We theoretically investigate the time-frequency characteristics of high-order harmonic generation (HHG) from Bloch-oscillating electrons in the band structure of a conventional bulk semiconductor driven by a single-optical-cycle two-color IR waveform. Spectrally filtering out the Bloch-HHG cutoff radiation allows the generation of an isolated Bloch-HHG pulse of $\sim$1.6-fs duration.

Transport and magnetic properties have been systematically investigated for SmMnAsO${}_{1-x}$ with controlled electron doping. As the electron band filling is increased with the increase of oxygen deficiency ($x$), the resistivity monotonically decreases, and the transition from Mott-insulator to metal occurs between $x=0.17$ and $0.2$. The Seebeck coefficient at room temperature abruptly changes around the critical doping level from a large value ($\sim$$-300$ $\mu$V/K) to a small one ($\sim$$-50$ $\mu$V/K) both with negative sign. In the metallic compounds with $x=0.2$ and $0.3$, Mn spins order antiferromagnetically around 30 K, and the Hall coefficient with the negative sign shows a reduction in magnitude upon the magnetic transition, indicating the change in the multiple Fermi surfaces. A gigantic positive magnetoresistance effect is observed in a wide range of temperatures, reaching up to 60% at 2 K for the $x=0.3$ compound. The effect is attributed to the field-induced change of the complex Fermi surfaces in this multiorbital correlated electron system.

We demonstrated optical spin control of a two-dimensional electron gas in a modulation-doped CdTe quantum well by driving a spin-flip Raman transition with a pair of phase-locked laser pulses. In contrast to single-pulse optical spin control, which features a fixed spin-rotation axis, manipulation of the initial relative phase of the pulse pair enables us to control the axis of the optical spin rotation. We show that the Raman pulse pair acts like an effective microwave field, mapping the relative optical phase onto the phase of the electron spin polarization and making possible ultrafast, all-optical, and full quantum control of the electron spins.

The detrimental effects of the fission gas Xe on the performance of oxide nuclear fuels are well known. However, less well known are the mechanisms that govern fission gas evolution. Here, to better understand bulk Xe behavior (diffusion mechanisms) in UO${}_{2\pm x}$ we calculate the relevant activation energies using density functional theory techniques. By analyzing a combination of Xe solution thermodynamics, migration barriers, and the interaction of dissolved Xe atoms with U, we demonstrate that Xe diffusion predominantly occurs via a vacancy-mediated mechanism. Since Xe transport is closely related to the diffusion of U vacancies, we have also studied the activation energy for this process. To best reproduce experimental data for the Xe and U activation energies, it is critical to consider the active charge-compensation mechanism for intrinsic defects in UO${}_{2\pm x}$. Due to the high thermodynamic cost of reducing U${}^{4+}$ ions, any defect formation occurring at a fixed composition, i.e., no change in UO${}_{2\pm x}$ stoichiometry, always avoids such reactions, which, for example, implies that the ground-state configuration of an O Frenkel pair in UO${}_2$ does not involve any explicit local reduction (oxidation) of U ions at the O vacancy (interstitial).

This work reviews and examines two particular issues related with the new technique of electrical detection of ferromagnetic resonance (FMR). This powerful technique has been broadly applied for studying magnetization and spin dynamics over the past ten years. The first issue is the relation and distinction between different mechanisms that give rise to a photovoltage via FMR in spintronic devices, and the second is the proper analysis of the FMR line shape, which has become the “Achilles heel” in interpreting experimental results, especially for either studying the spin pumping effect or quantifying spin Hall angles via the electrically detected FMR.

The superconducting Cu${}_x$Bi${}_2$Se${}_3$ is an electron-doped topological insulator and is a prime candidate of the topological superconductor which still awaits discovery. The electrochemical intercalation technique for synthesizing Cu${}_x$Bi${}_2$Se${}_3$ offers good control of restricting Cu into the van der Waals gap and yields samples with shielding fractions of up to $\sim$50%. We report essential details of this synthesis technique and present the established superconducting phase diagram of $T_c$ vs $x$, along with a diagram of the shielding fraction vs $x$. Intriguingly, those diagrams suggest that there is a tendency to spontaneously form small islands of optimum superconductor in this material.

