The specific heat capacity of mostly single-crystalline samples of 21 boracite compounds $M_3{\mathrm{B}}_7{\mathrm{O}}_{13}X$ with $M$ a $3d$ transition metal (Cr, Mn, Fe, Co, Ni, Cu, Zn) or Mg and $X$ a halogen (Cl, Br, I) is determined. In combination with magnetic susceptibility data the magnetic ordering of the $M^{2+}$ ions at $T_N$ is investigated in detail. The fully ferroelectric/fully ferroelastic structural phase transitions at higher temperatures are measured by differential scanning calorimetry. In the Cr-Br, Cr-I, Cu-Cl, and Cu-Br compounds, previously unknown magnetic phases were found. Magnetic order in the boracites is characterized by the quantum and classical spin states of the $M^{2+}$ ions, a variable degree of structural distortion, orbital effects, and competing exchange interactions. The Cu-Cl, Cu-Br, and Ni-Cl boracites exhibit broad maxima of magnetic specific heat and of magnetic susceptibility above $T_N$ caused by low-dimensional or frustrated magnetic interactions. Co boracites display additional broad anomalies below $T_N$ originating from continuous spin reorientations and effective $S=1/2$ ground states. Indications for spin reorientations are also observed for Fe boracites. New phases appear in high magnetic fields for some Co and Fe boracites, which is not the case for the Mn compounds. Stronger magnetic frustration is deduced for the cubic Cr compounds. For the latter compounds and Ni-I boracite magnetostructural phase transitions are observed.

The growth and magnetic domain structure of ultrathin Fe films epitaxially grown on face-centered cubic (fcc) Rh(001) is investigated by spin-polarized scanning tunneling microscopy (SP-STM) at low temperatures $(T=5.5\text{K})$. Our results indicate that the cleaning procedure applied to the Rh(001) substrate plays an important role for the formation of a clean and well-ordered Fe film. The Fe monolayer exhibits an out-of-plane antiferromagnetic $c(2\times 2)$ spin structure. Islands of the second Fe layer are found to be out-of-plane ferromagnetic. Coalescence of the double-layer islands results in the formation of larger domains, which extend over several hundred nanometers for a closed two layer film. Further increasing the Fe coverage leads to a reduction of the domain size and the formation of patterns that are reminiscent of stripe domains driven by the competition of surface/interface and shape anisotropy.

We present an x-ray orbital reflectometry and linear dichroism study of ${\text{PrNiO}}_3{\text{-PrAlO}}_3$ superlattices grown on substrates that induce either compressive or tensile strain. For superlattices under tensile strain, we observe a pronounced change of the local nickel electronic structure and a decrease of orbital polarization below the metal-insulator transition temperature, in qualitative agreement with theoretical scenarios for charge disproportionation. In contrast, the x-ray absorption spectra of the superlattice under compressive strain with suppressed metal-insulator transition show no temperature dependence at the magnetic transition, consistent with a spin density wave transition driven by epitaxial strain and spatial confinement of the conduction electrons. The layer-resolved orbital occupations indicate a linear orbital-strain coupling previously found in ${\text{LaNiO}}_3$-based superlattices and suggest that, when present, the charge order encompasses the entire ${\text{PrNiO}}_3$ layer stack.

We consider a model of strongly correlated electrons in 1D called the $t\text{−}J$ model, which was solved by the graded algebraic Bethe ansatz. We use it to design graded tensor networks which can be contracted approximately to obtain a matrix product state. As a proof of principle, we calculate observables of ground states and excited states of finite lattices up to 18 lattice sites.

Low temperature transport in multiwalled carbon nanotubes (MWNTs) has been studied at different diameters and lengths, within $2\text{–}10\mathrm{nm}$, and $0.3\text{–}3.5\mu \mathrm{m}$, respectively. In a majority of the samples, semiconductivity showed up as a transport gap in the gate voltage controlled conduction, but metallic MWNTs are found in all diameters. The transport gap is seen to be quantitatively determined by a diameter dependent band gap, and length dependent localization of charge carriers. The band gap of semiconducting MWNTs is estimated to be smaller than that extrapolated from the conventional expression applicable to semiconducting single wall carbon nanotubes. The results constitute a systematical study on size dependent transport and especially of semiconductivity in MWNTs.

We discuss the possibility of enhancing the chiroptical response from molecules uniformly distributed around nanostructures that sustain localized plasmon resonances. We demonstrate that the average optical chirality in the near field of any plasmonic nanostructure cannot be significantly higher than that in a plane wave. This conclusion stems from the quasistatic nature of the nanoparticle-enhanced electromagnetic fields and from the fact that, at optical frequencies, the magnetic response of matter is much weaker than the electric one.

