We report a domelike superconductivity and quantum criticality in the 12442-type hybrid-structure superconductor $\mathrm{KC}{\mathrm{a}}_2\mathrm{F}{\mathrm{e}}_4\mathrm{A}{\mathrm{s}}_4{\mathrm{F}}_2$ under hydrostatic pressures up to 15 GPa. It is found that the superconducting transition temperature ($T_c$) increases from $\sim 33.5\mathrm{K}$ at ambient pressure to a maximum of $\sim 36.5\mathrm{K}$ at 2 GPa in the background of enhanced resistivity of the $\left(\mathrm{K},\mathrm{Rb},\mathrm{Cs}\right)\mathrm{C}{\mathrm{a}}_2\mathrm{F}{\mathrm{e}}_4\mathrm{A}{\mathrm{s}}_4{\mathrm{F}}_2$ at ambient pressure (AP), and then shows a slope change near 8 GPa with anomalous characteristics of resistivity. $T_c$ is found to be inversely proportional to the characteristic temperature of the coherence-incoherence crossover. The plots of resistivity at various temperatures and pressures and high-pressure x-ray diffraction confirm no pressure-induced half collapse of the tetragonal structure below 15 GPa. The pressure-induced crossover from non-Fermi liquid (the temperature exponent $n\sim 1.05\pm 0.1$ at AP) to Fermi liquid ($n\sim 1.92\pm 0.1$ at 15 GPa) is accompanied by the reduction of the linear temperature coefficient of normal-state resistivity, nearly two orders of magnitude compared to AP. These characteristics are ascribed to the weakening electronic correlation and/or critical fluctuations by pressure.

In the layered iron-based superconductor ${\mathrm{YFe}}_2{\mathrm{Ge}}_2$, a high Sommerfeld ratio of $\sim 100\mathrm{mJ}/\mathrm{mol}{\mathrm{K}}^2$ and a $T^{3/2}$ temperature dependence of the electrical resistivity at low temperature $T$ indicate strong electronic correlations and point toward an unconventional pairing state. We have investigated the role of composition and annealing conditions in optimizing the growth of high-quality ${\mathrm{YFe}}_2{\mathrm{Ge}}_2$. Our findings confirm that bulk superconductivity is observed in samples with disorder scattering rates less than $2k_B{T}_c/\hslash$. Fe deficiency on the Fe site is identified as the dominant source of disorder, which can be minimized by precipitating from a slightly iron-rich melt, followed by annealing.

We have investigated the influence of electron beam generated defects on the structure of periodic lattice distortions (PLDs) which accompany charge density wave modulations in $1T\text{−}\mathrm{Ta}{\mathrm{S}}_2$ and $1T\text{−}\mathrm{TaS}{\mathrm{e}}_2$. Lattice defects were generated through irradiation with high-energy electrons in a transmission electron microscope (TEM). Using atomically resolved high-resolution TEM imaging, we investigate the PLD structure and the changes in this structure with prolonged exposure to the electron beam. We observe the formation of dislocationlike topological defects in the PLD structure. Prolonged exposure to the electron beam also leads to an increase in density of these defects. This is also accompanied by an increase in structural disorder of the PLD. Density functional theory calculations were also performed in order to understand sulfur (S) and selenium (Se) vacancy defect formation in $1T\text{−}\mathrm{TaS}{\mathrm{e}}_2$ and $1T\text{−}\mathrm{Ta}{\mathrm{S}}_2$ and their effects on the PLD structure. The formation energy of Se/S vacancies was calculated to be lowest for the highly displaced S/Se atoms in the vicinity of PLD maxima. Vacancies formed at the less displaced sites near the PLD minima were found to have lower formation energy. The calculations also showed that an increase in the S/Se vacancies leads to the formation of dislocations and an increase in disorder in the PLD structures. This supports the experimental observations.

A spring-mass model arranged in a diamond structure—mechanical diamond—is analyzed in terms of topology in detail. We find that additional springs connecting the next-nearest-neighbor (NNN) pairs of mass points and the modulation of the mass parameters to the pristine mechanical diamond generate multiple pairs of Weyl points in the frequency dispersion. Evolution of the Weyl point positions in the Brillouin zone against the uniform outward tension is tracked and explained by the point group symmetry, especially tetrahedral symmetry of the NNN springs. Interestingly, a rapid transmutation of the monopole charges of the Weyl points occurs as the tension varies. We also show surface Fermi arcs in the case with anisotropy in the NNN springs.

