We present femtosecond pump–probe spectroscopy studies of time-resolved optical reflectivity of all-oxide YBa${}_2$Cu${}_3$O${}_7$/La${}_{0.7}$Sr${}_{0.3}$MnO${}_3$ (YBCO/LSMO) superconductor/ferromagnet (S/F) bilayers consisting of a 100-nm-thick YBCO base layer and either 10 or 35 nm LSMO cap thickness. At temperatures far below the YBCO superconducting transition $T_C$, samples with a 10 nm F overlayer show a photoresponse that is similar to, but faster than, pure-YBCO, 100-nm-thick reference samples, while close to $T_C$ and above (up to ∼160 K) we observe a signature of both the electronic and spin response that cannot be interpreted as an incoherent sum of contributions from the two layers. The photoresponse of the S/F structures with the 35-nm LSMO layer always qualitatively follows that of the pure LSMO, but with a shorter relaxation time. In all cases, the YBCO/LSMO nonequilibrium dynamics can be modeled using a generalized multitemperature model, which is a superposition of the dynamics of the three-temperature models that are used to describe the superconductor and ferromagnet subsystems, respectively. The long term of the photoresponse signal of the S/F bilayer can be well fitted with the two characteristic relaxation times.

We report a single-crystal neutron diffraction study of the layered ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$. This work unambiguously determines the magnetic structure of the system and reveals that the spin orientation rigidly tracks the staggered rotation of the ${\mathrm{IrO}}_6$ octahedra in ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$. The long-range antiferromagnetic order has a canted spin configuration with an ordered moment of 0.208(3) ${\mu }_B$/Ir site within the basal plane; a detailed examination of the spin canting yields 0.202(3) and 0.049(2) ${\mu }_B$/site for the $a$ axis and the $b$ axis, respectively. It is intriguing that forbidden nuclear reflections of space group $I{4}_1/acd$ are also observed in a wide temperature range from 4 K to 600 K, which suggests a reduced crystal structure symmetry. This neutron-scattering work provides a direct, well-refined experimental characterization of the magnetic and crystal structures that are crucial to the understanding of the unconventional magnetism exhibited in this unusual magnetic insulator.

Gapped ${\mathbb{Z}}_2$ spin liquids have been proposed as candidates for the ground state of the $S=1/2$ quantum antiferromagnet on the kagome lattice. We extend the use of projected entangled pair states to construct (on the cylinder) resonating valence bond (RVB) states including both nearest-neighbor and next-nearest-neighbor singlet bonds. Our ansatz—dubbed “simplex spin liquid”—allows for an asymmetry between the two types of triangles (of order $2\%–3\%$ in the energy density after optimization) leading to the breaking of inversion symmetry. We show that the topological ${\mathbb{Z}}_2$ structure is still preserved and, by considering the presence or the absence of spinon and vison lines along an infinite cylinder, we explicitly construct four orthogonal RVB minimally entangled states. The spinon and vison coherence lengths are extracted from a finite size scaling with regard to the cylinder perimeter of the energy splittings of the four sectors and are found to be of the order of the lattice spacing. The entanglement spectrum of a partitioned (infinite) cylinder is found to be gapless, suggesting the occurrence, on a cylinder with real open boundaries, of gapless edge modes formally similar to Luttinger liquid (nonchiral) spin and charge modes. When inversion symmetry is spontaneously broken, the RVB spin liquid exhibits an extra Ising degeneracy, which might have been observed in recent exact diagonalization studies.

When PbTe, a good thermoelectric material, is doped with a few percent Tl, the figure of merit $ZT=T{S}^2/\left(\rho \kappa \right)$ [$S$ is the Seebeck coefficient, $\rho$ the electrical resistivity, and $\kappa$ the thermal conductivity] is dramatically improved. The maximum value of $ZT$ occurs for approximately 2$\%$ Tl, but the factors limiting $ZT$ are as yet poorly understood. From a detailed local structure study of PbTe:Tl using the extended x-ray absorption fine structure (EXAFS) technique, we find that Tl substitutes primarily as Tl(+1) on the Pb site, with no evidence for any solubility issue, any significant fraction of Tl(+3), or any Tl interstitials. However it is not a simple substitution as there is evidence for increasing Te vacancies on neighboring sites with increasing Tl concentration, $x$. In addition there is also increasing disorder with $x$—Tl-Te bond length disorder as well as the vacancy defects—that will scatter the hole carriers and begin to increase the electrical resistivity in spite of an increase in hole concentration. This increased disorder is likely an important factor limiting $ZT$.