The magnetization dynamics of ferromagnets is often formulated in terms of the Landau-Lifshitz-Gilbert (LLG) equation. The reactive part of this equation describes the response of the magnetization in terms of effective fields, whereas the dissipative part is parametrized by the Gilbert damping tensor. We formulate a scattering theory for the magnetization dynamics and map this description on the linearized LLG equation by attaching electric contacts to the ferromagnet. The reactive part can then be expressed in terms of the static scattering matrix. The dissipative contribution to the low-frequency magnetization dynamics can be described as an adiabatic energy pumping process to the electronic subsystem by the time-dependent magnetization. The Gilbert damping tensor depends on the time derivative of the scattering matrix as a function of the magnetization direction. By the fluctuation-dissipation theorem, the fluctuations of the effective fields can also be formulated in terms of the quasistatic scattering matrix. The theory is formulated for general magnetization textures and worked out for monodomain precessions and domain-wall motions. We prove that the Gilbert damping from scattering theory is identical to the result obtained by the Kubo formalism.

We study the quantum phase transition out of the Néel state in SU(3) and SU(4) generalizations of the Heisenberg antiferromagnet with a sign-problem-free four-spin coupling (so-called $JQ$ model), by extensive quantum Monte Carlo simulations. We present evidence that the SU(3) and SU(4) order parameters and the SU(3) and SU(4) stiffness go to zero continuously without any evidence of a first-order transition. However, we find considerable deviations from simple scaling laws for the stiffness even in the largest system sizes studied. We interpret these as arising from multiplicative scaling terms in these quantities that affect the leading behavior, i.e., they will persist in the thermodynamic limit, unlike the conventional additive corrections from irrelevant operators. We conjecture that these multiplicative terms arise from dangerously irrelevant operators whose contributions to the quantities of interest are nonanalytic.

The nonmagnetic impurity effect is studied on the Fe-based BaFe${}_{1.89-2x}$Co${}_{0.11}$As${}_2$ superconductor ($T_c$ $=$ 25 K) with Zn substitution for Fe up to 8 at. %, which is achieved by means of high-pressure and high-temperature heating. $T_c$ decreases almost linearly with increasing Zn content and disappears at ∼8 at. %. It is different in the shared phenomenology of the early Zn doping studies, where $T_c$ decreased little. The decreasing $T_c$ rate, however, remains much lower (3.63 K/%) than what is expected for the $s_{\pm }$-wave model, implying the model is inaccurate. Another symmetry model such as the non-sign-reversal $s$-wave model may better account for the result.

Hydrogen has been found to be an important source of $n$-type conductivity in the transparent conducting oxide SnO${}_2$. We have studied the properties of H in SnO${}_2$ single crystals with infrared spectroscopy. When H or D is introduced into SnO${}_2$ by annealing in an H${}_2$ or D${}_2$ ambient at elevated temperature, several O-H and O-D vibrational lines are produced, along with the low-frequency absorption characteristic of free carriers. To probe the relationship between H and the free carriers it introduces, the thermal stability of free carrier absorption and its relationship to the thermal stabilities of O-H lines have been examined. Two H-related donors are found: one is stable at room temperature on a time scale of weeks, and a second is stable up to 600 ${}^{\circ }$C. These electrically active defects are found to interact with other O-H centers and can be converted from one to another by thermal treatments. The vibrational modes have been found to have distinctive polarization properties that provide an important test of microscopic defect models for the several O-H and (O-H)${}_2$ centers we have observed.

We simulate the $t$-$J$ model in two dimensions by means of infinite projected entangled-pair states (iPEPS) generalized to arbitrary unit cells, finding results similar to those previously obtained by the density-matrix renormalization group (DMRG) for wide ladders. In particular, we show that states exhibiting stripes, that is, a unidirectional modulation of hole-density and antiferromagnetic order with a $\pi$-phase shift between adjacent stripes, have a lower variational energy than uniform phases predicted by variational and fixed-node Monte Carlo simulations. For a fixed unit-cell size the energy per hole is minimized for a hole density ${\rho }_l\sim 0.5$ per unit length of a stripe. The superconducting order parameter is maximal around ${\rho }_l\sim 0.75\text{--}0.8$.