Both structural glasses and disordered crystals are known to exhibit anomalous thermal, vibrational, and acoustic properties at low temperatures or low energies, what is still a matter of lively debate. To shed light on this issue, we studied the halomethane family ${\mathrm{CBr}}_n{\mathrm{Cl}}_{4-n}$ ($n=0,1,2$) at low temperature where, despite being perfectly translationally ordered stable monoclinic crystals, glassy dynamical features had been reported from experiments and molecular dynamics simulations. For $n=1,2$ dynamic disorder originates by the random occupancy of the same lattice sites by either Cl or Br atoms, but not for the ideal reference case of ${\mathrm{CCl}}_4$. Measurements of the low-temperature specific heat ($C_p$) for all these materials are here reported, which provide evidence of the presence of a broad peak in Debye-reduced $C_p\left(T\right)/T^3$ and in the reduced density of states ($g\left(\omega \right)/{\omega }^2$) determined by means of neutron spectroscopy, as well as a linear term in $C_p$ usually ascribed in glasses to two-level systems in addition to the cubic term expected for a fully ordered crystal. Being ${\mathrm{CCl}}_4$ a fully ordered crystal, we also performed density functional theory (DFT) calculations, which provide unprecedented detailed information about the microscopic nature of vibrations responsible for that broad peak, much alike the “'boson peak” of glasses, finding it to essentially arise from a piling up (at around $3–4$ meV) of low-energy optical modes together with acoustic modes near the Brillouin-zone limits.

The intermediate-valence compound CeNi experiences a pressure-induced first-order structural phase transition with an abrupt change in the unit cell volume, and as such constitutes an attractive system to study pressure-driven $f$-electron delocalization behavior in a system with an unstable $f$-electron shell. The inelastic neutron scattering study of the dynamic magnetic response of CeNi reveals that the structural transition increases the characteristic energy of magnetic fluctuations due to enhanced Ce $4f$–Ni $3d$ hybridization. At the same time, the inelastic $4f$ magnetic form factor remains unchanged.

Multiband superconductors have a sufficient number of degrees of freedom to allow topological excitations characterized by skyrmionic topological invariants. In the most common, clean $s$-wave multiband systems, the interband Josephson and magnetic couplings favor composite vortex solutions, without a skyrmionic topological charge. It was discussed recently that certain kinds of anisotropies lead to hybridization of the interband phase difference (Leggett) mode with magnetic modes, dramatically changing the hydromagnetostatics of the system. Here we report this effect for a range of parameters that substantially alter the nature of the topological excitations, leading to solutions characterized by a nontrivial skyrmionic topological charge. The solutions have a form of a coreless texture formed of spatially separated but bound excitations in each band, namely fractional vortices, each carrying a fraction of the flux quantum. We demonstrate that in this regime there is a rich spectrum of skyrmion solutions, with various topological charges, that are robust with respect to changes of parameters of the system and present for a wide range of anisotropies.

We show that a Fermi gas, in three dimensions, at temperatures above the superconducting phase transition $\left(T_c\right)$ but below the Fermi temperature $\left(T_F\right)$, cannot be described by Fermi liquid theory (FLT) in the unitary limit where the scattering length diverges. The result follows by showing that there are no effective field theory descriptions that both behave like a Fermi liquid and properly nonlinearly realize the spontaneously broken boost and conformal invariance of the system. We have also derived an exact result for the beta function for the coupling function in the unitary limit.

${\mathrm{CeMnNi}}_4$ exhibits an unusually large spin polarization, but its origin has baffled researchers for more than a decade. We use bulk sensitive hard x-ray photoelectron spectroscopy (HAXPES) and density functional theory based on the Green's function technique to demonstrate the importance of electron-electron correlations of both the Ni $3d$ ($U_{\text{Ni}}$) and Mn $3d$ ($U_{\text{Mn}}$) electrons in explaining the valence band of this multiply correlated material. We show that Mn-Ni antisite disorder as well as $U_{\text{Ni}}$ play a crucial role in enhancing its spin polarization: Antisite disorder broadens a Ni $3d$ minority-spin peak close to the Fermi level ($E_F$), while an increase in $U_{\text{Ni}}$ shifts it toward $E_F$, both leading to a significant increase of minority-spin states at $E_F$. Furthermore, the rare occurrence of a valence state transition between the bulk and the surface is demonstrated highlighting the importance of HAXPES in resolving the electronic structure of materials unhindered by surface effects.

We extend the formalism of matrix product states (MPS) to describe one-dimensional gapped systems of fermions with both unitary and antiunitary symmetries. Additionally, systems with orientation-reversing spatial symmetries are considered. The short-ranged entangled phases of such systems are classified by three invariants, which characterize the projective action of the symmetry on edge states. We give interpretations of these invariants as properties of states on the closed chain. The relationship between fermionic MPS systems at a renormalization group fixed point and equivariant algebras is exploited to derive a group law for the stacking of fermionic phases. The result generalizes known classifications to symmetry groups that are nontrivial extensions of fermion parity and time-reversal.