Based on ab initio calculations and metadynamics simulations, we predict that the electronic gap of $2H$-MoS${}_2$, a layered insulator, will close under pressures in excess of 25 to 35 GPa. In the same pressure range, simulations and enthalpy optimization predict a structural transition. Free mutual sliding of layers takes place at this transition, the original $2H_c$ stacking changing to a $2H_a$ stacking typical of $2H$-NbSe${}_2$, an event explaining for the first time previously mysterious x-ray diffraction and Raman data. Phonon and electron phonon calculations suggest that pristine MoS${}_2$ will remain semimetallic up to very high pressures and is thus unlikely to develop superconductivity as it does upon metal intercalation.

We report first-principles calculations of the lattice dynamics of KNi${}_2$Se${}_2$ together with Raman scattering study. We have observed three out of four Raman-active modes predicted by factor group analysis. Calculated phonon frequencies are in good agreement with experimental findings. Contrary to its iron counterpart (K${}_x$Fe${}_{2-y}$Se${}_2$), K${}_{0.95}$Ni${}_{1.86}$Se${}_2$ does not show vacancy ordering.

Carbon-13 nuclear magnetic resonance (${}^{13}$C NMR) measurements have been carried out in the quasi-one-dimensional organic conductor (TMTTF)${}_2$SbF${}_6$, which exhibits charge ordering (CO) transition and antiferromagnetic (AF) transition at 157 K and 6 K at ambient pressure, respectively. We estimated the degree of charge disproportionation, $\Delta \rho ={\rho }_{\mathrm{rich}}-{\rho }_{\mathrm{poor}}$, in the CO state to be 0.25$e$ from the analysis of the NMR shift and found the broadening of the NMR spectral peaks caused by the inhomogeneous distribution in association with the AF fluctuation at charge rich site below 20 K in the CO state. Moreover, we determined the magnetic structure with two kinds of staggered moments in the AF phase of (TMTTF)${}_2$SbF${}_6$, which suggests that the CO state remains even in the AF phase. Consequently, we found the difference between two AF phases appearing on both sides of the spin-Peierls phase in the universal $P-T$ phase diagram.

Lattice vibrations as well as electronic excitations are used to investigate LuFe${}_2$O${}_4$, which is intensively discussed in the context of electronic multiferroics. All phonon modes and an electronic continuum show drastic changes through a series of magnetic, structural, and charge ordering temperatures. In particular, the 634 cm${}^{-1}$ $A_g$ mode, corresponding to the relative out-of-plane motion of a combined LuO and Fe${}_2$O${}_3$ layer, is very susceptible to the charge ordering, indicative of antipolar ordering of the Fe${}_2$O${}_3$ layer. With decreasing temperature below $T_{\text{CO}}=320$ K, the electronic continuum is gradually suppressed while opening a charge gap of 790 cm${}^{-1}$. Its temperature dependence suggests that charge ordering is not totally frozen-in even at 5 K, signaling the importance of local charge fluctuations due to concerted charge and spin frustration.

We study the effects of quantum fluctuations on the magnetic properties of quantum frustrated ferromagnets in a magnetic field. It is shown that a nonclassical phase or a phase separation appears due to quantum fluctuations below the saturation field in a parameter range close to the classical zero-field phase boundary between ferromagnetic and antiferromagnetic phases, for the case that the classical antiferromagnetic state is not an eigenstate of the quantum model. As an example to which this argument is applicable, we study the $S=1/2$ $J_1$-$J_2$ Heisenberg model with ferromagnetic $J_1$ ($J_1<0$) on the bcc lattice using a dilute Bose gas approach. For $-1.50097\le J_1/J_2\le -1.389$, magnons form $f$-wave two-magnon bound states, leading to a spin nematic phase, and for $-1.389\le J_1/J_2\le -0.48$, a canted coplanar antiferromagnetic phase appears accompanied with a phase separation below the saturation field.

We analyze the evolution of the superconducting gap structure in strongly hole-doped Ba${}_{1-x}$K${}_x$Fe${}_2$As${}_2$ between $x=1$ and $x\sim 0.4$ (optimal doping). In the latter case, the pairing state is most likely $s\pm$, with different gap signs on hole and electron pockets, but with the same signs of the gap on the two $\Gamma$-centered hole pockets (a $++$ state on hole pockets). In a pure KFe${}_2$As${}_2$ ($x=1$), which has only hole pockets, laser ARPES data suggested another $s\pm$ state, in which the gap changes sign between hole pockets (a $+-$ state). We analyze how a $++$ gap transforms into a $+-$ gap as $x\to 1$. We found that this transformation occurs via an intermediate $s+is$ state in which the gaps on the two hole pockets differ in phase by $\varphi$, which gradually involves from $\varphi =\pi$ (the $+-$ state) to $\varphi =0$ (the $++$ state). This state breaks time-reversal symmetry and has huge potential for applications. We compute the dispersion of collective excitations and show that two different Leggett-type phase modes soften at the two end points of the time-reversal-symmetry-breaking state.

