We show, using an ab initio approach, that the quantum zero-point atomic motion has strong effects on the electronic properties of carbon and silicon nanoclusters. The ensuing band-gap renormalization is between 0.3 and 0.9 eV and is as large as the corresponding exciton binding energy. These results call for a critical reassessment of the quality of nanocluster calculations performed assuming frozen atom positions, even in the zero-temperature limit. Our approach allows to separate the effect into different spatial contributions (surface, core, passivants) and to quantify on which electronic states (according to their symmetries) the effect is strongest.

We present a synergic experimental and theoretical investigation of the plastic relaxation onset in low-temperature Ge growth on Si(001). High-resolution transmission electron microscopy reveals that misfit is released by pairs of coupled 60° dislocations. Atomic resolution proved to be key in distinguishing pairs from single 90° dislocations because of the revealed small intrapair dislocation distance (even less than 1 nm). By exploiting dislocation theory and molecular dynamics simulations, we demonstrate that the observed pairing naturally occurs as a result of the mutual interactions between the two dislocations. In particular, analytical models show that the stress field arising in a thin film when a dislocation segment lies at the interface with the substrate determines the most favored nucleation site for a new (complementary) dislocation that leads, after migration, to the coupling with the first in a stable position. At the growth temperature, further motion or recombination due to atomic scale effects is excluded by classical molecular dynamics simulations. A clear picture of the early stages in the strain relaxation emerges, gliding out of the interface and/or short-range climbing (as typically produced by annealing or higher temperature steps but not taking place under the present growth conditions) being required to transform pairs into edge dislocations. The present results also offer answers to the long-asked puzzling question about the mechanism originating 90° dislocation in high-mismatch Ge/Si systems.

We present theoretical investigations on surface elastic energy in spin-crossover (SC) solids studied in the frame of a microscopic elastic model, coupling spin, and lattice deformations. Although surface energy plays a crucial role in driving the SC transition, specific quantitative investigations on its effect have been neglected in most of the recent theoretical works based on atomistic descriptions of the SC transitions, resolved by Monte Carlo or by molecular dynamics simulations. Here, we perform a quantitative study of the surface energy resulting from an inserted high-spin (HS) domain in a low-spin (LS) lattice. This situation may be produced experimentally in SC solids, at low temperature, through a photoexcitation by a single pulse laser shot. We demonstrate that the surface energy depends on the ratio between the local molecular volume misfit (between the LS and HS sites) $\delta \upsilon$ and the lattice volume $V$, such as $E_{\mathrm{surf}}\sim \delta {\upsilon }^2/V$ for the HS atom at the center of lattice, while it is $E_{\mathrm{surf}}\sim \delta {\upsilon }^2/L$ ($L$ is the length of the lattice) in the case of the HS atom located on the edge of the lattice. We then derived the image pressure (negative in the case of embedded dilatation centers) through the work of the free surface atoms and evaluated the Eshelby constant, which was found equal to $\gamma \sim 1.90$, in very good agreement with the available data of literature. Energetic configuration diagrams in the homogeneous (HS and LS) and heterogeneous (coexistence of HS and LS) are calculated, from which estimations of the macroscopic bulk modulus were deduced.

We have investigated spin-wave excitations in a magnetic-field-induced 1/5-magnetization plateau phase in the triangular lattice antiferromagnet CuFeO${}_2$ (CFO), by means of inelastic neutron-scattering measurements under applied magnetic fields of up to 13.4 T. Comparing the observed spectra with the calculations in which spin-lattice coupling effects for the nearest-neighbor exchange interactions are taken into account, we have determined the Hamiltonian parameters in the field-induced 1/5-plateau phase, which directly show that CFO exhibits a bond order associated with the magnetic structure in this phase.

