We derive an efficiency bound for continuous quantum heat engines absorbing heat from squeezed thermal reservoirs. Our approach relies on a full-counting statistics description of nonequilibrium transport and it is not limited to the framework of irreversible thermodynamics. Our result, a generalized Carnot efficiency bound, is valid beyond the small-squeezing and high-temperature limit. Our findings are embodied in a prototype three-terminal quantum photoelectric engine where a qubit converts heat absorbed from a squeezed thermal reservoir into electrical power. We demonstrate that in the quantum regime, the efficiency can be greatly amplified by squeezing. From the fluctuation relation, we further receive other operational measures in linear response, for example, the universal maximum power efficiency bound.

We present a detailed analysis on the hopping between monolayer ${\mathrm{CuO}}_2$ and bulk ${\mathrm{CuO}}_2$ plane in the ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ substrate. With a two-band model, we demonstrate that the nodeless gap can only exist when the hole concentration in the monolayer ${\mathrm{CuO}}_2$ plane is very large. We argue that the possible phase separation may play an important role in the recent experimental observation of a nodeless gap.

We measured the optical conductivity of superconducting single crystals of ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ with $x$ ranging from 0.40 (optimal doping, $T_c=39$ K) down to 0.20 (underdoped, $T_c=16$ K), where a magnetic order coexists with superconductivity. In the normal state, the low-frequency optical conductivity can be described by an incoherent broad Drude component and a coherent narrow Drude component: the broad one is doping independent, while the narrow one shows strong scattering in the heavily underdoped compound. In the superconducting state, the formation of the condensate leads to a low-frequency suppression of the optical conductivity spectral weight. In the heavily underdoped region, the superfluid density is significantly suppressed, and the weight of unpaired carriers rapidly increases. We attribute these results to changes in the superconducting gap across the phase diagram, which could show a nodal-to-nodeless transition due to the strong interplay between magnetism and superconductivity in underdoped ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$.

The electronic structures of $\mathrm{U}{X}_3$ ($X=\mathrm{Al}, \mathrm{Ga}$, and $\mathrm{In}$) were studied by photoelectron spectroscopy to understand the relationship between their electronic structures and magnetic properties. The band structures and Fermi surfaces of ${\mathrm{UAl}}_3$ and ${\mathrm{UGa}}_3$ were revealed experimentally by angle-resolved photoelectron spectroscopy (ARPES), and they were compared with the result of band-structure calculations. The topologies of the Fermi surfaces and the band structures of ${\mathrm{UAl}}_3$ and ${\mathrm{UGa}}_3$ were explained reasonably well by the calculation, although bands near the Fermi level ($E_{\mathrm{F}}$) were renormalized owing to the finite electron correlation effect. The topologies of the Fermi surfaces of ${\mathrm{UAl}}_3$ and ${\mathrm{UGa}}_3$ are very similar to each other, except for some minor differences. Such minor differences in their Fermi surface or electron correlation effect might take an essential role in their different magnetic properties. No significant changes were observed between the ARPES spectra of ${\mathrm{UGa}}_3$ in the paramagnetic and antiferromagnetic phases, suggesting that ${\mathrm{UGa}}_3$ is an itinerant weak antiferromagnet. The effect of chemical pressure on the electronic structures of $\mathrm{U}{X}_3$ compounds was also studied by utilizing the smaller lattice constants of ${\mathrm{UAl}}_3$ and ${\mathrm{UGa}}_3$ than that of ${\mathrm{UIn}}_3$. The valence band spectrum of ${\mathrm{UIn}}_3$ is accompanied by a satellitelike structure on the high-binding-energy side. The core-level spectrum of ${\mathrm{UIn}}_3$ is also qualitatively different from those of ${\mathrm{UAl}}_3$ and ${\mathrm{UGa}}_3$. These findings suggest that the $\mathrm{U}5f$ states in ${\mathrm{UIn}}_3$ are more localized than those in ${\mathrm{UAl}}_3$ and ${\mathrm{UGa}}_3$.