We study the normal-state and superconducting properties of the NaFe${}_{1-x}$Co${}_x$As system by specific heat measurements. Both the normal-state Sommerfeld coefficient and superconducting condensation energy are strongly suppressed in the underdoped and heavily overdoped samples. The low-temperature electronic specific heat can be well fitted by either a one-gap or a two-gap BCS-type function for all the superconducting samples. The ratio ${\gamma }_N{T}_c^2/H_c^2\left(0\right)$ can nicely associate the neutron spin resonance as the bosons in the standard Eliashberg model. However, the value of $\Delta C/T_c{\gamma }_N$ near optimal doping is larger than the maximum value the model can obtain. Our results suggest that the high-$T_c$ superconductivity in the Fe-based superconductors may be understood within the framework of boson-exchange mechanism but significant modification may be needed to account for the finite-temperature properties.

We present high-resolution triple-axis neutron scattering studies of the high-temperature superconductor La${}_{1.88}$Sr${}_{0.12}$CuO${}_4$ ($T_c=27$ K). The temperature dependence of the low-energy incommensurate magnetic fluctuations reveals distinctly glassy features. The glassiness is confirmed by the difference between the ordering temperature $T_N\simeq T_c$ inferred from elastic neutron scattering and the freezing temperature $T_f\simeq 11$ K obtained from muon spin rotation studies. The magnetic field independence of the observed excitation spectrum as well as the observation of a partial suppression of magnetic spectral weight below $0.75$ meV for temperatures smaller than $T_f$, indicate that the stripe frozen state is capable of supporting a spin anisotropy gap, of a magnitude similar to that observed in the spin and charge stripe-ordered ground state of La${}_{1.875}$Ba${}_{0.125}$CuO${}_4$. The difference between $T_N$ and $T_f$ implies that the significant enhancement in a magnetic field of nominally elastic incommensurate scattering is caused by strictly inelastic scattering—at least in the temperature range between $T_f$ and $T_c$—which is not resolved in the present experiment. Combining the results obtained from our study of La${}_{1.88}$Sr${}_{0.12}$CuO${}_4$ with a critical reappraisal of published neutron scattering work on samples with chemical composition close to $p=0.12$, where local probes indicate a sharp maximum in $T_f\left(p\right)$, we arrive at the view that the low-energy fluctuations are strongly dependent on composition in this regime, with anisotropy gaps dominating only sufficiently close to $p=0.12$ and superconducting spin gaps dominating elsewhere.

We demonstrate how to parallelize the density matrix renormalization group (DMRG) algorithm in real space through a straightforward modification of serial DMRG. This makes it possible to apply at least an order of magnitude more computational power to challenging simulations, greatly accelerating investigations of two-dimensional systems and large parameter spaces. We discuss details of the algorithm and present benchmark results including a study of valence-bond-solid order within the square-lattice $Q_2$ model and Néel order within the triangular lattice Heisenberg model. The parallel DMRG algorithm also motivates an alternative canonical form for matrix product states.

We investigate characteristic features in the spin response of doped three-dimensional topological insulators with odd-parity unequal-spin superconducting pairing, which are predicted to have gapless Majorana surface modes. These Majorana modes contribute to the local spin susceptibility, giving rise to a characteristic temperature behavior of the Knight shift and the spin-lattice relaxation time in magnetic resonance experiments. Because of their different decay lengths, the Majorana modes can be observed and clearly distinguished from the Dirac modes of the topological insulator by local probes, which allow for a depth-controlled study of the electron spins on the nanometer length scale.

Using density functional theory, we determine parameters of tight-binding Hamiltonians for a variety of Fabre charge transfer salts, focusing, in particular, on the effects of temperature and pressure. Besides relying on previously published crystal structures, we experimentally determine two new sets of structures: (TMTTF)${}_2$SbF${}_6$ at different temperatures and (TMTTF)${}_2$PF${}_6$ under various hydrostatic pressures. We find that a few trends in the electronic behavior can be connected to the complex phase diagram shown by these materials. Decreasing temperature and increasing pressure cause the systems to become more two dimensional. We analyze the importance of correlations by considering an extended Hubbard model parameterized using Wannier orbital overlaps and show that while charge order is strongly activated by the intersite Coulomb interaction, the magnetic order is only weakly enhanced. Both orders are suppressed when the effective pressure is increased.