The microwave emission of magnetic tunnel junction nanopillars with symmetric CoFeB/MgO/CoFeB interfaces and a 9-nm-thick free layer was studied as a magnetic field was applied in plane at different angles ${\varphi }_H$ with respect to the easy axis. As ${\varphi }_H$ was increased, a more complicated mode spectrum containing modes of larger amplitude was observed. The character of the different modes was deduced by screening many devices fabricated from the same wafer, observing the variation of frequency both between different junctions and with ${\varphi }_H$, and through comparison with micromagnetic simulation. Within selected junctions at certain ${\varphi }_H$ values, the dependence of the frequency, amplitude, and linewidth of the lowest-frequency edge-localized mode upon the applied field and current can be interpreted in terms of combined thermal and spin-transfer-torque (STT) excitation. Fitting to a simple analytical model yielded values for the in-plane and out-of-plane spin STT. The out-of-plane STT was found to contain terms with both a linear and a quadratic dependence on the bias current. The linear term was often found to dominate, as expected for a tunnel junction with asymmetric rather than symmetric interfaces, suggesting that the fuller structure of the free and reference layers should be taken into account. The amplitude of the in-plane STT was somewhat larger than expected, suggesting that the dominant edge mode occupies only part of the area of the free layer but is subject to a proportionately larger fraction of the torque exerted upon the free layer as a whole. This might be expected when the resonant modes of the free layer interact and the lowest-frequency mode dominates.

We use unpolarized and polarized inelastic neutron scattering to study low-energy spin excitations in NaFeAs, which exhibits a tetragonal-to-orthorhombic lattice distortion at $T_s\approx 58$ K followed by a collinear antiferromagnetic (AF) order below $T_N\approx 45$ K. In the AF ordered state ($T<T_N$), spin waves are entirely $c$-axis polarized below $\sim$10 meV, exhibiting a gap of $\sim$4 meV at the AF zone center and disperse to $\sim$7 meV near the $c$-axis AF zone boundary. On warming to the paramagnetic state with orthorhombic lattice distortion ($T_N<T<T_s$), spin excitations become anisotropic within the FeAs plane. Upon further warming to the paramagnetic tetragonal state ($T>T_s$), spin excitations become more isotropic. Since similar magnetic anisotropy is also observed in the paramagnetic tetragonal phase of superconducting BaFe${}_{1.904}$Ni${}_{0.096}$As${}_2$, our results suggest that the spin excitation anisotropy in superconducting iron pnictides originates from similar anisotropy already present in their parent compounds.

In two-dimensional incompressible quantum spin liquids, a large enough magnetic field generically induces “doping” of polarized $S=1$ triplons or $S=1/2$ spinons. We review a number of cases such as spin-3/2 Affleck-Kennedy-Lieb-Tasaki (AKLT) or spin-1/2 resonating valence bond (RVB) liquids where the projected entangled pair states (PEPS) framework provides very simple and comprehensive pictures. On the bipartite honeycomb lattice, simple PEPS can describe Bose condensed triplon (AKLT) or spinon (RVB) superfluids with transverse staggered (Néel) magnetic order. On the kagome lattice, doping the RVB state with deconfined spinons or triplons (i.e., spinon bound pairs) yields uncondensed Bose liquids preserving $U\left(1\right)$ spin-rotation symmetry. We find that spinon (triplon) doping destroys (preserves) the topological ${\mathbb{Z}}_2$ symmetry of the underlying RVB state. We also find that spinon doping induces longer range interactions in the entanglement Hamiltonian, suggesting the emergence of (additive) log corrections to the entanglement entropy.

We analyze the essential role played by complex energy landscapes in the nanometer- to micron-scale inhomogeneities observed in perovskite manganites using a model expressed in terms of symmetrized atomic-scale lattice distortion modes. We also examine the stability of large metal and insulator domains in the absence of defects. Our results demonstrate that an intrinsic mechanism, which involves long-range interactions between strain fields, the Peierls-Nabarro energy barrier, and complex energy landscapes with multiple metastable states, rather than an extrinsic mechanism such as chemical randomness, is responsible for the inhomogeneity in perovskite manganites.

