We present a detailed study of the phase diagram of copper-intercalated ${\mathrm{TiSe}}_2$ single crystals, combining local Hall-probe magnetometry, tunnel diode oscillator technique (TDO), and specific-heat and angle-resolved photoemission spectroscopy measurements. A series of the ${\mathrm{Cu}}_x{\mathrm{TiSe}}_2$ samples from three different sources with various copper content $x$ and superconducting critical temperatures $T_c$ have been investigated. We first show that the vortex penetration mechanism is dominated by geometrical barriers enabling a precise determination of the lower critical field, $H_{c1}$. We then show that the temperature dependence of the superfluid density deduced from magnetic measurements (both $H_{c1}$ and TDO techniques) clearly suggests the existence of a small energy gap in the system, with a coupling strength $2{\mathrm{\Delta }}_s\sim \left[2.4\text{–}2.8\right]k_B{T}_c$, regardless of the copper content, in puzzling contradiction with specific-heat measurements which can be well described by one single large gap $2{\mathrm{\Delta }}_l\sim \left[3.7\text{–}3.9\right]k_B{T}_c$. Finally, our measurements reveal a nontrivial doping dependence of the condensation energy, which remains to be understood.

Using a field-effect transistor (FET) configuration with solid Li-ion conductor (SIC) as gate dielectric, we have successfully tuned carrier density in ${\mathrm{FeSe}}_{0.5}{\mathrm{Te}}_{0.5}$ thin flakes, and the electronic phase diagram has been mapped out. It is found that electron doping controlled by SIC-FET leads to a suppression of the superconducting phase, and eventually gives rise to an insulating state in ${\mathrm{FeSe}}_{0.5}{\mathrm{Te}}_{0.5}$. During the gating process, the (001) peak in x-ray diffraction patterns stays at the same position and no new diffraction peak emerges, indicating no evident ${\mathrm{Li}}^{+}$ ions intercalation into the ${\mathrm{FeSe}}_{0.5}{\mathrm{Te}}_{0.5}$. It indicates that a systematic change of electronic properties in ${\mathrm{FeSe}}_{0.5}{\mathrm{Te}}_{0.5}$ arises from the electrostatic doping induced by the accumulation of ${\mathrm{Li}}^{+}$ ions at the interface between ${\mathrm{FeSe}}_{0.5}{\mathrm{Te}}_{0.5}$ and solid ion conductor in the devices. It is striking that these findings are drastically different from the observation in FeSe thin flakes using the same SIC-FET, in which $T_c$ is enhanced from 8 K to larger than 40 K, then the system goes into an insulating phase accompanied by structural transitions.

Tuning the polarization of a periodic array of magnetic stripes on top of a superconducting film allows control of Abrikosov vortex motion. Using direct magneto-optical imaging of the vortex patterns, we demonstrate that the proximity of the magnetic stripe ends to the edges of the superconducting film can strongly alter the vortex dynamics. We observe qualitatively different vortex behavior when the stripes overlap with the film edges. From the resulting unique magnetic flux patterns, we calculate the magnetic pinning strength of our stripe array and study effects of the modified edge barrier on vortex guidance and gating that result from different polarizations of the stripes.

