We extend the range of quantum dot (QD) emission energies where electron and hole $g$ factors have been measured to the practically important telecom range. The spin dynamics in ${\text{InAs/In}}_{0.53}{\mathrm{Al}}_{0.24}{\mathrm{Ga}}_{0.23}\mathrm{As}$ self-assembled QDs with emission wavelengths at about 1.6 $\mu \mathrm{m}$ grown on InP substrate is investigated by pump-probe Faraday rotation spectroscopy in a magnetic field. Pronounced oscillations on two different frequencies, corresponding to the QD electron and hole spin precessions about the field, are observed from which the corresponding $g$ factors are determined. The electron $g$ factor of about $-1.9$ has the largest negative value so far measured for III-V QDs by optical methods. This value, as well as the $g$ factors reported for other III-V QDs, differ from those expected for bulk semiconductors at the same emission energies, and this difference increases significantly for decreasing energies.

The effect of the number of stacking layers and the type of stacking on the electronic and optical properties of bilayer and trilayer black phosphorus are investigated by using first-principles calculations within the framework of density functional theory. We find that inclusion of many-body effects (i.e., electron-electron and electron-hole interactions) modifies strongly both the electronic and optical properties of black phosphorus. While trilayer black phosphorus with a particular stacking type is found to be a metal by using semilocal functionals, it is predicted to have an electronic band gap of 0.82 eV when many-body effects are taken into account within the $G_0{W}_0$ scheme. Though different stacking types result in similar energetics, the size of the band gap and the optical response of bilayer and trilayer phosphorene are very sensitive to the number of layers and the stacking type. Regardless of the number of layers and the type of stacking, bilayer and trilayer black phosphorus are direct band gap semiconductors whose band gaps vary within a range of 0.3 eV. Stacking arrangements that are different from the ground state structure in both bilayer and trilayer black phosphorus exhibit significant modified valence bands along the zigzag direction and result in larger hole effective masses. The optical gap of bilayer (trilayer) black phosphorus varies by 0.4 (0.6) eV when changing the stacking type. The calculated binding energy of the bound exciton hardly changes with the type of stacking and is found to be 0.44 (0.30) eV for bilayer (trilayer) phosphorous.

The nonequilibrium dynamics of diffusion-mediated plasticity and creep in materials subjected to constant load at high homologous temperatures is studied atomistically using phase field crystal (PFC) methods. Creep stress and grain size exponents obtained for nanopolycrystalline systems, $m\simeq 1.02$ and $p\simeq 1.98$, respectively, closely match those expected for idealized diffusional Nabarro-Herring creep. These exponents are observed in the presence of significant stress-assisted diffusive grain boundary migration, indicating that Nabarro-Herring creep and stress-assisted boundary migration contribute in the same manner to the macroscopic constitutive relation. When plastic response is dislocation-mediated, power-law stress exponents inferred from dislocation climb rates are found to increase monotonically from $m\simeq 3$, as expected for generic climb-mediated natural creep, to $m\simeq 5.8$ as the dislocation density ${\rho }_d$ is increased beyond typical experimental values. Stress exponents $m\gtrsim 3$ directly measured from simulations that include dislocation nucleation, climb, glide, and annihilation are attributed primarily to these large ${\rho }_d$ effects. Extrapolation to lower ${\rho }_d$ suggests that $m\simeq 4–4.5$ should be obtained from our PFC description at typical experimental ${\rho }_d$ values, which is consistent with expectations for power-law creep via mixed climb and glide. The anomalously large stress exponents observed in our atomistic simulations at large ${\rho }_d$ may nonetheless be relevant to systems in which comparable densities are obtained locally within heterogeneous defect domains such as dislocation cell walls or tangles.