Crystal structure, electronic structure, and magnetic behavior of the spin-$\frac{1}{2}$ quantum magnet Na${}_{1.5}$VOPO${}_4$F${}_{0.5}$ are reported. The disorder of Na atoms leads to a sequence of structural phase transitions revealed by synchrotron x-ray powder diffraction and electron diffraction. The high-temperature second-order $\alpha \leftrightarrow \beta$ transition at 500 K is of the order-disorder type, whereas the low-temperature $\beta \leftrightarrow \gamma +{\gamma }^{\prime }$ transition around 250 K is of the first order and leads to a phase separation toward the polymorphs with long-range ($\gamma$) and short-range (${\gamma }^{\prime }$) order of Na. Despite the complex structural changes, the magnetic behavior of Na${}_{1.5}$VOPO${}_4$F${}_{0.5}$ probed by magnetic susceptibility, heat capacity, and electron spin resonance measurements is well described by the regular frustrated square lattice model of the high-temperature $\alpha$-polymorph. The averaged nearest-neighbor and next-nearest-neighbor couplings are ${\overline{J}}_1\simeq -3.7$ K and ${\overline{J}}_2\simeq 6.6$ K, respectively. Nuclear magnetic resonance further reveals the long-range ordering at $T_N=2.6$ K in low magnetic fields. Although the experimental data are consistent with the simplified square-lattice description, band structure calculations suggest that the ordering of Na atoms introduces a large number of inequivalent exchange couplings that split the square lattice into plaquettes. Additionally, the direct connection between the vanadium polyhedra induces an unusually strong interlayer coupling having effect on the transition entropy and the transition anomaly in the specific heat. Peculiar features of the low-temperature crystal structure and the relation to isostructural materials suggest Na${}_{1.5}$VOPO${}_4$F${}_{0.5}$ as a parent compound for the experimental study of tetramerized square lattices as well as frustrated square lattices with different values of spin.

We have used large pulsed magnetic fields up to 60 T to suppress the contribution of superconducting fluctuations (SCFs) to the $ab$-plane conductivity above $T_c$ in a series of YBa${}_2$Cu${}_3$O${}_{6+x}$ from the deep pseudogapped state to slight overdoping. Accurate determinations of the SCF contribution to the conductivity versus temperature and magnetic field have been achieved. Their joint quantitative analyses with respect to Nernst data allow us to establish that thermal fluctuations following the Ginzburg-Landau scheme are dominant for nearly optimally doped samples. The deduced coherence length $\xi \left(T\right)$ is in perfect agreement with a Gaussian (Aslamazov-Larkin) contribution for $1.01T_c\lesssim T\lesssim 1.2T_c$. A phase-fluctuation contribution might be invoked for the most underdoped samples in a $T$ range which increases when controlled disorder is introduced by electron irradiation. For all dopings we evidence that the fluctuations are highly damped when increasing $T$ or $H$. This behavior does not follow the Ginzburg-Landau approach, which should be independent of the microscopic specificities of the superconducting state. The data permits us to define a field $H_c^{\prime }\left(T\right)$ and a temperature $T_c^{\prime }$ above which the SCFs are fully suppressed. The analysis of the fluctuation magnetoconductance in the Ginzburg-Landau approach allows us to determine the critical field $H_{c2}\left(0\right)$. The actual values of $H_c^{\prime }\left(0\right)$ and $H_{c2}\left(0\right)$ are found to be quite similar and both increase with hole doping. These depairing fields, which are directly connected to the magnitude of the superconducting gap, do therefore follow the $T_c$ variation which is at odds with the sharp decrease of the pseudogap $T^{*}$ with increasing hole doping. This is on line with our previous evidence that $T^{*}$ is not the onset of pairing. So the large gap seen by spectroscopic experiments in the underdoped regime has to be associated with the pseudogap. We finally propose here a three-dimensional phase diagram including a disorder axis, which makes it possible to explain most peculiar observations done so far on the diverse cuprate families.