The origin of the 90-K nematic transition in the chalcogenide FeSe, which displays no magnetic order down to $T=0$, remains a major puzzle for a unifying theory for the iron-based superconductors. We analyze this problem in light of recent experimental data which reveal very small Fermi pockets in this material. We show that the smallness of the Fermi energy leads to a near degeneracy between magnetic fluctuations and fluctuations in the charge-current density-wave channel. Although the two fluctuation modes cooperate to promote the same preemptive Ising-nematic order, they compete for primary order. We argue that this explains why in FeSe the nematic order emerges when the magnetic correlation length is smaller than in other Fe-based materials. We argue that pressure lifts this near degeneracy and causes nonmonotonic behavior of the nematic transition.

Rephasing and nonrephasing two-dimensional coherent spectra map the anticrossing associated with normal-mode splitting in a semiconductor microcavity. For a 12-meV detuning range near zero detuning, it is observed that there are two diagonal features related to the intra-action of exciton-polariton branches and two off-diagonal features related to coherent interaction between the polaritons. At negative detuning, the line shape properties of the diagonal intra-action features are distinguishable and can be associated with cavitylike and excitonlike modes. A biexcitonic companion feature is observed, shifted from the exciton feature by the biexciton binding energy. Closer to zero detuning, all features are enhanced and the diagonal intra-action features become nearly equal in amplitude and linewidth. At positive detuning the excitonlike and cavitylike characteristics return to the diagonal intra-action features. Off-diagonal interaction features exhibit asymmetry in their amplitudes throughout the detuning range. The amplitudes are strongly modulated (and invert) at small positive detuning, as the lower polariton branch crosses the bound biexciton energy determined from negatively detuned spectra.

Consider two quantum critical Hamiltonians $H$ and $\tilde{H}$ on a $d$-dimensional lattice that only differ in some region $\mathcal{R}$. We study the relation between holographic representations, obtained through real-space renormalization, of their corresponding ground states $|\psi \rangle$ and $|\tilde{\psi }\rangle$. We observe that, even though $|\psi \rangle$ and $|\tilde{\psi }\rangle$ disagree significantly both inside and outside region $\mathcal{R}$, they still admit holographic descriptions that only differ inside the past causal cone $\mathcal{C}\left(\mathcal{R}\right)$ of region $\mathcal{R}$, where $\mathcal{C}\left(\mathcal{R}\right)$ is obtained by coarse-graining region $\mathcal{R}$. We argue that this result follows from a notion of directed influence in the renormalization group flow that is closely connected to the success of Wilson's numerical renormalization group for impurity problems. At a practical level, directed influence allows us to exploit translation invariance when describing a homogeneous system with, e.g., an impurity, in spite of the fact that the Hamiltonian is no longer invariant under translations.

We explore the effect of isotropic strain on the electronic structure of graphene. It is shown that the interplay between one-particle band narrowing and sizable multiorbital Coulomb interactions induces a transition from a Dirac-liquid semimetal to an orbital selective metal characterized by narrow $\pi$-orbital Kondo clouds and abrupt downshift of ${\sigma }^{*}$ states. The correlated electronic structure we derive is promising in the sense that it leads to results that might explain the reshaped electronic structure of graphene nanobubbles probed in scanning tunneling microscopy. Our proposal is a key step in understanding the intricate and interdependent changes in orbital and electronic degrees of freedom of strained materials with hexagonal lattice structure.

We present a detailed theoretical study of bilayer-graphene's electronic properties in the presence of electric and magnetic fields. Using group-theoretical methods, we derive an invariant expansion of the Hamiltonian for electron states near the $\mathrm{K}$ point of the Brillouin zone. In contrast to known materials, including single-layer graphene, any possible coupling of physical quantities to components of the external electric (magnetic) field has a counterpart where the analogous component of the magnetic (electric) field couples to exactly the same combination of quantities. For example, a purely electric spin splitting appears as the magnetoelectric analog of the familiar magnetic Zeeman spin splitting. The measurable thermodynamic response induced by magnetic and electric fields is thus completely symmetric. The Pauli magnetization induced by a magnetic field takes exactly the same functional form as the polarization induced by an electric field. Our findings thus reveal unconventional behavior of spin and pseudospin degrees of freedom in their coupling to external fields. We explain how these counterintuitive couplings are consistent with fundamental principles such as time reversal symmetry. For example, only a magnetic field can give rise to a macroscopic spin polarization, whereas only a perpendicular electric field can induce a macroscopic polarization of the sublattice-related pseudospin degree of freedom characterizing the intravalley orbital motion in bilayer graphene. These rules enforced by symmetry for the matter-field interactions clarify the nature of spins versus pseudospins. We also provide numerical values of prefactors for relevant coupling terms. While our theoretical arguments use bilayer graphene as an example, they are generally valid for any material with similar symmetries. The unusual equivalence of magnetic and electric fields discussed here can provide the basis for designing more versatile device architectures for creating polarizations and manipulating the orientation of spins and pseudospins.

