonon density of states, we estimate the thermal conductivity of LiF at high temperatures (1000–4000 K) and pressures (100–400 GPa) with the Green-Kubo method. We also compare these estimates to those derived directly from ab initio data. To ascertain the correct phase of LiF at these extreme conditions, we calculate the (relative) phase stability of the B1 and B2 structures using a quasiharmonic ab initio model of the free energy. We also estimate the thermal conductivity of LiF in an uniaxial loading state that emulates initial stages of compression in high-stress ramp loading experiments and show the degree of anisotropy induced in the conductivity due to deformation.

The quantum excitations of the collective magnetization dynamics in a ferromagnet (F)—magnons—enable spin transport without an associated charge current. This pure spin current can be transferred to electrons in an adjacent nonmagnetic conductor (N). We evaluate the finite temperature noise of the magnon-mediated spin current injected into N by an adjacent F driven by a coherent microwave field. We find that the dipolar interaction leads to squeezing of the magnon modes, giving them wave-vector-dependent nonintegral spin, which directly manifests itself in the shot noise. For temperatures higher than the magnon gap, the thermal noise is dominated by large-wave-vector magnons which exhibit negligible squeezing. The noise spectrum is white up to the frequency corresponding to the maximum of the temperature or the magnon gap. At larger frequencies, the noise is dominated by vacuum fluctuations. The shot noise is found to be much larger than its thermal counterpart over a broad temperature range, making the former easier to be measured experimentally.

Miniaturization of magnon-based devices into the nanometer range would require the utilization of exchange-dominated spin waves of nanometer wavelength. In experiment and theory we show that the intrinsic lifetime and mean free path of the homogeneous acoustic spin wave in ultrathin cobalt films is sufficiently long for such applications provided that the films are atomically flat. The presence of surface steps, however, shortens lifetime and mean free path. The experimental data are consistent with a model which assumes that steps act as perfect sinks for spin waves.

We investigate the effect of a unidirectional quenched random field on the anisotropic quantum spin-1/2 $XY$ model, which magnetizes spontaneously in the absence of the random field. We adopt a mean-field approach for this analysis. In general, the models considered have Ising symmetry, and as such they exhibit ferromagnetic order in two and three dimensions in the presence of not too large disorder. Even in the special case when the model without disorder has $\text{U}\left(1\right)$ symmetry, a small $\text{U}\left(1\right)$-symmetry-breaking random field induces ferromagnetic long-range order in two dimensions. The mean-field approach, consequently, provides a rather good qualitative and even quantitative description when applied not too close to the criticality. We show that spontaneous magnetization persists even in the presence of the random field, but the magnitude of magnetization gets suppressed due to disorder, and the system magnetizes in the directions parallel and transverse to the random field. Our results are obtained via analytical calculations within a perturbative framework and by numerical simulations. Interestingly, we show that it is possible to enhance a component of magnetization in the presence of the disorder field provided that we apply an additional constant field in the $XY$ plane. Moreover, we derive generalized expressions for the critical temperature and the scalings of the magnetization near the critical point for the $XY$ spin system with arbitrary fixed quantum spin angular momentum.

Quantum phase slips (QPSs) generate voltage fluctuations in superconducting nanowires. Employing the Keldysh technique and making use of the phase-charge duality arguments, we develop a theory of QPS-induced voltage noise in such nanowires. We demonstrate that quantum tunneling of the magnetic flux quanta across the wire yields quantum shot noise which obeys Poisson statistics and is characterized by a power-law dependence of its spectrum $S_{\mathrm{\Omega }}$ on the external bias. In long wires, $S_{\mathrm{\Omega }}$ decreases with increasing frequency $\mathrm{\Omega }$ and vanishes beyond a threshold value of $\mathrm{\Omega }$ at $T\to 0$. The quantum coherent nature of QPS noise yields nonmonotonous dependence of $S_{\mathrm{\Omega }}$ on $T$ at small $\mathrm{\Omega }$.

