We investigate a three-component fermion mixture in the presence of weak attractive interactions. We use a combination of the equation of motion and the Gaussian variational mean-field approaches, which both allow for simultaneous superfluid and magnetic ordering in an unbiased way, and capture the interplay between the two order parameters. This interplay significantly modifies the phase diagram, especially the superfluid-normal phase boundaries. In the close vicinity of the critical temperature and for small chemical potential imbalances, strong particle-hole symmetry breaking leads to a phase diagram similar to the one predicted by Cherng et al. [Phys. Rev. Lett. 99, 130406 (2007)], however, the overall phase diagram is markedly different: additional chemical potential-driven first- and second-order transitions and triple points emerge as well as more exotic second-order multicritical points, and bicritical lines with $\text{O}(2,2)$ symmetry. We identify the terms which are necessary to capture this complex phase diagram in a Ginzburg-Landau approach, and determine the corresponding coefficients.

We propose an improved scheme to construct many-body trial wave functions for fractional Chern insulators (FCI), using one-dimensional localized Wannier basis. The procedure borrows from the original scheme on a continuum cylinder, but is adapted to finite-size lattice systems with periodic boundaries. It fixes several issues of the continuum description that made the overlap with the exact ground states insignificant. The constructed lattice states are translationally invariant, and have the correct degeneracy as well as the correct relative and total momenta. Our prescription preserves the (possible) inversion symmetry of the lattice model, and is isotropic in the limit of flat Berry curvature. By relaxing the maximally localized hybrid Wannier orbital prescription, we can form an orthonormal basis of states which, upon gauge fixing, can be used in lieu of the Landau orbitals. We find that the exact ground states of several known FCI models at $\nu =1/3$ filling are well captured by the lattice states constructed from the Laughlin wave function. The overlap is higher than $0.99$ in some models when the Hilbert space dimension is as large as $3\times {10}^4$ in each total momentum sector.

Local symmetry breaking in complex materials is emerging as an important contributor to materials properties but is inherently difficult to study. Here we follow up an earlier structural observation of such a local symmetry broken phase in the technologically important compound PbTe with a study of the lattice dynamics using inelastic neutron scattering (INS). We show that the lattice dynamics are responsive to the local symmetry broken phase, giving key insights in the behavior of PbTe, but also revealing INS as a powerful tool for studying local structure. The new result is the observation of the unexpected appearance upon warming of a new zone center phonon branch in PbTe. In a harmonic solid the number of phonon branches is strictly determined by the contents and symmetry of the unit cell. The appearance of the new mode indicates a crossover to a dynamic lower symmetry structure with increasing temperature. No structural transition is seen crystallographically, but the appearance of the new mode in inelastic neutron scattering coincides with the observation of local Pb off-centering dipoles observed in the local structure. The observation resembles relaxor ferroelectricity, but since there are no inhomogeneous dopants in pure PbTe this anomalous behavior is an intrinsic response of the system. We call such an appearance of dipoles out of a nondipolar ground-state “emphanisis” meaning the appearance out of nothing. It cannot be explained within the framework of conventional phase transition theories such as soft-mode theory and challenges our basic understanding of the physics of materials.

We studied the defects of Bi${}_2$Se${}_3$ generated from Bridgman growth of stoichiometric and nonstoichiometric self-fluxes. Growth habit, lattice size, and transport properties are strongly affected by the types of defects generated. Major defect types of the Bi${}_{\text{Se}}$ antisite and partial Bi${}_2$-layer intercalation are identified through combined studies of direct atomic-scale imaging with scanning transmission electron microscopy in conjunction with energy-dispersive x-ray spectroscopy, x-ray diffraction, and Hall effect measurements. We propose a consistent explanation to the origin of defect type, growth morphology, and transport property.