The frequency and wave-vector-dependent memory function in the longitudinal conductivity tensor of weakly interacting electronic systems is calculated by using an approach based on quantum transport equations. In this paper, we show that there is a close relation between the single-electron self-energy, the electron-hole pair self-energy, and the memory function. It is also shown in which way singular long-range Coulomb interactions, together with other $\mathbf{q}\approx \mathbf{0}$ scattering processes, drop out of both the memory function and the related transport equations. The theory is illustrated on heavily doped graphene, which is the prototype of weakly interacting single-band electron-phonon systems. A steplike increase of the width of the quasiparticle peak in angle-resolved photoemission spectra at frequencies of the order of the frequency of in-plane optical phonons is shown to be consistent with the behavior of an intraband plasmon peak in the energy loss spectroscopy spectra. Both anomalies can be understood as a direct consequence of weak electron scattering from in-plane optical phonons.

The physics of thermal transport on strained, two-dimensional (2D) materials graphene, boron nitride, and their superlattices is analyzed by molecular dynamics, lattice dynamics, and numerical solutions to Boltzmann transport equation. The thermal conductivity of these materials is found to be highly sensitive to tensile strain, and the strain dependence itself is also highly dependent on the sample total length. Both finite-sized systems (varying from ∼100 to 300 nm long) as well as infinitely long systems are considered. In contrast to the typical reduction of thermal conductivity with strain exhibited by bulk 3D materials, the thermal conductivity initially increases to a peak value, after which it then decreases with further strain. Modal decomposition of the phonon spectrum shows that the nonmonotonic behavior arises from a competition between in-plane softening and flexural stiffening of phonons. The length sensitivity arises from the nature of the linear/quadratic dispersion of the in-plane/flexural modes and their distinct scattering selection rules: longer systems favor out-of-plane flexural phonon stiffening while smaller systems favor in-plane phonon softening. Additionally, we show that this competition occurs in concert with a strain-induced transition in the nature of the phonon flow from ballistic dominant to diffusive dominant. Overall these effects give rise to a complex dependence of thermal conductivity on strain and sample size.

The structure and stability of defect clusters in ${\mathrm{LiNbO}}_3$, as well as their influence on the linear and nonlinear optical susceptibilities, are calculated within density functional theory (DFT) using semilocal and hybrid exchange-correlation functionals. In particular, the complexes modeling the Li shortage during the crystal growth, the Li-vacancy model and the Nb-vacancy model, are examined in detail. It is found that clustering significantly decreases the formation energies of all considered defects with respect to the dilute limit. The Li-vacancy model is energetically preferred with respect to the total formation energy, while the Nb-vacancy model has the lowest formation energy per single point defect. The independent-particle approximation based on the hybrid DFT electronic structure describes the ${\mathrm{LiNbO}}_3$ optical response much better than semilocal DFT. A further improvement between the calculated optical absorption and second-harmonic generation spectra with experiment is achieved if the calculations take defect complexes into account. Nb antisite polarons give rise to optical absorption within the band gap.

NMR relaxation rates $\left(1/T_1\right)$, magnetic susceptibility, and electrical conductivity studies in doped poly-3-methylthiophene are reported in this paper. The magnetic susceptibility data show the contributions from both Pauli and Curie spins, with the size of the Pauli term depending strongly on the doping level. Proton and fluorine NMR relaxation rates have been studied as a function of temperature (3–300 K) and field (for protons at 0.9, 9.0, 16.4, and 23.4 T, and for fluorine at 9.0 T). The temperature dependence of $T_1$ is classified into three regimes: (a) For $T<\left(g{\mu }_B\mathbf{B}/\mathbf{2}{\mathbf{k}}_{\mathbf{B}}\right)$, the relaxation mechanism follows a modified Korringa relation due to electron-electron interactions and disorder. ${}^1\mathrm{H}-T_1$ is due to the electron-nuclear dipolar interaction in addition to the contact term. (b) For the intermediate temperature range $\left(g{\mu }_B\mathbf{B}/\mathbf{2}{\mathbf{k}}_{\mathbf{B}}\right)<T<T_{\mathrm{BPP}}$ (the temperature where the contribution from the reorientation motion to the $T_1$ is insignificant) the relaxation mechanism is via spin diffusion to the paramagnetic centers. (c) In the high-temperature regime and at low Larmor frequency the relaxation follows the modified Bloembergen, Purcell, and Pound model. $T_1$ data analysis has been carried out in light of these models depending upon the temperature and frequency range of study. Fluorine relaxation data have been analyzed and attributed to the $\mathrm{P}{\mathrm{F}}_6$ reorientation. The cross relaxation among the ${}^1\mathrm{H}$ and ${}^{19}\mathrm{F}$ nuclei has been observed in the entire temperature range suggesting the role of magnetic dipolar interaction modulated by the reorientation of the symmetric molecular subgroups. The data analysis shows that the enhancement in the Korringa ratio is greater in a less conducting sample. Intra- and interchain hopping of charge carriers is found to be a dominant relaxation mechanism at low temperature. Frequency dependence of $T_1^{-1}$ on temperature shows that at low temperature $[T<\left(g{\mu }_B\mathbf{B}/\mathbf{2}{\mathbf{k}}_{\mathbf{B}}\right)]$ the system shows three dimensions and changes to quasi one dimension at high temperature. Moreover, a good correlation between electrical conductivity, magnetic susceptibility, and NMR $T_1$ data has been observed.