We report on the uniaxial strain and thermal conductivity of well-ordered, suspended silicon nanowire arrays between 10 to 20 nm width and 22 nm half-pitch, fabricated by extreme-ultraviolet (UV) interference lithography. Laser-power-dependent Raman spectroscopy showed that nanowires connected monolithically to the bulk had a consistent strain of $\sim 0.1\%$, whereas nanowires clamped by metal exhibited variability and high strain of up to 2.3%, having implications in strain engineering of nanowires. The thermal conductivity at room temperature was measured to be $\sim 1\mathrm{W}/\mathrm{m}\mathrm{K}$ for smooth nanowires and $\sim 0.1\mathrm{W}/\mathrm{m}\mathrm{K}$ for rougher ones, similar to results by other investigators. We found no modification of the bulk properties in terms of intrinsic scattering, and therefore, the decrease in thermal conductivity is mainly due to boundary scattering. Different types of surface roughness, such as constrictions and line-edge roughness, may play roles in the scattering of phonons of different wavelengths. Such low thermal conductivities would allow for very efficient thermal energy harvesting, approaching and passing values achieved by state-of-the-art thermoelectric materials.

We study the dynamics of the biased sub-Ohmic spin-boson model by means of a time-dependent variational matrix product state (TDVMPS) algorithm. The evolution of both the system and the environment is obtained in the weak- and the strong-coupling regimes, respectively characterized by damped spin oscillations and by a nonequilibrium process where the spin freezes near its initial state, which are explicitly shown to arise from a variety of reactive environmental quantum dynamics. We also explore the rich phenomenology of the intermediate-coupling case, a nonperturbative regime where the system shows a complex dynamical behavior, combining features of both the weakly and the strongly coupled case in a sequential, time-retarded fashion. Our work demonstrates the potential of TDVMPS methods for exploring otherwise elusive, nonperturbative regimes of complex open quantum systems, and points to the possibilities of exploiting the qualitative, real-time modification of quantum properties induced by nonequilibrium bath dynamics in ultrafast transient processes.

We numerically study amorphous analogs of a two-dimensional photonic Chern insulator. The amorphous lattices consist of gyromagnetic rods that break time-reversal symmetry, with the lattice sites generated by a close-packing algorithm. The level of short-range order is adjustable, and there is no long-range order. The topologically nontrivial gaps of the photonic Chern insulator are found to persist into the amorphous regime, so long as there is sufficient short-range order. Strongly nonreciprocal robust transmission occurs via edge states, which are shown to propagate ballistically despite the absence of long-range order, and to be exponentially localized along the lattice edge. Interestingly, there is an enhancement of nonreciprocal transmission even at very low levels of short-range order, where there are no discernible spectral gaps.

For a generic lattice Hamiltonian of the electron states in Weyl materials, we calculate analytically the chiral (or, equivalently, valley) charge and current densities in the first order in background electromagnetic and strain-induced pseudoelectromagnetic fields. We find that the chiral response induced by the pseudoelectromagnetic fields is not topologically protected. Although our calculations reproduce qualitatively the anomalous chiral Hall effect, the actual result for the conductivity depends on the definition of the chirality as well as on the parameters of the lattice model. In addition, while for the well-separated Fermi surfaces surrounding the individual Weyl nodes the current induced by the magnetic field coincides almost exactly with the current of the chiral separation effect in linearized models, there are clear deviations when the Fermi surfaces undergo the Lifshitz transition. In general, we find that all chiral response coefficients vanish at large chemical potential.