We use polarized Raman scattering to study the structural phase transition in ${\mathrm{EuFe}}_2{\mathrm{As}}_2$, the parent compound of the 122-ferropnictide superconductors. The in-plane lattice anisotropy is characterized by measurements of the side surface with different strains induced by different preparation methods. We show that while a fine surface polishing leaves the samples free of residual internal strain, in which case the onset of the $C_4$ symmetry breaking is observed at the nominal structural phase transition temperature $T_S$, cutting the side surface induces a permanent fourfold rotational symmetry breaking spanning tens of degrees above $T_S$.

We report an antiferromagnetic (AF) ordering at ambient pressure and a superconducting transition under pressure for $\kappa \text{−}{\left(\mathrm{ET}\right)}_2\mathrm{C}{\mathrm{F}}_3\mathrm{S}{\mathrm{O}}_3$ $\mathrm{[}\mathrm{ET}=\mathrm{bis}$(ethylenedithio)tetrathiafulvalene], which has a two-dimensional electronic system with quasi-one-dimensional triangular spin lattice. At ambient pressure, AF ordering was detected at $T_N=2.5\mathrm{K}$ by ${}^1\mathrm{H}$ NMR, subsequent to two structural phase transitions at 230 and 190 K. Under hydrostatic pressures, metallic behavior appeared above ∼1.1 GPa, and a superconducting transition (maximum onset $T_c=4.8\mathrm{K}$ at ∼1.3 GPa) was observed up to 2.2 GPa. Superconductivity was also found under $c$-axis strain, which reduced $t^{\prime }/t$, but was absent under $b$-axis strain which increased $t^{\prime }/t$.

The wavelength dependence of the threshold for femtosecond optical breakdown in water provides information on the interplay of multiphoton, tunneling, and avalanche ionization and is of interest for parameter selection in laser surgery. We measured the bubble threshold from ultraviolet to near-infrared wavelengths and found a continuous decrease of the irradiance threshold with increasing wavelength $\lambda$. Results are compared to the predictions of a numerical model that assumes a band gap of 9.5 eV and considers the existence of a separate initiation channel via excitation of valence band electrons into a solvated state followed by rapid upconversion into the conduction band. Fits to experimental data yield an electron collision time of $\approx 1\mathrm{fs}$ and an estimate for the capacity of the initiation channel. Using that collision time, the breakdown dynamics were explored up to $\lambda =2\mu \mathrm{m}$. The irradiance threshold first continues to decrease but levels out for wavelengths longer than 1.3 $\mu \mathrm{m}$. This opens promising perspectives for laser surgery at wavelengths around 1.3 and 1.7 $\mu \mathrm{m}$, which are attractive because of their large penetration depth into scattering tissues.

The ${\mathrm{V}}_3\mathrm{Al}$ alloy with $\mathrm{D}{\mathrm{O}}_3$ crystal structure belongs to the family of the very few metallic materials that exhibit a magnetically ordered state with a high ordering temperature (∼600 K) and consist only of nonmagnetic elements. We show that, similarly to the ferromagnetism in the fcc Ni (with ordering temperature at about 630 K), the antiferromagnetism in ${\mathrm{V}}_3\mathrm{Al}$ has itinerant character, and the high value of the Néel temperature is the result of the strong longitudinal spin fluctuations in the paramagnetic state. In order to develop an ab initio–based theory of the magnetic ordering at finite temperatures, we employ an effective magnetic Heisenberg-like Hamiltonian with varying values of the on-site magnetic moments. Using a set of approximations we map this model onto the results of the first-principle-based disordered local moment formalism and the magnetoforce theorem applied in the framework of the Korringa-Kohn-Rostoker method. Our high-temperature approach is shown to describe the experimental Néel temperature of ${\mathrm{V}}_3\mathrm{Al}$ very well and thus underlines the importance of the longitudinal spin-fluctuation mechanism of formation of the vanadium magnetic moment at high temperatures.