We consider microwave spectroscopy of Josephson junctions composed of hybridized Majorana states in topological one-dimensional superconductors. We point out how spectroscopic features of the junction appear in the current phase relation under microwave irradiation. Moreover, we discuss a way to directly probe the nonequilibrium state associated with the $4\pi$-periodic Josephson effect. In particular, we show how the microwave driving can be used to switch from a $4\pi$ to a $2\pi$ Shapiro step in the current voltage relation.

We have performed conductivity measurements on a Si-MOSFET sample with a slot in the upper gate, allowing for different electron densities $n_1$ and $n_2$ across the slot. Dynamic longitudinal resistance was measured by a standard lock-in technique, while maintaining a large dc current through the source-drain channel. We find that in a parallel magnetic field, the resistance of the sample $R\left(I_{\mathrm{dc}}\right)$ is asymmetric with respect to the direction of the dc current. The asymmetry becomes stronger with an increase of either the magnetic field or the difference between $n_1$ and $n_2$. These observations are interpreted in terms of the effective spin injection: the degree of spin polarization is different in the two parts of the sample, implying different magnitudes of spin current away from the slot. The carriers thus leave the excess spin (of the appropriate sign) in the region around the slot, leading to spin accumulation (or depletion) and to the spin-drift-diffusion phenomena. Due to the positive magnetoresistance of the two-dimensional electron gas, this change in a local magnetization affects the resistivity near the slot and the measured net resistance, giving rise to an asymmetric contribution. We further observe that the value of $R\left(I_{\mathrm{dc}}\right)$ saturates at large $I_{\mathrm{dc}}$; we suggest that this is due to electron tunneling from the two-dimensional $n$-type layer into the $p$-type silicon (or into another “spin reservoir”) at the slot.

We find clear signatures of spin-dependent negative differential resistance in compound systems comprising a graphene nanoribbon and a set of ferromagnetic insulator strips deposited on top of it. The periodic array of ferromagnetic strips induces a proximity exchange splitting of the electronic states in graphene, resulting in the appearance of a superlattice with a spin-dependent energy spectrum. The electric current through the device can be highly polarized and both the current and its polarization manifest nonmonotonic dependence on the bias voltage. The device operates therefore as an Esaki spin diode, which opens possibilities to design new spintronic circuits.

We describe some exact high-energy properties of a single Anderson impurity connected to two noninteracting leads in a nonequilibrium steady state. In the limit of high bias voltages, and also in the high-temperature limit at thermal equilibrium, the model can be mapped onto an effective non-Hermitian Hamiltonian consisting of two sites, which correspond to the original impurity and its image that is defined in a doubled Hilbert space referred to as Liouville-Fock space. For this, we provide a heuristic derivation using a path-integral representation of the Keldysh contour and the thermal field theory, in which the time evolution along the backward contour is replicated by extra degrees of freedom corresponding to the image. We find that the effective Hamiltonian can also be expressed in terms of charges and currents. From this, it can be deduced that the dynamic susceptibilities for the charges and the current fluctuations become independent of the Coulomb repulsion $U$ in the high bias limit. Furthermore, the equations of motion for the Green's function and two other higher-order correlation functions constitute a closed system. The exact solution obtained from the three coupled equations extends the atomic-limit solution such that the self-energy correctly captures the imaginary part caused by the relaxation processes at high energies. The spectral weights of the upper and lower Hubbard levels depend sensitively on the asymmetry in the tunneling couplings to the left and right leads.

We provide analytical composite pulse sequences that perform dynamical decoupling concurrently with arbitrary rotations for a qubit coded in the spin state of a triple quantum dot. The sequences are designed to respect realistic experimental constraints such as strictly nonnegative couplings. Logical errors and leakage errors are simultaneously corrected. A short pulse sequence is presented to compensate nuclear noise and a longer sequence is presented to simultaneously compensate nuclear and charge noise. The capability developed in this work provides a clear prescription for combatting the relevant sources of noise that currently hinder exchange-only qubit experiments.