We theoretically study transport properties of voltage-biased one-dimensional superconductor–normal metal–superconductor tunnel junctions with arbitrary junction transparency where the superconductors can have trivial or nontrivial topology. Motivated by recent experimental efforts on Majorana properties of superconductor-semiconductor hybrid systems, we consider two explicit models for topological superconductors: (i) spinful $p$-wave, and (ii) spin-split spin-orbit-coupled $s$-wave. We provide a comprehensive analysis of the zero-temperature dc current $I$ and differential conductance $dI/dV$ of voltage-biased junctions with or without Majorana zero modes (MZMs). The presence of an MZM necessarily gives rise to two tunneling conductance peaks at voltages $eV=\pm {\mathrm{\Delta }}_{\mathrm{lead}}$, i.e., the voltage at which the superconducting gap edge of the lead aligns with the MZM. We find that the MZM conductance peak probed by a superconducting lead without a BCS singularity has a nonuniversal value, which decreases with decreasing junction transparency. This is in contrast to the MZM tunneling conductance measured by a superconducting lead with a BCS singularity, where the conductance peak in the tunneling limit takes the quantized value $G_M=(4-\pi )2e^2/h$ independent of the junction transparency. We also discuss the “subharmonic gap structure”, a consequence of multiple Andreev reflections, in the presence and absence of MZMs. Finally, we show that for finite-energy Andreev bound states (ABSs), the conductance peaks shift away from the gap bias voltage $eV=\pm {\mathrm{\Delta }}_{\mathrm{lead}}$ to a larger value set by the ABSs energy. Our work should have important implications for the extensive current experimental efforts toward creating topological superconductivity and MZMs in semiconductor nanowires proximity coupled to ordinary $s$-wave superconductors.

Infinite projected entangled pair states simulations of the $S=1$ bilinear-biquadratic Heisenberg model on the square lattice reveal an emergent Haldane phase in between the previously predicted antiferromagnetic and three-sublattice ${120}^{\circ }$ magnetically ordered phases. This intermediate phase preserves SU(2) spin and translational symmetry but breaks lattice rotational symmetry, and it can be adiabatically connected to the Haldane phase of decoupled $S=1$ chains. Our results contradict previous studies which found a direct transition between the two magnetically ordered states.

We use time-resolved magneto-optic Kerr effect and ultrathin Co/Pt transducer films to perform thermal-transport experiments with higher sensitivity and greater time resolution than typically available in studies of interfacial thermal transport by time-domain thermoreflectance. We measure the interface conductance between Pt and amorphous ${\mathrm{SiO}}_2$ using Pt/Co/Pt ferromagnetic transducer films with thicknesses between 4.2 and 8.2 nm and find an average value of $G_{\mathrm{Pt}}\approx 0.3\mathrm{GW}{\mathrm{m}}^{-2}{\mathrm{K}}^{-1}$. This result demonstrates that interfaces between metals and amorphous dielectrics can have a conductance corresponding to Kapitza lengths of the order of 4 nm, and are thus of relevance when engineering nanoscale devices. For thin ${\mathrm{SiO}}_2$ layers, our method also provides sensitivity to the interface conductance between ${\mathrm{SiO}}_2$ and Si and we find $G_{\mathrm{Si}}=0.6\mathrm{GW}{\mathrm{m}}^{-2}{\mathrm{K}}^{-1}$ as the lower limit.

We study the current-driven skyrmion expulsion from magnetic nanostrips using micromagnetic simulations and analytic calculations. We explore the threshold current density for the skyrmion expulsion, and show that this threshold is determined by the critical boundary force as well as the spin-torque parameters. We also find the dependence of the critical boundary force on the magnetic parameters; the critical boundary force decreases with increasing the exchange stiffness and perpendicular anisotropy constants, while it increases with increasing Dzyaloshinskii-Moriya interaction and saturation magnetization constants. Using a simple model describing the skyrmion and locally-tilted edge magnetization, we reveal the underlying physics of the dependence of the critical boundary force on the magnetic parameters based on the relation between the scaled Dzyaloshinskii-Moriya-interaction parameter and the critical boundary force. This work provides a fundamental understanding of the skyrmion expulsion and the interaction between the skymion and boundaries of devices and shows that the stability of the skyrmion in devices can be related to the scaled Dzyaloshinskii-Moriya-interaction parameter of magnetic materials.