We report on the study of spin-polarized electric currents in diluted magnetic semiconductor (DMS) quantum wells subjected to an in-plane external magnetic field and illuminated by microwave or terahertz radiation. The effect is studied in (Cd,Mn)Te/(Cd,Mg)Te quantum-wells (QWs) and (In,Ga)As/InAlAs:Mn QWs belonging to the well-known II-VI and III-V DMS material systems, as well as in heterovalent AlSb/InAs/(Zn,Mn)Te QWs, which represent a promising combination of II-VI and III-V semiconductors. Experimental data and developed theory demonstrate that the photocurrent originates from a spin-dependent scattering of free carriers by static defects or phonons in the Drude absorption of radiation and subsequent relaxation of carriers. We show that in DMS structures, the efficiency of the current generation is drastically enhanced compared to nonmagnetic semiconductors. The enhancement is caused by the exchange interaction of carrier spins with localized spins of magnetic ions resulting, on the one hand, in the giant Zeeman spin splitting, and, on the other hand, in the spin-dependent carrier scattering by localized Mn${}^{2+}$ ions polarized by an external magnetic field.

A series of measurements using a technique called electrostatic-manipulation scanning tunneling microscopy (EM-STM) were performed on a highly oriented pyrolytic graphite surface. The electrostatic interaction between the STM tip and the sample can be tuned to produce both reversible and irreversible large-scale movement of the graphite surface. Under this influence, atomic-resolution STM images reveal that a continuous electronic transition from triangular symmetry, where only alternate atoms are imaged, to hexagonal symmetry can be systematically controlled. Density functional theory (DFT) calculations reveal that this transition can be related to vertical displacements of the top layer of graphite relative to the bulk. Evidence for horizontal shifts in the top layer of graphite is also presented. Excellent agreement is found between experimental STM images and those simulated using DFT.

Based on specific heat and magnetoresistance measurements, we report that a “heavy” electronic state exists below $T$ ≈ 20 K in KNi${}_2$Se${}_2$, with an increased carrier mobility and enhanced effective electronic band mass, $m^{*}$ = 6$m_{\mathrm{b}}$ to 18$m_{\mathrm{b}}$. This heavy state evolves into superconductivity at $T_{\mathrm{c}}$ = 0.80(1) K. These properties resemble that of a many-body heavy-fermion state, which derives from the hybridization between localized magnetic states and conduction electrons. Yet, no evidence for localized magnetism or magnetic order is found in KNi${}_2$Se${}_2$ from magnetization measurements or neutron diffraction. Instead, neutron pair-distribution-function analysis reveals the presence of local charge-density-wave distortions that disappear on cooling, an effect opposite to what is typically observed, suggesting that the low-temperature electronic state of KNi${}_2$Se${}_2$ arises from cooperative Coulomb interactions and proximity to, but avoidance of, charge order.

We successfully obtained Sn-doped SrTiO${}_3$ (SSTO) perovskites, and clarified their ferroelectricity and structural properties by using first-principles theoretical calculations. The ferroelectricity of SSTO was confirmed by the appearance of a dielectric permittivity maximum and a clear hysteresis loop of the relationship between the external electric field and the electric flux density below 180 K. X-ray diffraction and Raman spectra revealed the structural phase transition of SSTO at approximately 200 K. We directly observed by spherical aberration corrected scanning transmission electron microscopy with energy-dispersive x-ray spectroscopy that Sn ions are doped into both Sr and Ti sites (Sn${}_A$ and Sn${}_B$), and that Sn${}_A$ is located at an off-centered position. We also performed theoretical analyses of SSTO and related perovskites, and found that Sn${}_A$ is preferentially located in an off-centered position and that Sn${}_A$ and the O${}_6$ octahedron, which includes Sn${}_B$ in its center, oscillate along the antiphase direction in the soft mode. Thus, we propose that the ferroelectricity of SSTO originates from the antiphase off-centering, which induces ferroelectric nanoregions in paraelectric SrTiO${}_3$.

SrYb${}_2$O${}_4$  is a geometrically frustrated rare-earth magnet, which presents a variety of interrelated magnetic phenomena. The magnetic Yb${}^{3+}$ ions ($J=7/2$) form potentially frustrated “zigzag” chains along the $c$ axis, arranged in a honeycomb fashion in the $ab$ plane. Heat capacity reveals a magnetic phase transition at $T_{\mathrm{N}}=0.9$ K. The magnetic structure was solved by polarized neutron diffraction and found to be noncollinear with a reduction of the ordered spin moment from the full ionic moment. The low-energy excitations, which were measured by inelastic neutron scattering reveal diffuse scattering both above and below $T_{\mathrm{N}}$. Heat capacity and magnetocaloric effect were performed to map out the magnetic phase diagram as a function of magnetic field and temperature and show a complicated series of states. Altogether, the results suggest that the magnetic interactions in SrYb${}_2$O${}_4$  compete with each other and with the single-ion anisotropy to produce a highly degenerate ground state manifold that suppresses the magnetic order, broadens the excitations and gives rise to a complex phase diagram.