The crystallographic structure, magnetization, specific heat, and electrical resistivity of polycrystalline CeTi${}_{1-x}$V${}_x$Ge${}_3$ ($0\le x\le 0.42$ and $x$ $=$ 1) have been studied. CeTiGe${}_3$ has been reported as one of the rare ferromagnetic Kondo systems with a Curie temperature $T_c$ $=$ 14 K. We present a detailed study of the low-temperature properties on this compound, including magnetization, specific heat, and electrical resistivity in magnetic fields up to 5 T. Elastic neutron-scattering experiments confirm ferromagnetic and exclude ferrimagnetic order of CeTiGe${}_3$. Analysis of the specific heat gives evidence of a strong magnetic anisotropy in this system as evidenced by a gap. Replacing Ti by V reduces $T_c$, with $T_c\to 0$ for $x\approx 0.35$, suggesting a possible ferromagnetic quantum critical point.

The halide perovskites CsSn$X_3$, with $X=$ Cl, Br, I, are investigated using quasiparticle self-consistent $GW$ electronic structure calculations. These materials are found to have an “inverted” band structure from most semiconductors with a nondegenerate $s$-like valence band maximum (VBM) and triply degenerate $p$-like conduction band minimum (CBM). The small hole effective mass results in high hole mobility, in agreement with recent reports for CsSnI${}_3$. The relatively small band gap changes from Cl to Br to I result from the intra-atomic Sn $s$ and Sn $p$ characters of the VBM and CBM, respectively. The latter is also responsible for the high oscillator strength of the optical transition in these direct-gap semiconductors and hence a strong luminescence and absorption. The band gap change with lattice constant is also anomalous. It increases with increasing lattice constant, and this results from the decreasing valence band width due to the decreased Sn $s$ with anion $p$ interaction. It leads to an anomalous temperature dependence of the gap. The changes in band gap in different lower-symmetry crystallographic phases is studied. The exciton binding energy of the free exciton, estimated from the Wannier-Mott exciton theory and the calculated dielectric constants and effective masses, is found to be two orders of magnitude smaller than previously claimed in literature, or of the order of 0.1 meV. The photoluminescence peak previously assigned to the free exciton is instead ascribed to an acceptor bound exciton. The phonons at the $\Gamma$ point are calculated as well as the related enhancement of the dielectric constants.

The impact of polymer-polymer interactions of various types on the thermodynamics, structure, and accommodation of topological constraints is addressed for systems comprising many directed polymers in two spatial dimensions. The approach is predicated on the well-known equivalence between the classical equilibrium statistical mechanics of directed polymers in two spatial dimensions and the imaginary-time quantum dynamics of particles in one spatial dimension, originally exploited by P.-G. de Gennes [P.-G. de Gennes, J. Chem. Phys. 48, 2257 (1968)]. Known results concerning two exactly solvable microscopic models of quantum particles moving in one spatial dimension—the Lieb-Liniger model of contact interactions and the Calogero-Sutherland model of long-range interactions—are used to shed light on the behavior of the corresponding polymeric systems. In addition, the technique of bosonization is used to reveal how generic polymer interactions give rise to an emergent polymer fluid that has universal collective excitations. Additionally, the response of the system to topological constraints such as pins though which polymers cannot pass is explored. Immediately on the compressed side of a pin there is a divergent pile-up in polymer density, while on the other side there is a gap of finite area in which polymer density is negligible. Comparison of this response to that of a system of simply noncrossing (i.e., noncrossing but otherwise noninteracting) directed polymers, explored in a companion paper, reveals that generic interactions leave the structure quantitatively unchanged on the line transverse to the pin, and leave it qualitatively unchanged throughout the two dimensions of the system's extent. Furthermore, the free-energy cost associated with a pin that partitions a system having generic interactions is found to be proportional to the pin-partitioning cost for a system of simply noncrossing polymers.