Ergodicity in quantum many-body systems is—despite its fundamental importance—still an open problem. Many-body localization provides a general framework for quantum ergodicity and may therefore offer important insights. However, the characterization of many-body localization through simple observables is a difficult task. In this article, we introduce a measure for distances in Hilbert space for spin-$\frac{1}{2}$ systems that can be interpreted as a generalization of the Anderson localization length to many-body Hilbert space. We show that this many-body localization length is equivalent to a simple local observable in real space, which can be measured in experiments of superconducting qubits, polar molecules, Rydberg atoms, and trapped ions. By using the many-body localization length and a necessary criterion for ergodicity that it provides, we study many-body localization and quantum ergodicity in power-law-interacting Ising models subject to disorder in a transverse field. Based on the nonequilibrium dynamical renormalization group, numerically exact diagonalization, and an analysis of the statistics of resonances, we find a many-body localized phase at infinite temperature for small power-law exponents. Within the applicability of these methods, we find no indications of a delocalization transition.

The magnetic ground state of two isostructural coordination polymers, (i) the quasi-two-dimensional $S=1/2$ square-lattice antiferromagnet $\left[\mathrm{Cu}\right({\mathrm{HF}}_2\left)\right({\mathrm{pyrazine})}_2]{\mathrm{SbF}}_6$ and (ii) a related compound $\left[\mathrm{Co}\right({\mathrm{HF}}_2\left)\right({\mathrm{pyrazine})}_2]{\mathrm{SbF}}_6$, was examined with neutron powder diffraction measurements. We find that the ordered moments of the Heisenberg $S=1/2$ Cu(II) ions in $\left[\mathrm{Cu}\right({\mathrm{HF}}_2\left)\right({\mathrm{pyrazine})}_2]{\mathrm{SbF}}_6$ are $0.6\left(1\right){\mu }_b$, while the ordered moments for the Co(II) ions in $\left[\mathrm{Co}\right({\mathrm{HF}}_2\left)\right({\mathrm{pyrazine})}_2]{\mathrm{SbF}}_6$ are $3.02\left(6\right){\mu }_b$. For Cu(II), this reduced moment indicates the presence of quantum fluctuations below the ordering temperature. We show from heat capacity and electron spin resonance measurements that due to the crystal electric field splitting of the $S=3/2$ Co(II) ions in $\left[\mathrm{Co}\right({\mathrm{HF}}_2\left)\right({\mathrm{pyrazine})}_2]{\mathrm{SbF}}_6$, this isostructual polymer also behaves as an effective spin-half magnet at low temperatures. The Co moments in $\left[\mathrm{Co}\right({\mathrm{HF}}_2\left)\right({\mathrm{pyrazine})}_2]{\mathrm{SbF}}_6$ show strong easy-axis anisotropy, neutron diffraction data, which do not support the presence of quantum fluctuations in the ground state, and heat capacity data, which are consistent with 2D or close to 3D spatial exchange anisotropy.

Magnetocaloric (MC) materials present an avenue for chemical-free, solid-state refrigeration through cooling via adiabatic demagnetization. We have used inelastic neutron scattering to measure the lattice dynamics in the MC material $\mathrm{N}{\mathrm{i}}_{45}\mathrm{C}{\mathrm{o}}_5\mathrm{M}{\mathrm{n}}_{36.6}\mathrm{I}{\mathrm{n}}_{13.4}$. Upon heating across the Curie temperature $\left(T_C\right)$, the material exhibits an anomalous increase in phonon entropy of $0.22\pm 0.04k_B/\mathrm{atom}$, which is ten times larger than expected from conventional thermal expansion. This transition is accompanied by an abrupt softening of the transverse optic phonon. We present first-principles calculations showing a strong coupling between lattice distortions and magnetic excitations.

Elastic neutron scattering, ac susceptibility, and specific heat experiments on the pyrochlores ${\mathrm{Er}}_2{\mathrm{Ge}}_2{\mathrm{O}}_7$ and ${\mathrm{Yb}}_2{\mathrm{Ge}}_2{\mathrm{O}}_7$ show that both systems are antiferromagnetically ordered in the ${\mathrm{\Gamma }}_5$ manifold. The ground state is a ${\psi }_3$ phase for the Er sample and a ${\psi }_2$ or ${\psi }_3$ phase for the Yb sample, which suggests “Order by Disorder” physics. Furthermore, we unify the various magnetic ground states of all known $R_2{X}_2{\mathrm{O}}_7$ ($R=\text{Er}$, Yb; $X=\text{Sn}$, Ti, Ge) compounds through the enlarged $XY$-type exchange interaction $J_{\pm }$ under chemical pressure. The mechanism for this evolution is discussed in terms of the phase diagram proposed in the theoretical study by Wong et al. [Phys. Rev. B 88, 144402 (2013)].