We investigate second-harmonic generation, low-threshold multistability, all-optical switching, and inherently nonlocal effects due to the free-electron gas pressure in an epsilon-near-zero (ENZ) metamaterial slab made of cylindrical, plasmonic nanoshells illuminated by TM-polarized light. Damping compensation in the ENZ frequency region, achieved by using gain medium inside the nanoshells’ dielectric cores, enhances the nonlinear properties. Reflection is inhibited, and the electric field component normal to the slab interface is enhanced near the effective pseudo-Brewster angle, where the effective $\varepsilon \approx 0$ condition triggers a nonresonant, impedance-matching phenomenon. We show that the slab displays a strong effective, spatial nonlocality associated with leaky modes that are mediated by the compensation of damping. The presence of these leaky modes then induces further spectral and angular conditions, where the local fields are enhanced, thus opening new windows of opportunity for the enhancement of nonlinear optical processes.

The high-pressure behavior of magnetite has been widely debated in the literature. Experimental measurements have found conflicting high-pressure transitions: a charge reordering in magnetite from inverse-spinel to normal-spinel [Pasternak et al., J. Phys. Chem. Solids 65, 1531 (2004); Rozenberg et al., Phys. Rev. B 75, 020102 (2007)], iron high-spin to intermediate-spin transition in magnetite [Ding et al., Phys. Rev. Lett. 100, 045508 (2008)], electron delocalization in magnetite [Baudelet et al., Phys. Rev. B 82, 140412 (2010); Glazyrin et al., Am. Mineral. 97, 128 (2012)], and a structural phase transition from magnetite to $h$-Fe${}_3$O${}_4$ [Dubrovinsky et al., J. Phys.: Condens. Matter 15, 7697 (2003); Fei et al., Am. Mineral. 84, 203 (1999); Haavik et al., Am. Mineral. 85, 514 (2000)]. We present ab initio calculations of iron's spin state in magnetite and $h$-Fe${}_3$O${}_4$, which help resolve the high-pressure debate. The results of the calculations find that iron remains high spin in both magnetite and $h$-Fe${}_3$O${}_4$; intermediate-spin iron is not stable. In addition, magnetite remains inverse-spinel but undergoes a phase transition to $h$-Fe${}_3$O${}_4$ near 10 GPa. Magnetite has a complex magnetic ordering, multiple valence states (Fe${}^{2+}$ and Fe${}^{3+}$), charge ordering, and different local Fe site environments, all of which were accounted for in the calculations. The lack of intermediate-spin iron in magnetite helps resolve the spin state of iron in perovskite, the major mineral in the lower mantle. In both magnetite and perovskite, x-ray emission spectroscopy (XES) measurements in the literature show a drop in satellite peak intensity by approximately half, which is interpreted as intermediate-spin iron. In both minerals, calculations give no indication of intermediate-spin iron and predict high-spin iron to be stable for defect-free crystals. The results question the interpretation of a nonzero drop in XES satellite peak intensities as intermediate-spin iron.

We show that the work function (Φ) of ZnO can be increased by up to 2.8 eV by depositing the molecular electron acceptor 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). On metals, already much smaller Φ increases involve significant charge transfer to F4TCNQ. No indication of negatively charged F4TCNQ on ZnO is found by photoemission spectroscopy. This fundamental difference is explained by a simple electrostatic model that identifies the bulk doping and band bending in ZnO as key parameters. Varying Φ of the inorganic semiconductor enables tuning the energy-level alignment at ZnO/organic semiconductor interfaces.

We study the effects of Landau level mixing in the limit of weak electron interaction. We use a numerical method to obtain the two- and three-body corrections to quantum Hall pseudopotentials, which are exact to lowest order in the Landau level mixing parameter. Our results are in general agreement with certain analytic results (some derived here; some derived by other authors) in the thermodynamic limit. We find that the convergence to this thermodynamic limit can be slow. This suggests that errors could occur if one tries to use pseudopotentials derived in a thermodynamic limit for numerical work on finite systems.

It is commonly assumed that after an electron-hole pair is created in a semiconductor by absorption of a photon the electron and hole rapidly relax to their respective lowest-energy states before recombining with one another. In semiconductor heterostructure nanocrystals, however, intraband relaxation can be inhibited to the point where recombination occurs primarily from an excited state. We demonstrate this effect using time-resolved optical measurements of CdSe/CdS core/shell nanorods. For nanorods with large CdSe cores, an electron photoexcited into the lowest-energy state in the core remains in the core, and an electron photoexcited into an excited state in the CdS shell remains in the shell, until the electron recombines with the hole. This provides a means of controlling the spatial location of photoexcited electrons by excitation energy. The control over electron localization is explained in terms of slow relaxation into the lowest-energy electron state in the nanorods, on time scales slower than electron-hole recombination. The observation of inhibited relaxation suggests that a simple picture of band alignment is insufficient for understanding charge separation in semiconductor heterostructures.