We determine the upper and lower critical fields, the penetration depth and the vortex pinning characteristics of single crystals of overdoped $\mathrm{B}{\mathrm{a}}_{0.2}{\mathrm{K}}_{0.8}\mathrm{F}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$ with $T_c\sim 10\mathrm{K}$. We find that bulk vortex pinning is weak and vortex dynamics to be dominated by the geometrical surface barrier. The temperature dependence of the lower critical field, $H_{c1}$, displays a distinctive upturn at low temperatures, which is suggestive of two distinct superconducting gaps. Furthermore, the penetration depth, λ, varies linearly with temperature below 4 K indicative of line nodes in the superconducting gap. These observations can be well described in a model based on a multiband nodal superconducting gap.

Single crystals of ${\mathrm{Bi}}_2{\mathrm{Rh}}_3{\mathrm{S}}_2$ and ${\mathrm{Bi}}_2{\mathrm{Rh}}_{3.5}{\mathrm{S}}_2$ were synthesized by solution growth, and the crystal structures and thermodynamic and transport properties of both compounds were studied. In the case of ${\mathrm{Bi}}_2{\mathrm{Rh}}_3{\mathrm{S}}_2$, a structural first-order transition at around 165 K is identified by single-crystal diffraction experiments, with clear signatures visible in resistivity, magnetization, and specific heat data. No superconducting transition for ${\mathrm{Bi}}_2{\mathrm{Rh}}_3{\mathrm{S}}_2$ was observed down to 0.5 K. In contrast, no structural phase transition at high temperature was observed for ${\mathrm{Bi}}_2{\mathrm{Rh}}_{3.5}{\mathrm{S}}_2$; however, bulk superconductivity with a critical temperature, $T_c\approx 1.7$ K, was observed. The Sommerfeld coefficient $\gamma$ and the Debye temperature $({\mathrm{\Theta }}_{\mathrm{D}})$ were found to be 9.41 mJ ${\mathrm{mol}}^{-1}{\mathrm{K}}^{-2}$ and 209 K, respectively, for ${\mathrm{Bi}}_2{\mathrm{Rh}}_3{\mathrm{S}}_2$, and 22 mJ ${\mathrm{mol}}^{-1}{\mathrm{K}}^{-2}$ and 196 K, respectively, for ${\mathrm{Bi}}_2{\mathrm{Rh}}_{3.5}{\mathrm{S}}_2$. Study of the specific heat in the superconducting state of ${\mathrm{Bi}}_2{\mathrm{Rh}}_{3.5}{\mathrm{S}}_2$ suggests that ${\mathrm{Bi}}_2{\mathrm{Rh}}_{3.5}{\mathrm{S}}_2$ is a weakly coupled, BCS superconductor.

A ternary compound, MgPtSi, was synthesized by solid-state reaction. An examination of the compound by powder x-ray diffraction revealed that it crystallizes in the orthorhombic TiNiSi-type structure with the $Pnma$ space group. The structure comprises alternately stacked layers of Mg and PtSi honeycomb network, which is reminiscent of ${\mathrm{MgB}}_2$, and the buckling of the honeycomb network causes orthorhombic distortion. Electrical and magnetic studies revealed that MgPtSi exhibited superconductivity with a transition temperature of 2.5 K. However, its isostructural compounds, namely, MgRhSi and MgIrSi, were not found to exhibit superconductivity.

We present a detailed study of an electric-field-induced phase transition of a single layer of Fe on a Ni(111) substrate. Scanning tunneling microscopy at 4 K substrate temperature is used to provide the necessary electric field and to follow the transition from face-centered cubic to hexagonal close-packed stacking with atomic resolution.