We isolated flux disorder effects on the transport at the critical point of the quantum magnetic field tuned superconductor-to-insulator transition (BSIT). The experiments employed films patterned into geometrically disordered hexagonal arrays. Spatial variations in the flux per unit cell, which grow in a perpendicular magnetic field, constitute flux disorder. The growth of flux disorder with magnetic field limited the number of BSITs exhibited by a single film due to flux matching effects. The critical metallic resistance at successive BSITs grew with flux disorder contrary to predictions of its universality. These results open the door for controlled studies of disorder effects on the universality class of an ubiquitous quantum phase transition.

Ammonium trichlorocuprate, ${\mathrm{NH}}_4{\mathrm{CuCl}}_3$, a member of the family of quantum antiferromagnets ${\mathrm{M}}_3{\mathrm{CuCl}}_3(\text{M}=\text{K},\text{Tl},{\mathrm{NH}}_4^{+})$, well known for its gapless magnetization with unusual plateaus, is shown here to exhibit ferroelectricity below 67 K based on anomalies of the specific heat and dielectric constant, along with the appearance of spontaneous polarization and electric-field reversible pyroelectric current, making it a rare example of a Cu(II)-based ferroelectric. This suggests that the general ${\mathrm{M}}_3{\mathrm{CuCl}}_3$ family could be rendered multiferroic by alloying its alkali metals with ${\mathrm{NH}}_4^{+}$ ions.

Using a realistic low-energy model, derived from the first-principles electronic structure calculations, we investigate the behavior of interatomic exchange interactions in ${\mathrm{CrO}}_2$, which is regarded to be one of the canonical half-metallic (HM) ferromagnetics. For these purposes we employ the dynamical mean-field theory (DMFT), based on the exact diagonalization of the effective Anderson impurity Hamiltonian, which was further supplemented with the theory of infinitesimal spin rotations for the exchange interactions. In order to elucidate the relative roles played by static and dynamic electron correlations, we compare the obtained results with several static techniques, including the unrestricted Hartree-Fock (HF) approximation, static DMFT (corresponding to the infinite frequency limit for the self-energy), and optimized effective potential method for treating the correlation interactions in the random-phase approximation. Our results demonstrate that the origin of the HM ferromagnetism in ${\mathrm{CrO}}_2$ is highly nontrivial. As far as the interactions in the neighboring coordination spheres are concerned, HF and DMFT methods produce very similar results, due to the partial cancellation of ferromagnetic (FM) double-exchange and antiferromagnetic (AFM) superexchange contributions, which represent two leading terms in the ${\left(\mathrm{\Delta }\widehat{\mathrm{\Sigma }}\right)}^{-1}$ expansion for the exchange interactions ($\mathrm{\Delta }\widehat{\mathrm{\Sigma }}$ being the intra-atomic spin splitting). Both contributions are weaker in the HF approximation due to, respectively, additional orbital polarization of the $t_{2g}$ states and neglect of dynamic correlations. The role of higher-order terms in the ${\left(\mathrm{\Delta }\widehat{\mathrm{\Sigma }}\right)}^{-1}$ expansion is twofold. On the one hand, they give rise to additional FM contributions to the neighboring exchange interactions, which tend to stabilize the FM state. On the other hand, they produce AFM long-range interactions, which make the FM state unstable in the single-site DMFT calculations for the minimal model, consisting of the $t_{2g}$ bands. Thus, the robust ferromagnetism in the minimal model, which can be easily obtained using static approximations, is fortuitous and this picture is largely revised at the level of more rigorous DMFT approach. We argue that the main ingredients, which are missing in the minimal model, are the direct exchange interactions and the magnetic polarization of the oxygen $2p$ band. We evaluate these contributions in the local-spin-density approximation and argue that they play a very important role in stability of the FM ground state in ${\mathrm{CrO}}_2$.