We present a detailed nuclear magnetic resonance (NMR) study of ${}^{239}\mathrm{Pu}$ in bulk and powdered single-crystal plutonium tetraboride (${\mathrm{PuB}}_4$), which has recently been investigated as a potential correlated topological insulator. This study constitutes the second-ever observation of the ${}^{239}\mathrm{Pu}$ NMR signal, and provides unique on-site sensitivity to the rich $f$-electron physics and insight into the bulk gaplike behavior in ${\mathrm{PuB}}_4$. The ${}^{239}\mathrm{Pu}$ NMR spectra are consistent with axial symmetry of the shift tensor showing for the first time that ${}^{239}\mathrm{Pu}$ NMR can be observed in an anisotropic environment and up to room temperature. The temperature dependence of the ${}^{239}\mathrm{Pu}$ shift, combined with a relatively long spin-lattice relaxation time ($T_1$), indicate that ${\mathrm{PuB}}_4$ adopts a nonmagnetic state with gaplike behavior consistent with our density functional theory calculations. The temperature dependencies of the NMR Knight shift and $T_1^{-1}$–microscopic quantities sensitive only to bulk states–imply bulk gaplike behavior confirming that ${\mathrm{PuB}}_4$ is a good candidate topological insulator. The large contrast between the ${}^{239}\mathrm{Pu}$ orbital shifts in the ionic insulator ${\mathrm{PuO}}_2$ ($\sim +24.7$%) and ${\mathrm{PuB}}_4$ ($\sim -0.5$%) provides a new tool to investigate the nature of chemical bonding in plutonium materials.

PbTe is a leading mid-range thermoelectric material with a $zT$ that has been enhanced by, amongst other methods, band engineering. Here we present an experimental study of the Hall effect, quantum oscillations, specific heat, and electron microprobe analysis that explores the evolution of the electronic structure of PbTe heavily doped with the “ideal” acceptor Na up to the solubility limit. We identify two phenomenological changes that onset as the electronic structure deviates from a Kane-type dispersion at around 180 meV; a qualitative change in the field dependence of the Hall effect indicative of an increase in the high-field limit and a change in the Fermiology, and a sharp increase in the density of states as a function of energy. Following consideration of three possible origins for the observed phenomenology we conclude that the most likely source is nonellipsoidicity of the $L$ pocket upon approach to a saddle point in the band structure, which is evidenced directly by our quantum oscillation measurements. Comparison to density functional theory calculations imply that this evolution of the electronic structure may be a key contributor to the large thermopower in PbTe.

We propose an experimental method to examine the in-plane anisotropy of electronic structure in layered conductors. In the method, we measure the interlayer magnetoresistance as a function of in-plane magnetic field orientation. We applied it to an organic Dirac fermion system $\alpha \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2{\mathrm{I}}_3$ to experimentally determine the orientation of the anisotropic Dirac cones. It is concluded that the long axis of the elliptic constant-energy contours of the Dirac cone is tilted by approximately −30° from the crystalline $a$ axis to the $b$ axis under hydrostatic pressures. Additionally, we observed a signature of Van Hove singularity (which is a saddle point of the band dispersion) at 30–40 K above or below the Dirac point. The ridgeline of the saddle point is estimated as almost parallel to the crystalline $b$ axis.

Multiple ${\mathrm{Mn}}^{2+}$ spin-flip Raman scattering (SFRS) in Voigt geometry was observed in self-organized disk-shaped quantum dots (QDs) of $\mathrm{CdSe}/{\mathrm{Zn}}_{0.99}{\mathrm{Mn}}_{0.01}\mathrm{Se}$, where magnetic ions and QD carriers are spatially separated and therefore the exchange interaction between them is expected to be weak. Many lines (about ten) were observed in SFRS spectra, yet the overlapping of the hole wave function with ${\mathrm{Mn}}^{2+}$ ions is very small, in agreement with both the absence of observable Zeeman splitting of the photoluminescence line and the calculation. Interesting polarization properties of SFRS spectra were observed which could be affected by tilting the sample out of normal alignment and changing the temperature. These polarization properties were attributed to the selection rules in SFRS in Voigt geometry. It has been found that the theoretical model suggested by Stühler et al. [J. Cryst. Growth 159, 1001 (1996)] does not describe the SFRS spectra in systems with weak exchange interaction between charge carriers and magnetic ions. A qualitative model is suggested here for description of SFRS in such systems.