The quantum spin liquid is an enigmatic quantum state in insulating magnets, in which conventional long-range order is suppressed by strong quantum fluctuations. Recently, an unconventional phase transition was reported between the low-temperature quantum spin liquid and the high-temperature paramagnet in the Kitaev model on a three-dimensional hyperhoneycomb lattice. Here, we show that a similar “liquid-gas” transition takes place in another three-dimensional lattice, the hyperoctagon lattice. We investigate the critical phenomena by adopting the Green-function based Monte Carlo technique with the kernel polynomial method, which enables systematic analysis of up to 2048 sites. The critical temperature is lower than that in the hyperhoneycomb case, reflecting the smaller flux gap. We also discuss the transition on the basis of an effective model in the anisotropic limit.

Coherent phonon (CP) generation in an undoped Si crystal is theoretically investigated to shed light on unexplored quantum-mechanical effects in the early-time region immediately after the irradiation of ultrashort laser pulses. We examine time signals attributed to an induced charge density of an ionic core, placing the focus on the effects of the Rabi frequency ${\mathrm{\Omega }}_{0cv}$ on the signals; this frequency corresponds to the peak electric-field of the pulse. It is found that at specific ${\mathrm{\Omega }}_{0cv}$'s, where the energy of plasmon caused by photoexcited carriers coincides with the longitudinal-optical phonon energy, the energetically resonant interaction between these two modes leads to striking anticrossings, revealing irregular oscillations with anomalously enhanced amplitudes in the observed time signals. Also, the oscillatory pattern is subject to the Rabi flopping of the excited carrier density that is controlled by ${\mathrm{\Omega }}_{0cv}$. These findings show that the early-time region is enriched with quantum-mechanical effects inherent in the CP generation, though experimental signals are more or less masked by the so-called coherent artifact due to nonlinear optical effects.

Quantum-dot (QD) intermediate-band (IB) materials are regarded as promising candidates for high-efficiency photovoltaics. The sequential two-step two-photon absorption processes that take place in these materials have been proposed to develop high-efficiency solar cells and infrared (IR) photodetectors. In this work, we experimentally and theoretically study the interrelation of the absorptivity with transitions of carriers to and from the IB in type-II GaSb/GaAs QD devices. Our devices exhibit three optical band gaps with: $E_{\mathrm{L}}=0.49\mathrm{eV},E_{\mathrm{H}}=1.02\mathrm{eV}$, and $E_{\mathrm{G}}=1.52\mathrm{eV}$, with the IB located 0.49 eV above the valence band. These values are well supported by semiempirical calculations of the QDs electronic structure. Through intensity-dependent two-photon photocurrent experiments, we are able to vary the filling state of the IB, thus modifying the absorptivity of the transitions to and from this band. By filling the IB with holes via $E=1.32\mathrm{eV}$ or $E=1.93\mathrm{eV}$ monochromatic illumination, we demonstrate an increase in the $E_{\mathrm{L}}$-related absorptivity of more than two orders of magnitude and a decrease in the $E_{\mathrm{H}}$-related absorptivity of one order of magnitude. The antisymmetrical evolution of those absorptivities is quantitatively explained by a photoinduced shift of the quasi-Fermi level of the IB. Furthermore, we report the observation of a two-photon photovoltage, i.e., the contribution of subband gap two-photon absorption to increase the open-circuit voltage of solar cells. We find that the generation of the two-photon photovoltage is related, in general, to the production of a two-photon photocurrent. However, while photons with energy close to $E_{\mathrm{L}}$ participate in the production of the two-photon photocurrent, they are not effective in the production of a two-photon photovoltage. We also report the responsivity of GaSb/GaAs QD devices performing as optically triggered photodetectors. These devices exhibit an amplification factor of almost 400 in the IR spectral region. This high value is achieved by minimizing—via doping—the absorptivity in the IR range of the QDs under equilibrium conditions.