We have performed magnetic susceptibility, heat capacity, muon spin relaxation, and neutron-scattering measurements on three members of the family ${\mathrm{Ba}}_{3}M{\mathrm{Ru}}_2{\mathrm{O}}_9$, where $M=\mathrm{In}$, Y, and Lu. These systems consist of mixed-valence Ru dimers on a triangular lattice with antiferromagnetic interdimer exchange. Although previous work has argued that charge order within the dimers or intradimer double exchange plays an important role in determining the magnetic properties, our results suggest that the dimers are better described as molecular units due to significant orbital hybridization, resulting in one spin-1/2 moment distributed equally over the two Ru sites. These molecular building blocks form a frustrated, quasi-two-dimensional triangular lattice. Our zero- and longitudinal-field $\mu \mathrm{SR}$ results indicate that the molecular moments develop a collective, static magnetic ground state, with oscillations of the zero-field muon spin polarization indicative of long-range magnetic order in the Lu sample. The static magnetism is much more disordered in the Y and In samples, but they do not appear to be conventional spin glasses.

Thermodynamic properties of the proton-mediated single component dimer-Mott insulator of $\kappa \text{−}{\mathrm{H}}_3{\left(\mathrm{Cat}\text{−}\mathrm{EDT}\text{−}\mathrm{TTF}\right)}_2$, which has a two-dimensional triangle lattice structure of $S=1/2$ spins, are reported. The extraordinary large electronic heat capacity coefficient $\gamma =58.8\mathrm{mJ}{\mathrm{K}}^{-2}\mathrm{mo}{\mathrm{l}}^{-1}$ observed by the low-temperature heat capacity measurements up to 6 T suggests the formation of the gapless spin liquid ground state. Although the magnetic interaction $J/k_{\mathrm{B}}$ is quite different from those of other dimer-Mott spin liquids, the thermodynamic feature scales well with the Wilson ratio of 1.4–1.6. The heat capacity measurements also detected that the deuteration of the proton-linkage changes the ground state to the nonmagnetic one with almost vanishing γ. Using the data of the deuterated compound, the accurate temperature dependence of the magnetic heat capacity reflecting on the low-energy excitations from the gapless spin liquids ground state is discussed.

Specific heat and magnetization measurements of the compound $\left[\mathrm{C}{\mathrm{u}}_2{\left(\mathrm{apyhist}\right)}_2\mathrm{C}{\mathrm{l}}_2\right]{\left(\mathrm{Cl}{\mathrm{O}}_4\right)}_2$, where apyhist = (4-imidazolyl)ethylene-2-amino-1-ethylpyridine), were used to identify a magnetic-field-induced long-range antiferromagnetic ordered phase at low temperatures $(T<0.36\mathrm{K})$ and magnetic fields $(1.6<H<5.3\mathrm{T})$. This system consists of a Schiff base copper(II) complex, containing chloro-bridges between adjacent copper ions in a dinuclear arrangement, with an antiferromagnetic intradimer interaction $|J_{\mathrm{intra}}|/k_B\approx 3.65\mathrm{K}$ linked by an antiferromagnetic coupling $|J_{\mathrm{inter}}|z/k_B\approx 2.7\mathrm{K}$. The magnetic-field-induced ordering behavior was analyzed using the mean-field approximation and Monte Carlo simulation results. The obtained physical properties of the system are consistent with the description of the ordered phase as a Bose-Einstein condensation (BEC) of magnetic excitations. We present the phase diagram of this compound, which shows one of the lowest critical magnetic field among all known members of the family of BEC quantum magnets.

Measurements of the current-voltage characteristics ($I-V$) were performed on ${\mathrm{Ba}(\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{)}_2{\mathrm{As}}_2$ single crystals with doping level $0.044\le x\le 0.100$. An unconventional increase in the flux-flow resistivity ${\rho }_{\mathrm{ff}}$ with decreasing magnetic field $H$ was observed across this doping range. Such an abnormal field dependence of ${\rho }_{\mathrm{ff}}$ is in contrast with the linear ${\rho }_{\mathrm{ff}}\left(H\right)$ of conventional type-II superconductors, but similar to the behavior recently observed in the heavy-fermion superconductor ${\mathrm{CeCoIn}}_5$. A significantly enhanced ${\rho }_{\mathrm{ff}}$ was found for the $x=0.06$ single crystals, implying a strong single-particle energy dissipation around the vortex cores. At different temperatures and fields and for a given doping concentration, the normalized ${\rho }_{\mathrm{ff}}$ scales with normalized field and temperature. The doping level dependence of the scaling parameters strongly suggests that the abnormal upturn in ${\rho }_{\mathrm{ff}}$ is likely related to the enhancement of spin fluctuations around the vortex cores of the samples with $x\approx 0.06$.