Diamond has been extensively investigated recently due to a wide range of potential applications of nitrogen-vacancy (NV) defect centers existing in a diamond lattice. The applications include magnetometry and quantum information technologies, and long decoherence time $\left(T_2\right)$ of NV centers is critical for those applications. Although it has been known that $T_2$ highly depends on the concentration of paramagnetic impurities in diamond, precise measurement of the impurity concentration remains challenging. In the present work we show a method to determine a wide range of the nitrogen concentration $\left(n\right)$ in diamond using a wide-band high-frequency electron spin resonance and double electron-electron resonance spectrometer. Moreover, we investigate $T_2$ of the nitrogen impurities and show the relationship between $T_2$ and $n$. The method developed here is applicable for various spin systems in solid and implementable in nanoscale magnetic resonance spectroscopy with NV centers to characterize the concentration of the paramagnetic spins within a microscopic volume.

We demonstrate that thin polycrystalline films of antiferromagnet CoO, in bilayers with ferromagnetic Permalloy, exhibit slow power-law aging of their magnetization state. The aging characteristics are remarkably similar to those previously observed in thin epitaxial ${\mathrm{Fe}}_{50}{\mathrm{Mn}}_{50}$ films, indicating that these behaviors are likely generic to ferromagnet/antiferromagnet bilayers. In very thin films, aging is observed over a wide temperature range. In thicker CoO, aging effects become reduced at low temperatures. Aging entirely disappears for large CoO thicknesses. We also investigate the dependence of aging characteristics on temperature and magnetic history. Analysis shows that the observed behaviors are inconsistent with the Neel-Arrhenius model of thermal activation, and are instead indicative of cooperative aging of the antiferromagnet. Our results provide new insights into the mechanisms controlling the stationary states and dynamics of ferromagnet/antiferromagnet bilayers, and potentially other frustrated magnetic systems.

The dynamics of driven pancake vortices in layered superconductors is studied using molecular-dynamics simulations. We found that, with increasing driving force, for strong interlayer coupling, the preexisted vortex lines either directly depin or first transform to two-dimensional (2D) pinned states before they are depinned, depending on the pinning strength. In a narrow region of pinning strengths, we found an interesting repinning process, which results in a negative differential resistance. For weak interlayer coupling, individually pinned pancake vortices first form disordered 2D flow and then transform to ordered three-dimensional (3D) flow with increasing driving force. However, for extremely strong pinning, the random pinning-induced thermal-like Langevin forces melt 3D vortex lines, which results in a persistent 2D flow in the fast-sliding regime. In the intermediate regime, the peak effect is found: With increasing driving force, the moving pancake vortices first crystallize to moving 3D vortex lines, and then these 3D vortex lines are melted, leading to the appearance of a reentrant 2D flow state. Our results are summarized in a dynamical phase diagram.

The ground-state electronic configurations of the correlated organometallic metallocenes, $M{\mathrm{Cp}}_2,M=\mathrm{V}$, Cr, Mn, Fe, Co, and Ni, are investigated using constraint density functional theory combined with nonempirical $U_{\mathrm{eff}}$ parameters determined from linear-response theory. The relative stability of the various $d$-orbital electronic configurations of these organometallic molecules is found to be sensitive to the amount of correlation. Using nonempirical values of $U_{\mathrm{eff}}$, the calculated electronic configurations are in agreement with the experiments: ${}^4{A}_{2g},{}^3{E}_{2g},{}^6{A}_{1g},{}^1{A}_{1g},{}^2{E}_{1g}$, and ${}^3{A}_{2g}$ for the ${\mathrm{VCp}}_2,{\mathrm{CrCp}}_2,{\mathrm{MnCp}}_2,{\mathrm{FeCp}}_2,{\mathrm{CoCp}}_2$, and ${\mathrm{NiCp}}_2$, respectively.

We introduce new three-dimensional (3D) topological phases of two-band models using the Pontryagin-Thom construction. In symmetry class $A$ these are the infinitely many Hopf-Chern topological insulators, which are hybrids of layered Chern insulators and Hopf insulators. Being constructed by a modification of the experimentally observed Chern insulators, these provide promising candidates for the observation of a genuinely 3D topological phase in class $A$. In symmetry class $C$, there is a ${\mathbb{Z}}_2$ classification with the nontrivial topological phase, the Hopf superconductor, being realized by a construction that doubles a Hopf insulator in momentum space. For these new topological phases we introduce concrete tight-binding Hamiltonians and investigate their energy spectra in the presence of a boundary, revealing gapless surface modes in accordance with the bulk-boundary correspondence.