Single-chain magnets (SCMs) are one-dimensional coordination polymers or spin chains that display slow relaxation of the magnetization. Typically their static magnetic properties are described by the Heisenberg model, while the description of their dynamic magnetic properties is based on an Ising-like model. The types of excitations predicted by these models (collective vs localized) are quite different. Therefore we probed the nature of the elementary excitations for two SCMs abbreviated ${\mathrm{Mn}}_2\mathrm{Ni}$ and ${\mathrm{Mn}}_2\mathrm{Fe}$, as well as a mononuclear derivative of the ${\mathrm{Mn}}_2\mathrm{Fe}$ chain, by means of high-frequency electron paramagnetic resonance spectroscopy (HFEPR) and inelastic neutron scattering (INS). We find that the HFEPR spectra of the chains are clearly distinct from those of the monomer. The momentum transfer dependence of the INS intensity did not reveal significant dispersion, indicating an essentially localized nature of the excitations. At the lowest temperatures these are modified by the occurrence of short-range correlations.

The synthesis of hydrogen sulfides, with the potential of high-temperature superconductivity, was recently proposed at high $T_c$ = 203 K. It motivated us to employ an ab initio approach for the predictions of crystal structures to find the stable scandium hydrides. In addition to the earlier predicted three stoichiometries of ScH, ${\mathrm{ScH}}_2$, and ${\mathrm{ScH}}_3$, we identify three other metallic stoichiometries of ${\mathrm{ScH}}_4, {\mathrm{ScH}}_6$, and ${\mathrm{ScH}}_8$, which show superconductivity at significantly higher temperatures. The phases of ${\mathrm{ScH}}_4$ and ${\mathrm{ScH}}_6$, whose stability does not require extremely high pressures ($<150$ GPa with ZPE), are primarily ionic compounds containing exotic quasimolecular ${\mathrm{H}}_2$ arrangements. The present electron-phonon calculations revealed the superconductive potential of ${\mathrm{ScH}}_4$ and ${\mathrm{ScH}}_6$ with estimated $T_c$ of 98 K and 129 K at 200 GPa and 130 GPa, respectively. The superconductivity of ${\mathrm{ScH}}_n$ stems from the large electron-phonon coupling associated with the wagging, bending, and intermediate-frequency modes attributed mainly to the hydrogen atoms.

High-pressure neutron powder diffraction, muon-spin rotation, and magnetization studies of the structural, magnetic, and the superconducting properties of the Ce-underdoped superconducting (SC) electron-doped cuprate system with the ${\mathrm{Nd}}_2{\mathrm{CuO}}_4$ (the so-called ${\mathrm{T}}^{{}^{\prime }}$) structure ${\mathrm{T}}^{{}^{\prime }}\text{−}{\mathrm{Pr}}_{1.3-x}{\mathrm{La}}_{0.7}{\mathrm{Ce}}_x{\mathrm{CuO}}_4$ with $x=0.1$ are reported. A strong reduction of the in-plane and out-of-plane lattice constants is observed under pressure. However, no indication of any pressure-induced phase transition from ${\mathrm{T}}^{{}^{\prime }}$ to the ${\mathrm{K}}_2{\mathrm{NiF}}_4$ (the so-called T) structure is observed up to the maximum applied pressure of $p$ = 11 GPa. Large and nonlinear increase of the short-range magnetic order temperature $T_{\mathrm{so}}$ in ${\mathrm{T}}^{{}^{\prime }}\text{−}{\mathrm{Pr}}_{1.3-x}{\mathrm{La}}_{0.7}{\mathrm{Ce}}_x{\mathrm{CuO}}_4$ ($x=0.1$) was observed under pressure. Simultaneous pressure causes a nonlinear decrease of the SC transition temperature $T_{\mathrm{c}}$. All these experiments establish the short-range magnetic order as an intrinsic and competing phase in SC ${\mathrm{T}}^{{}^{\prime }}\text{−}{\mathrm{Pr}}_{1.3-x}{\mathrm{La}}_{0.7}{\mathrm{Ce}}_x{\mathrm{CuO}}_4$ ($x=0.1$). The observed pressure effects may be interpreted in terms of the improved nesting conditions through the reduction of the in-plane and out-of-plane lattice constants upon hydrostatic pressure.