We introduce a topological boundary condition to study the surface states of topological insulators within a long-wavelength four-band model. We find that the Dirac point energy, the band curvature, and the spin texture of surface states are crystal-face dependent. For an arbitrary termination of a bulk crystal, the energy of the symmetry protected Dirac point is determined by the bulk physics that breaks particle-hole symmetry in the surface normal direction and is tunable by surface potentials that preserve time reversal symmetry. For a model appropriate to ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ the constant energy contours are generically elliptical with spin textures that are helical on the cleavage surface, collapsed to one dimension on any side face, and tilted out of plane otherwise. Our findings identify a route to engineering the Dirac point physics on the surfaces of real materials.

We show that the plasmon spectrum of an ordinary two-dimensional electron gas (2DEG) hosted in a GaAs heterostructure is significantly modified when a graphene sheet is placed on the surface of the semiconductor in close proximity to the 2DEG. Long-range Coulomb interactions between massive electrons and massless Dirac fermions lead to a set of optical and acoustic intrasubband plasmons. Here we compute the dispersion of these coupled modes within the random phase approximation, providing analytical expressions in the long-wavelength limit that shed light on their dependence on the Dirac velocity and Dirac-fermion density. We also evaluate the resistivity in a Coulomb-drag transport setup. These Dirac-Schrödinger hybrid electron systems are experimentally feasible and open research opportunities for fundamental studies of electron-electron interaction effects in two spatial dimensions.

A class of three-dimensional classical lattice systems with macroscopic ground state degeneracies, most famously the spin ice system, are known to exhibit “Coulomb” phases wherein long wavelength correlations within the ground state manifold are described by an emergent Maxwell electrodynamics. We discuss a more elaborate example of this phenomenon—the four-state Potts model on the pyrochlore lattice—where the long wavelength description now involves three independent gauge fields, as we confirm via simulation. The excitations above the ground state manifold are bions, defects that are simultaneously charged under two of the three gauge fields, and they exhibit an entropic interaction dictated by these charges. We also show that the distribution of flux loops exhibits a scaling with loop length and system size previously identified as characteristic of Coulomb phases.

We report on measurements of the linewidth $\Delta f$ of terahertz radiation emitted from intrinsic Josephson junction stacks, using a Nb/AlN/NbN integrated receiver for detection. Previous resolution-limited measurements indicated that $\Delta f$ may be below 1 GHz—much smaller than expected from a purely cavity-induced synchronization. While at low bias we found $\Delta f$ to be not smaller than $\sim$500 MHz, at high bias, where a hot spot coexists with regions which are still superconducting, $\Delta f$ turned out to be as narrow as 23 MHz. We attribute this to the hot spot acting as a synchronizing element. $\Delta f$ decreases with increasing bath temperature, a behavior reminiscent of motional narrowing in NMR or electron spin resonance (ESR), but hard to explain in standard electrodynamic models of Josephson junctions.

We present a theoretical study of the effect of spin filtering on the Josephson and dissipative quasiparticle currents in a superconducting tunnel junction. By combining the quasiclassical Green's functions and the tunneling Hamiltonian method, we describe the transport properties of a generic junction consisting of two superconducting leads with an effective exchange field $\mathbf{h}$ separated by a spin-filter insulating barrier. We show that in addition to the tunneling of Cooper pairs with total spin projection $S_z=0$ there is another contribution to the Josephson current due to triplet Cooper pairs with total spin projection $S_z\ne 0$. The latter is finite and not affected by the spin-filter effect provided that the fields $\mathbf{h}$ and the magnetization of the barrier are noncollinear. We also determine the quasiparticle current for a symmetric junction and show that the differential conductance may exhibit peaks at different values of the voltage depending on the polarization of the spin filter, and the relative angle between the exchange fields and the magnetization of the barrier. Our findings provide a plausible explanation for existing experiments on Josephson junctions with magnetic barriers, predict further effects, and show how spin-polarized supercurrents in hybrid structures can be created.