Arrays of coupled phonon cavities each including an impurity qubit in silicon are considered. We study experimentally feasible architectures that can exhibit quantum many-body phase transitions of phonons, e.g., Mott insulator and superfluid states, due to a strong phonon-phonon interaction (which is mediated by the impurity qubit-cavity phonon coupling). We investigate closed equilibrium systems as well as driven dissipative nonequilibrium systems at zero and nonzero temperatures. Our results indicate that quantum many-body phonon systems are achievable both in on-chip nanomechanical systems in silicon and distributed Bragg reflector phonon cavity heterostructures in silicon-germanium. Temperature and driving field are shown to play a critical role in achieving these phonon superfluid and insulator states, results that are also applicable to polariton systems. Experimental procedures to detect these states are also given. Cavity-phoniton systems enable strong phonon-phonon interactions as well as offering long wavelengths for forming extended quantum states; they may have some advantage in forming truly quantum many-body mechanical states as compared to other optomechanical systems.

We investigate the low temperature structural and electronic properties of the type-I clathrate Eu${}_8$Ga${}_{16}$Ge${}_{30}$ under pressure using x-ray powder diffraction (XRD), x-ray absorption near-edge structure (XANES), and x-ray magnetic circular dichroism (XMCD) techniques. The XRD measurements reveal a transition to an amorphous phase above 18 GPa. Unlike previous reports on other clathrate compounds, no volume collapse is observed prior to the crystalline-amorphous phase transition which takes place when the unit cell volume is reduced to 81$\%$ of its ambient pressure value. Fits of the pressure-dependent relative volume to a Murnaghan equation of state yield a bulk modulus $B_0=65\pm 3$ GPa and a pressure derivative $B_0^{\prime }=3.3\pm 0.5$. The Eu $L_2$-edge XMCD data shows quenching of the magnetic order at a pressure coincident with the crystalline-amorphous phase transition. This information along with the persistence of an Eu${}^{2+}$ valence state observed in the XANES spectra up to the highest pressure point (22 GPa) indicates that the suppression of XMCD intensity is due to the loss of long range magnetic order. When compared with other clathrates, the results point to the importance of guest ion-cage interactions in determining the mechanical stability of the framework structure and the critical pressure for amorphization. Finally, the crystalline structure is not found to recover after pressure release, resulting in an amorphous material that is at least metastable at ambient pressure and temperature.

We present measurements of the temperature-dependent conductance for series arrays of small-capacitance superconducting quantum interference devices (SQUIDs). At low-bias voltages, the arrays exhibit a strong Coulomb blockade, which we study in detail as a function of temperature and Josephson energy $E_J$. We find that the zero-bias conductance is dominated by a thermally activated hopping of Cooper pairs between neighboring superconductive islands with the activation energy of the order of $\Lambda E_C$, where $\Lambda$ is the charge screening length in the array and $E_C$ is the charging energy of a single SQUID.

Near field radiative heat transfer and vacuum friction are just two instances of topics of technological and fundamental interest studied via the formalism of fluctuational electrodynamics. From the perspective of experiment and simulations, it is hard to precisely control and probe such nonequilibrium situations. Fluctuations in equilibrium are easier to measure, and typically can be related to nonequilibrium response functions by Green-Kubo relations. We consider a collection of arbitrary objects in vacuum, perturbed by changing the temperature or velocity of one object. Developing a method for computation of higher order correlation functions in fluctuational electrodynamics, we explicitly compare linear response and equilibrium fluctuations. We obtain a Green-Kubo relation for the radiative heat transfer, as well as a closed formula for the vacuum friction in arbitrary geometries in the framework of scattering theory. We comment on the signature of the radiative heat conductivity in equilibrium fluctuations.