Vortex lattice states occurring in noncentrosymmetric superconductors with a spin-orbit coupling of Rashba type under a magnetic field parallel to the symmetry plane are examined by assuming the $s$-wave pairing case and in an approach combining the quasiclassical theory with the Landau level expansion of the superconducting order parameter. The resulting field-temperature phase diagrams include not only a discontinuous transition but a continuous crossover between different vortex lattice structures, and, further, a critical end point of a structural transition line is found at an intermediate field and a low temperature in the present approach. It is pointed out that the strange field dependence of the vortex lattice structure is a consequence of that of its anisotropy stemming from the Rashba spin-orbit coupling, and that the critical end point is related to the helical phase modulation peculiar to these materials in the ideal Pauli-limited case. Furthermore, calculation results on the local density of states detectable in STM experiments are also presented.

We perform a first-principles band calculation for quasi-two-dimensional organic superconductors $\beta -(\mathrm{BDA}-{\mathrm{TTP})}_2{\mathrm{I}}_3$ and $\beta -(\mathrm{BDA}-{\mathrm{TTP})}_2{\mathrm{SbF}}_6$. The first-principles band structures between the ${\mathrm{I}}_3$ and ${\mathrm{SbF}}_6$ salts are apparently different. We construct a tight-binding model for each material which accurately reproduces the first-principles band structure. The obtained transfer energies give the differences as follows: (i) larger dimerization in the ${\mathrm{I}}_3$ salt than the ${\mathrm{SbF}}_6$ salt, and (ii) different signs and directions of the interstacking transfer energies. To decompose the origin of the difference into the dimerization and the interstacking transfer energies, we adopt a simplified model by eliminating the dimerization effect and focus only on the difference caused by the interstacking transfer energies. From the analysis using the simplified model, we find that the difference of the band structure comes mainly from the strength of the dimerization. To compare the strength of the electron correlation having roots in the band structure, we calculate the physical properties originating from the effect of the electron correlation such as the spin susceptibility applying the two-particle self-consistent method. We find that the maximum value of the spin susceptibility for the ${\mathrm{I}}_3$ salt is larger than that of the ${\mathrm{SbF}}_6$ salt. Hypothetically decreasing the dimerization within the model of the ${\mathrm{I}}_3$ salt, the spin susceptibility takes almost the same value as that of the ${\mathrm{SbF}}_6$ salt for the same magnitude of the dimerization. We expect that the different ground state between the ${\mathrm{I}}_3$ and ${\mathrm{SbF}}_6$ salt mainly comes from the strength of the dimerization which is apparently masked in the band calculation along a particular $k$ path.

By means of first-principles calculations, we identified the structural phase transition of ${\mathrm{Na}}_3\mathrm{Bi}$ from the hexagonal ground state to the cubic $cF16$ phase above 0.8 GPa, in agreement with the experimental findings. Upon the releasing of pressure, the $\mathit{cF}16$ phase of ${\mathrm{Na}}_3\mathrm{Bi}$ is mechanically stable at ambient condition. The calculations revealed that the $cF16$ phase is topological semimetal (TS), similar to well-known HgTe, and it even exhibits an unusually low $C^{\prime }$ modulus (only about 1.9 GPa) and a huge anisotropy $A^u$ of as high as 11, which is the third-highest value among all known cubic crystals in their elastic behaviors. These facts render $\mathit{cF}16$-type ${\mathrm{Na}}_3\mathrm{Bi}$ very soft with a liquidlike elastic deformation in the (110)$\langle 1\overline{1}0\rangle$ slip system. Importantly, accompanying this deformation, ${\mathrm{Na}}_3\mathrm{Bi}$ shows a topological phase transition from a TS state at its strain-free cubic phase to a topological insulator (TI) at its distorted phase. Because the $C^{\prime }$ elastic deformation has almost no energy cost in a reversible and liquidlike soft manner, $\mathit{cF}16$-type ${\mathrm{Na}}_3\mathrm{Bi}$ would potentially provide a fast on/off switching method between TS and TI, which would be beneficial to quantum electronic devices for practical applications.