The entanglement entropy in Kondo impurity systems is studied analytically using conformal field theory. From the impurity contribution to the scaling corrections of the entanglement entropy we extract information about the screening cloud profile for general non-Fermi-liquid fixed points. By also considering the finite-temperature corrections to scaling of the von Neumann entropy we point out a direct connection between the long-distance screening cloud profile and thermodynamic observables such as the specific heat.

We investigate the optical properties of bromine-intercalated highly orientated pyrolytic graphite (Br-HOPG) and provide an interpretation of the data. We observe absorption features below 620 meV which are absent in the absorption spectrum of graphite. Comparing our results with those of theoretical studies on graphite, single- and bilayer graphene, as well as recent optical studies of multilayer graphene, we conclude that Br-HOPG contains the signatures of ultrapure bilayer graphene, ultrapure single-layer graphene, and graphite. The observed supermetallic conductivity of Br-HOPG is identified with the presence of very high mobility ($\simeq$121 000 cm${}^2$V${}^{-1}$s${}^{-1}$ at room temperature and at very high carrier density), multilayer graphene components in our sample. This could provide an avenue for single- and multilayer graphene research.

We study electron spin decoherence in a two-electron double quantum dot due to the hyperfine interaction, under spin-echo conditions as studied in recent experiments. We develop a semiclassical model for the interaction between the electron and nuclear spins, in which the time-dependent Overhauser fields induced by the nuclear spins are treated as classical vector variables. Comparison of the model with experimentally obtained echo signals allows us to quantify the contributions to the nuclear spin evolution of various processes such as coherent Larmor precession and spin diffusion.

We have measured the evolution of the tunneling density of states (DOS) in superconductor/ferromagnet (S/F) bilayers with increasing F-layer thickness, where F in our experiment is the strong ferromagnet Ni. As a function of increasing Ni thickness, we detect multiple oscillations in the DOS at the Fermi energy from differential conductance measurements. The features in the DOS associated with the proximity effect change from normal to inverted twice as the Ni thickness increases from 1 to 5 nm.

A newly discovered family of Fe-based superconductors is isostructural with the so-called 122 family of Fe pnictides but has a qualitatively different doping state. Early experiments indicate that superconductivity is nodeless, yet prerequisites for the $s_{\pm }$ nodeless state (generally believed to be realized in Fe superconductors) are missing. It is tempting to assign a $d$-wave symmetry to the new materials, and it does seem, at first glance, that such a state may be nodeless. Yet a more careful analysis shows that it is not possible, given the particular 122 crystallography. If indeed superconductivity in this system is nodeless, the possible choice of admissible symmetries is severely limited: it is either a conventional single-sign $s_{+}$ state or another $s_{\pm }$ state, different from the one believed to be present in other Fe-based superconductors.

Lifting a single molecular wire off the surface with a combined frequency-modulated atomic force and tunneling microscope it is possible to monitor the evolution of both the wire configuration and the contacts simultaneously with the transport conductance experiment. In particular, critical points where individual bonds to the surface are broken and instabilities where the wire is prone to change its contact configuration can be identified in the force gradient and dissipation responses of the junction. This additional mechanical information can be used to unambiguously determine the conductance of a true molecular wire, that is, of a molecule that is contacted via a pointlike “crocodile clip” to each of the electrodes but is otherwise free.

We measured the anomalous Nernst effect and anisotropic magnetoresistive heating in a lateral multiterminal permalloy/copper spin valve using all-electrical lock-in measurements. To interpret the results, a three-dimensional thermoelectric finite-element model is developed. Using this model, we extract the heat profile which we use to determine the anomalous Nernst coefficient of permalloy $R_N=0.13$ and also determine the maximum angle $\theta =8^{\circ }$ of the magnetization prior to the switching process when an opposing noncollinear 10${}^{\circ }$ magnetic field is applied.