We investigate femtosecond spin injection from an optically excited Ni top layer into an Au bottom layer using time-resolved complex magneto-optical Kerr effect (C-MOKE) measurements. Employing the C-MOKE formalism, we are able to follow layer-resolved demagnetization in Ni and the simultaneous spin injection into the adjacent Au film, both occurring within $\sim 40\mathrm{fs}$. We confirm the ballistic to diffusive propagation of the spin transfer process with ab initio theory and superdiffusive transport calculations. In particular, our combined experimental-theoretical effort does allow us to quantify the so far elusive amount of spin injection, and therefore the spin injection efficiency at the interface.

We propose a new efficient permanent magnet, ${\mathrm{SmCoNiFe}}_3$, which is a development of the well-known ${\mathrm{SmCo}}_5$ prototype. More modern neodymium magnets of the Nd-Fe-B type have an advantage over ${\mathrm{SmCo}}_5$ because of their greater maximum energy products due to their iron-rich stoichiometry. Our new magnet, however, removes most of this disadvantage of ${\mathrm{SmCo}}_5$ while preserving its superior high-temperature efficiency over the neodymium magnets. We show by means of first-principles electronic-structure calculations that ${\mathrm{SmCoNiFe}}_3$ has very favorable magnetic properties and could therefore potentially replace ${\mathrm{SmCo}}_5$ or Nd-Fe-B types in various applications.

We investigate the temperature dependence of the lower critical field $H_{c1}\left(T\right)$, the field at which vortices penetrate into the sample, of a high-quality fluorine-doped NdFeAsO single crystal under static magnetic fields $H$ parallel to the $c$ axis. The temperature dependence of the first vortex penetration field has been experimentally obtained and pronounced changes of the $H_{c1}\left(T\right)$ curvature are observed, which are attributed to the multiband superconductivity. Using a two-band model with $s$-wave-like gaps, the temperature dependence of the lower critical field $H_{c1}\left(T\right)$ can be well described. These observations clearly show that the superconducting energy gap in fluorine-doped NdFeAsO is nodeless. The values of the penetration depth at $T=0\mathrm{K}$ have been determined and confirm that the pnictide superconductors obey an Uemura-style relationship between $T_c$ and ${\lambda }_{ab}{\left(0\right)}^{-2}$.

We investigate Gilbert damping, spectroscopic gyromagnetic ratio, and current-induced torques in the one-dimensional Rashba model with an additional noncollinear magnetic exchange field. We find that the Gilbert damping differs between left-handed and right-handed Néel-type magnetic domain walls due to the combination of spatial inversion asymmetry and spin-orbit interaction (SOI), consistent with recent experimental observations of chiral damping. Additionally, we find that also the spectroscopic $g$ factor differs between left-handed and right-handed Néel-type domain walls, which we call chiral gyromagnetism. We also investigate the gyromagnetic ratio in the Rashba model with collinear magnetization, where we find that scattering corrections to the $g$ factor vanish for zero SOI, become important for finite spin-orbit coupling, and tend to stabilize the gyromagnetic ratio close to its nonrelativistic value.

The emission of pure spin currents by mechanical deformations, the piezospintronic effect, in antiferromagnets is studied. We characterize the piezospintronic effect in an antiferromagnetic honeycomb monolayer in response to external strains. It is shown that the strain tensor components can be evaluated in terms of the spin Berry phase. In addition, we propose an experimental setup to detect the piezospin current generated in the piezospintronic material through the inverse spin Hall effect. Our results apply to a wide family of two-dimensional antiferromagnetic materials without inversion symmetry, such as the transition-metal chalcogenophosphate materials $M\mathrm{P}{X}_3$ ($M=\mathrm{V}$, Mn; $X=\mathrm{S}$, Se, Te) and ${\mathrm{NiPSe}}_3$.