The manifestation of the spin-wave interaction in the low-temperature series of the partition function has been investigated extensively over more than seven decades in the case of the three-dimensional ferromagnet. Surprisingly, the same problem regarding ferromagnets in two spatial dimensions, to the best of our knowledge, has not been addressed in a systematic way so far. In the present paper the low-temperature properties of two-dimensional ideal ferromagnets are analyzed within the model-independent method of effective Lagrangians. The low-temperature expansion of the partition function is evaluated up to two-loop order and the general structure of this series is discussed, including the effect of a weak external magnetic field. Our results apply to two-dimensional ideal ferromagnets which exhibit a spontaneously broken spin rotation symmetry $\mathrm{O}\left(3\right)\to \mathrm{O}\left(2\right)$ and are defined on a square, honeycomb, triangular, or kagome lattice. Remarkably, the spin-wave interaction only sets in at three-loop order. In particular, there is no interaction term of order $T^3$ in the low-temperature series for the free energy density. This is the analog of the statement that, in the case of three-dimensional ferromagnets, there is no interaction term of order $T^4$ in the free energy density. We also provide a careful discussion of the implications of the Mermin-Wagner theorem in the present context and thereby put our low-temperature expansions on safe grounds.

The interface between the insulating oxides LaAlO${}_3$ and SrTiO${}_3$ exhibits a superconducting two-dimensional electron system that can be modulated by a gate voltage. While the conductivity has been probed extensively and gating of the superconducting critical temperature has been demonstrated, the question as to whether, and if so how, the gate tunes the superfluid density and superconducting order parameter needs to be answered. We present local magnetic susceptibility, related to the superfluid density, as a function of temperature, gate voltage, and location. We show that the temperature dependence of the superfluid density at different gate voltages collapses to a single curve that is characteristic of a full superconducting gap. Further, we show that the dipole moments observed in this system are not modulated by the gate voltage.

Two-dimensional superconductors with time-reversal symmetry have a ${\mathbb{Z}}_2$ topological invariant, which distinguishes phases with and without helical Majorana edge states. We study the topological phase transition in a class-DIII network model and show that it is associated with a metal-insulator transition for the thermal conductance of the helical superconductor. The localization length diverges at the transition with critical exponent $\nu \approx 2.0$, about twice the known value in a chiral superconductor.

We report polarization resolved photoluminescence from monolayer ${\mathrm{MoS}}_2$, a two-dimensional, noncentrosymmetric crystal with direct energy gaps at two different valleys in momentum space. The inherent chiral optical selectivity allows exciting one of these valleys, and close to 90$\%$ polarized emission at 4 K is observed with 40$\%$ polarization remaining at 300 K. The high polarization degree of the emission remains unchanged in transverse magnetic fields up to 9 T indicating robust, selective valley excitation.

We report on ab initio predictions of environmental embrittlement in aluminum. We have used an atomistic-continuum multiscale framework to simulate the behavior of a loaded crack tip in the presence of oxygen and hydrogen. The multiscale simulations and subsequent analysis suggest that electronegative surface impurities can inhibit dislocation nucleation from a loaded crack tip, thus raising the likelihood for incremental brittle crack growth to occur during near-threshold fatigue. The metal-impurity bonding characteristics have been analyzed using a Bader charge transfer approximation, and the effect of this bond on the theoretical slip distribution has been investigated using a continuum Peierls model. The Peierls model, which is a function of the position dependent stacking fault energy along the slip plane, was used to estimate the effects of several common environmental impurities.

We argue that the low-frequency quantum oscillations observed recently in the vortex state of underdoped ortho-II YBa${}_2$Cu${}_3$O${}_{6,5}$ have the same origin as in other strongly correlated electronic systems. Superconductivity driven by strong interactions on several leading Fermi surfaces creates the proximity gaps on the other. The gap that is transferred on a small-sized pocket from larger Fermi surfaces is small. In the case of unconventional pairing symmetry the induced gap is proportional to the ratio of the oscillations’ frequencies for small and large Fermi surfaces. The gap is in inverse proportion to the mass enhancement on the latter. We share the view that small pockets in and of themselves exist as a certain band feature among all Fermi surfaces.