We have successfully synthesized the single crystal of $\mathrm{A}{\mathrm{g}}_2\mathrm{P}{\mathrm{d}}_3\mathrm{S}$, which exhibits superconductivity with the transition temperature of $T_{\mathrm{c}}=2.25\mathrm{K}$. $\mathrm{A}{\mathrm{g}}_2\mathrm{P}{\mathrm{d}}_3\mathrm{S}$ crystallizes in the space group $P{4}_{1}32$ with the filled $\beta -\mathrm{Mn}$ structure, which has no inversion symmetry. The value of the Ginzburg-Landau parameter ${\kappa }_{\mathrm{GL}}$ indicates that $\mathrm{A}{\mathrm{g}}_2\mathrm{P}{\mathrm{d}}_3\mathrm{S}$ is a type-II superconductor. $\mathrm{\Delta }C\left(T_{\mathrm{c}}\right)/{\gamma }_{\mathrm{n}}{T}_{\mathrm{c}}=1.50$ and $2\mathrm{\Delta }/k_B{T}_{\mathrm{c}}=3.48$ from the heat-capacity analyses indicate that $\mathrm{A}{\mathrm{g}}_2\mathrm{P}{\mathrm{d}}_3\mathrm{S}$ is a weak-coupling Bardeen-Cooper-Schrieffer (BCS) superconductor with an isotropic superconducting gap. On the other hand, the violation of the Werthamer-Helfand-Hohenberg curve in the $H-T$ phase diagram implies $\mathrm{A}{\mathrm{g}}_2\mathrm{P}{\mathrm{d}}_3\mathrm{S}$ is not a typical BCS superconductor.

Electron-doped $\mathrm{Eu}{\left({\mathrm{Fe}}_{0.93}{\mathrm{Rh}}_{0.07}\right)}_2{\mathrm{As}}_2$ has been systematically studied by high-pressure investigations of the magnetic and electrical transport properties in order to unravel the complex interplay of superconductivity and magnetism. The compound reveals an exceedingly broad reentrant transition to the superconducting state between $T_{\mathrm{c},\mathrm{on}}=19.8$ K and $T_{\mathrm{c},0}=5.2$ K due to a canted A-type antiferromagnetic ordering of the ${\mathrm{Eu}}^{2+}$ moments at $T_{\mathrm{N}}=16.6$ K and a reentrant spin glass transition at $T_{\mathrm{SG}}=14.1$ K. At ambient pressure evidences for the coexistence of superconductivity and ferromagnetism could be observed, as well as a magnetic-field-induced enhancement of the zero-resistance temperature $T_{\mathrm{c},0}$ up to 7.2 K with small magnetic fields applied parallel to the $ab$ plane of the crystal. We attribute the field-induced-enhancement of superconductivity to the suppression of the ferromagnetic component of the ${\mathrm{Eu}}^{2+}$ moments along the $c$ axis, which leads to a reduction of the orbital-pair-breaking effect. Application of hydrostatic pressure suppresses the superconducting state around 14 kbar along with a linear temperature dependence of the resistivity, implying that a non-Fermi liquid region is located at the boundary of the superconducting phase. At intermediate pressure, an additional feature in the resistivity curves is identified, which can be suppressed by external magnetic fields and competes with the superconducting phase. We suggest that the effect of negative pressure by the chemical Rh substitution in $\mathrm{Eu}{\left({\mathrm{Fe}}_{0.93}{\mathrm{Rh}}_{0.07}\right)}_2{\mathrm{As}}_2$ is partially reversed, leading to a reactivation of the spin density wave.