Magnetic impurities deposited on the surface of a thin film experience oscillating-in-space indirect exchange interaction even if the chemical potential is pinned to a middle gap position in the surface electron spectrum, preventing free electrons and holes to exist in the ground state. Oscillating interaction mediated by massive Dirac fermions is a signature of the topologically nontrivial state.

The significant electron-electron interactions that characterize the $\pi$ electrons of graphene nanoribbons (GNRs) necessitate going beyond one-electron tight-binding description. Existing theories of electron-electron interactions in GNRs take into account one electron–one hole interactions accurately but miss higher-order effects. We report highly accurate density matrix renormalization group (DMRG) calculations of the ground-state electronic structure, the relative energies of the lowest one-photon versus two-photon excitations, and the charge gaps in three narrow GNRs within the correlated Pariser-Parr-Pople model for $\pi$-conjugated systems. We have employed the symmetrized DMRG method to investigate the zigzag nanoribbon 3-ZGNR and two armchair nanoribbons 6-AGNR and 5-AGNR, respectively. We predict bulk magnetization of the ground state of 3-ZGNR, and a large spin gap in 6-AGNR in their respective thermodynamic limits. Nonzero charge gaps and semiconducting behavior, with moderate to large exciting binding energies, are found for all three nanoribbons, in contradiction to the prediction of tight-binding theory. The lowest two-photon gap in 3-ZGNR vanishes in the thermodynamic limit, while this gap is smaller than the one-photon gap in 5-AGNR. However, in 6-AGNR the one-photon gap is smaller than the two-photon gap and it is predicted to be fluorescent.

Exciton, trion, and electron spin dynamics in a 20-nm-wide modulation-doped GaAs single quantum well are investigated using resonant ultrafast two-color Kerr rotation spectroscopy. Excitons and trions are selectively detected by resonant probe pulses while their relative spectral weight is controlled by adjusting the gate voltage which tunes the carrier density. Tuning the carrier density markedly influences the spin decay time of the two-dimensional electron gas. The spin decay time can be enhanced by a factor of 3 at an intermediate carrier concentration in the quantum well where excitons and trions coexist in the system. In addition, we explore the capability to tune the $g$ factor of the electron gas via the carrier density.

Using first-principles calculations based on density functional theory, we study the energetics and charge transfer effects in ${\mathrm{MgB}}_x$ nanotubes and two-dimensional (2D) sheets. The behavior of adsorbed Mg on 2D boron sheets is found to depend on the amount of electron transfer between the two subsystems. The amount is determined by both the density of adsorbed Mg as well as the atomic-scale structure of the boron subsystem. The degree of transfer can lead to repulsive or attractive Mg-Mg interactions. In both cases, model ${\mathrm{MgB}}_x$ nanotubes built from 2D ${\mathrm{MgB}}_x$ sheets can display negative curvature energy: a relatively unusual situation in nanosystems where the energy cost to curve the parent 2D sheet into a small-diameter nanotube is negative. Namely, the small-diameter nanotube is energetically preferred over the corresponding flat sheet. We also discuss how these findings may manifest themselves in experimentally synthesized ${\mathrm{MgB}}_x$ nanotubes.

Emergence of ferromagnetism in nonmagnetic semiconductors is strongly desirable, especially in topological materials because of the possibility of achieving the quantum anomalous Hall effect. Based on first-principles calculations, we propose that for Si thin film grown on metal substrate, the pristine Si(111)-$\sqrt{3}\times \sqrt{3}$ surface with a spontaneous weak reconstruction has a strong tendency toward ferromagnetism and nontrivial topological properties, characterized by spin-polarized Dirac-fermion surface states. In contrast to conventional routes relying on introduction of alien charge carriers or specially patterned substrates, the spontaneous magnetic order and spin-orbit coupling on the pristine silicon surface together give rise to the quantized anomalous Hall effect with a finite Chern number $\mathcal{C}=-1$. This work suggests opportunities in silicon-based spintronics and quantum computing free from alien dopants or proximity effects.