In this study, we examine effective field theories of superconducting phases with topological order, making a connection to proposed realizations of exotic topological phases (including those hosting Ising and Fibonacci anyons) in superconductor-quantum Hall heterostructures. Our effective field theories for the non-Abelian superconducting states are non-Abelian Chern-Simons theories in which the condensation of vortices carrying non-Abelian gauge flux leads to the associated Abelian quantum Hall states. This Chern-Simons-Higgs condensation process is dual to the emergence of superconducting non-Abelian topological phases in coupled chain constructions. In such transitions, the chiral central charge of the system generally changes, so they fall outside the description of bosonic condensation transitions put forth by Bais and Slingerland [F. A. Bais and J. K. Slingerland, Phys. Rev. B 79, 045316 (2009)] (though the two approaches agree when the described transitions coincide). Our condensation process may be generalized to Chern-Simons theories based on arbitrary Lie groups, always describing a transition from a Lie algebra to its Cartan subalgebra. We include several instructive examples of such transitions.

Photoresistivity and its spectral response has been systematically studied in oxygen-deficient ${\mathrm{SrTiO}}_3$ single crystals for a wide range of resistivities $\rho$ and carrier densities $n$. At room temperature we found a persistent photoresistance that gradually decreases as $\rho$ is diminished or $n$ is increased, in addition to relaxation times of seconds to a few minutes, suggesting that trapping of carriers is playing a major role. An analysis of the photoresistance excitation spectra showed two distinctive features that are related to the indirect gap of ${\mathrm{SrTiO}}_3$ at $(3.25\pm 0.04)\mathrm{eV}$ and to a direct transition at $(3.40\pm 0.03)\mathrm{eV}$. The photoresistive crystals present a temperature-dependent resistivity under illumination that experiences a metal-insulator transition below $T\sim 85\mathrm{K}$. The low-temperature photoresistance spectrum reveals a suitable technique to understand the origin of this transition, pointing to an enhanced efficiency of the $\sim 3.25\mathrm{eV}$ gap to promote electrons to the bottom of the conduction band.

Twisted bilayer graphene (tBLG) forms a quasicrystal whose structural and electronic properties depend on the angle of rotation between its layers. Here, we present a scanning tunneling microscopy study of gate-tunable tBLG devices supported by atomically smooth and chemically inert hexagonal boron nitride (BN). The high quality of these tBLG devices allows identification of coexisting moiré patterns and moiré super-superlattices produced by graphene-graphene and graphene-BN interlayer interactions. Furthermore, we examine additional tBLG spectroscopic features in the local density of states beyond the first van Hove singularity. Our experimental data are explained by a theory of moiré bands that incorporates ab initio calculations and confirms the strongly nonperturbative character of tBLG interlayer coupling in the small twist-angle regime.

We study cascaded harmonic generation of hybrid surface plasmons in integrated planar waveguides composed of a graphene layer and a doped-semiconductor slab. We derive a comprehensive model of cascaded third-harmonic generation through phase-matched nonlinear interaction of fundamental, second-harmonic, and third-harmonic plasmonic modes supported by the structure. We show that hybrid graphene-semiconductor waveguides can simultaneously phase-match these three interacting harmonics, increasing the total third-harmonic output by a factor of 5 compared to the noncascaded regime.

We analyze the form of static charge susceptibility $\chi \left(q\right)$ in underdoped cuprates near axial momenta $(Q,0)$ and $(0,Q)$ at which short-range static charge order has been observed. We show that the momentum dependence of $\chi \left(q\right)$ is anisotropic, and the correlation length in the longitudinal direction is larger than in the transverse direction. We show that correlation lengths in both directions decrease once the system evolves into a superconductor, as a result of the competition between superconductivity and charge order. These results are in agreement with resonant x-ray scattering data [R. Comin et al., Science 347, 1335 (2015)]. We also argue that density and current components of the charge order parameter are affected differently by superconductivity: the charge density component is reduced less than the current component and hence extends deeper into the superconducting state. This gives rise to two distinct charge order transitions at zero temperature.