Based on the density matrix renormalization group (DMRG), strongly correlated quantum many-body systems at finite temperatures can be simulated by sampling over a certain class of pure matrix product states (MPS) called minimally entangled typical thermal states (METTS). When a system features symmetries, these can be utilized to substantially reduce MPS computation costs. It is conceptually straightforward to simulate canonical ensembles using symmetric METTS. In practice, it is important to alternate between different symmetric collapse bases to decrease autocorrelations in the Markov chain of METTS. To this purpose, we introduce symmetric Fourier and Haar-random block bases that are efficiently mixing. We also show how grand-canonical ensembles can be simulated efficiently with symmetric METTS. We demonstrate these approaches for spin-$1/2$ $XXZ$ chains and discuss how the choice of the collapse bases influences autocorrelations as well as the distribution of measurement values and, hence, convergence speeds.

I study the experimentally observed structural phase transition in ${\mathrm{NaTiO}}_2$ using density functional calculations. I do not find the previously proposed low-temperature structure energetically favorable with respect to the high-temperature rhombohedral structure. The calculated phonon dispersions of the rhombohedral phase show dynamical instabilities at several inequivalent parts of the Brillouin zone, including at the wave vector $(\frac{1}{2},\frac{1}{5},\frac{1}{5})$. These instabilities lead to monoclinic structures without inversion symmetry that are modulated along all three directions. The calculated electronic structures show that a local bonding instability of the Ti $3d$ states is associated with the structural transition.

In this work we have investigated the exciton reflectance spectra of a high quality heterostructure with a GaAs/AlGaAs quantum well in a transverse magnetic field (Voigt geometry). It has been shown that application of the magnetic field leads to a decrease of energy distance between spectral features related to the excitonlike polariton modes. This effect has been treated as the magneto-induced increase of the exciton mass. We have shown that the hydrogenlike and diamagnetic exciton models are insufficient to describe the exciton behavior in the intermediate magnetic fields studied. Considering the symmetry of the problem, we have developed a phenomenological model which adequately describes the experimental data.

Silicon (Si) nanocrystals (NCs) are of great interest for many applications, ranging from photovoltaics to optoelectonics. The photoluminescence quantum yield of Si NCs dispersed in ${\mathrm{SiO}}_2$ is limited, suggesting the existence of very efficient processes of nonradiative recombination, among which the formation of a self-trapped exciton state on the surface of the NC. In order to improve the external quantum efficiency of these systems, the carrier relaxation and recombination need to be understood more thoroughly. For that purpose, we perform transient-induced absorption spectroscopy on Si NCs embedded in a ${\mathrm{SiO}}_2$ matrix over a broad probe range for NCs of average sizes from 2.5 to 5.5 nm. The self-trapping of free excitons on surface-related states is experimentally and theoretically discussed and found to be dependent on the NC size. These results offer more insight into the self-trapped exciton state and are important to increase the optical performance of Si NCs.

We present the results of a diagrammatic, fully ab initio calculation of the $G$ peak intensity of graphene. The flexibility and generality of our approach enables us to go beyond the previous analytical calculations in the low-energy regime. We study the laser and Fermi energy dependence of the $G$ peak intensity and analyze the contributions from resonant and nonresonant electronic transitions. In particular, we explicitly demonstrate the importance of quantum interference and nonresonant states for the $G$ peak process. Our method of analysis and computational concept is completely general and can easily be applied to study other materials as well.

When a semiconductor absorbs light, the resulting electron-hole superposition amounts to a uncontrolled quantum ripple that eventually degenerates into diffusion. If the conformation of these excitonic superpositions could be engineered, though, they would constitute a new means of transporting information and energy. We show that properly designed laser pulses can be used to create such excitonic wave packets. They can be formed with a prescribed speed, direction, and spectral make-up that allows them to be selectively passed, rejected, or even dissociated using superlattices. Their coherence also provides a handle for manipulation using active, external controls. Energy and information can be conveniently processed and subsequently removed at a distant site by reversing the original procedure to produce a stimulated emission. The ability to create, manage, and remove structured excitons comprises the foundation for optoexcitonic circuits with application to a wide range of quantum information, energy, and light-flow technologies. The paradigm is demonstrated using both tight-binding and time-domain density functional theory simulations.