Bulk inversion asymmetry (BIA) of III-V and II-VI semiconductor quantum wells is demonstrated by reflection experiments in magnetic field oriented in the structure plane. The linear in the magnetic field contribution to the reflection coefficients is measured at oblique incidence of $s$ and $p$ polarized light in the vicinity of exciton resonances. We demonstrate that this contribution to the reflection is caused by magnetogyrotropy of quantum wells, i.e., by the terms in the optical response which are linear in both the magnetic field strength and light wave vector. Theory of magnetogyrotropic effects in light reflection is developed accounting for linear in momentum BIA induced terms in the electron and hole effective Hamiltonians. Theoretical estimates agree with the experimental findings. We have found the electron BIA splitting constant in both GaAs and CdTe based quantum wells is about three times smaller than that for heavy holes.

Inhomogeneous strain effects on exciton dynamics have become particularly attractive with the emergence of two-dimensional flexible semiconductors. However, a solid understanding of the shear strain effect is lacking. Here, we study tunable exciton dynamics in two-dimensional semiconductors that have inhomogeneous shear strain. The state-of-the-art first-principles calculations based on the GW plus Bethe-Salpeter equation formalism show that in monolayer molybdenum disulfide, an isotropic semiconductor, the exciton excitation energy can be dramatically tuned as an even function of shear strain with a deformation potential of $-50$ meV/1%, despite the induced anisotropy of optical absorption with respect to the polarization direction of incident light. For an anisotropic semiconductor such as phosphorene, the modulation of exciton excitation energy is found to be shear-direction dependent. The finite-element simulations further reveal a shear-induced funnel effect of excitons in a micrometer scale ${\mathrm{MoS}}_2$ disk. The enhanced understanding of inhomogeneous strain engineering presented in this study has the potential to be instrumental in the design of novel optoelectronic devices.

Graphene/ferromagnet interface promises a plethora of new science and technology. The interfacial Dzyaloshinskii-Moriya interaction (iDMI) is essential for stabilizing chiral spin textures, which are important for future spintronic devices. Here, we report direct observation of iDMI in graphene/${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$/Ta heterostructures from nonreciprocity in spin-wave dispersion using Brillouin light-scattering technique. Linear scaling of iDMI with the inverse of ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ thicknesses suggests primarily interfacial origin of iDMI. Both iDMI and spin-mixing conductance increase with the increase in defect density of graphene obtained by varying argon pressure during sputter deposition of ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$. This suggests that the observed iDMI originates from defect-induced extrinsic spin-orbit coupling at the interface. The direct observation of iDMI at graphene/ferromagnet interface without perpendicular magnetic anisotropy opens a route in designing thin-film heterostructures based on two-dimensional materials for controlling chiral spin structure such as skyrmions and bubbles, and magnetic domain-wall-based storage and memory devices.

The spin orientation of electronic wave functions in crystals is an internal degree of freedom, typically insensitive to electrical knobs. We argue from a general symmetry analysis and a $\mathbf{k}·\mathbf{p}$ perspective, that monolayer $1T^{\prime }-{\mathrm{WTe}}_2$ possesses a gate-activated canted spin texture that produces an electrically tunable bulk band quantum geometry. In particular, we find that due to its out-of-plane asymmetry, an applied out-of-plane electric field breaks inversion symmetry to induce both in-plane and out-of-plane electric dipoles. These in-turn generate spin-orbit coupling to lift the spin degeneracy and enable a bulk band Berry curvature and magnetic moment distribution to develop. Further, due to its low symmetry, Berry curvature and magnetic moment in $1T^{\prime }-{\mathrm{WTe}}_2$ possess a dipolar distribution in momentum space, and can lead to unconventional effects such as a current induced magnetization and quantum nonlinear anomalous Hall effect. These render $1T^{\prime }-{\mathrm{WTe}}_2$ a rich two-dimensional platform for all-electrical control over quantum geometric effects.

Fluorescence blinking of single colloidal semiconductor quantum dots (QDs) has been extensively studied, and several sophisticated models have been proposed. In this work, we derive Heisenberg equations of motion to carefully study principal transition processes, i.e., photoexcitation, energy relaxation, impact ionization and Auger recombination, radiative and nonradiative recombinations, and tunneling between core states and surface states, of the electron-hole pair in single CdSe-CdS/ZnS core-multishell QDs and show that the on-state probability density distribution of the QD fluorescence obeys the random telegraph signal theory because of the random radiative recombination of the photoexcited electron-hole pair in the QD core, while the off-state probability density distribution obeys the inverse power law distribution due to the series of random walks of the photoexcited electron in the two-dimensional surface-state network after the electron tunnels from the QD core to the QD surface. These two different blinking characteristics of the single QD are resolved experimentally by properly adjusting the optical excitation power and the bin time.