Here, by conducting a systematic ${}^{89}\mathrm{Y}$ NMR study, we explore the nature of the magnetic ground state in a newly discovered iron-based superconductor ${\mathrm{YFe}}_2{\mathrm{Ge}}_2$. An incoherent-to-coherent crossover due to the Hund's coupling induced electronic correlation is revealed below the crossover temperature $T^{*}\sim 75\pm 15\mathrm{K}$. During the electronic crossover, both the Knight shift ($K$) and the bulk magnetic susceptibility ($\chi$) exhibit a similar nonmonotonic temperature dependence, and a so-called Knight shift anomaly is also revealed by a careful $K-\chi$ analysis. Such an electronic crossover has been also observed in heavily hole-doped pnictide superconductors $A{\mathrm{Fe}}_2{\mathrm{As}}_2$ ($A=\text{K}$, Rb, and Cs), which is ascribed to the Hund's coupling induced electronic correlation. Below $T^{*}$, the spin-lattice relaxation rate divided by temperature ($1/T_{1}T$) shows a similar suppression as the Knight shift, suggesting the absence of critical spin fluctuations. This seems to be in conflict with a predicted magnetic quantum critical point (QCP) near this system. However, considering a $\mathbf{q}$-dependent “filter” effect on the transferred hyperfine field, a predominant spin fluctuation with A-type correlation would be perfectly filtered out at ${}^{89}\mathrm{Y}$ sites, which is consistent with the recent inelastic neutron scattering results. Therefore, our results confirm that, through a Hund's coupling induced electronic crossover, the magnetic ground state of ${\mathrm{YFe}}_2{\mathrm{Ge}}_2$ becomes close to an itinerant magnetic QCP with A-type spin fluctuations. In addition, the possible superconducting pairing due to spin fluctuations is also discussed.

We discuss superconductivity in a model on a cubic lattice for a ${\mathrm{\Gamma }}_3$ non-Kramers system. In previous studies, it is revealed that $d$-wave superconductivity with $E_g$ symmetry occurs in a wide parameter range in a ${\mathrm{\Gamma }}_3$ system. Such anisotropic superconductivity can break the cubic symmetry of the lattice. In a ${\mathrm{\Gamma }}_3$ system, the quadrupole degrees of freedom are active and the effect of the cubic symmetry breaking should be important. Here, we investigate the coexisting states of the $d$-wave superconductivity and quadrupole order by a mean-field theory. In particular, we discuss possible competition and cooperation between the superconductivity and quadrupole order depending on types of them. We find nematic superconductivity breaking the cubic symmetry and coexisting with quadrupole order. In the present model, we also find $d+id$ superconductivity, which breaks time-reversal symmetry but retains the cubic symmetry. We also discuss the effects of uniaxial stress on these superconducting states.

We examine a basic lattice model of interacting fermions that exhibits quadratic band crossing points (QBCPs) in the noninteracting limit. In particular, we consider spinless fermions on the honeycomb lattice with nearest-neighbor hopping $t$ and third-nearest-neighbor hopping $t^{\prime \prime }$, which exhibits fine-tuned QBCPs at the corners of the Brillouin zone for $t^{\prime \prime }=t/2$. In this situation, the density of states remains finite at the Fermi level of the half-filled band and repulsive nearest-neighbor interactions $V$ lead to a charge-density-wave (CDW) instability at infinitesimally small $V$ in the random-phase approximation or mean-field theory. We examine the fragility of the QBCPs against dispersion renormalizations in the $t-t^{\prime \prime }-V$ model using perturbation theory and find that the $t^{\prime \prime }$ value needed for the QBCPs increases with $V$ due to the hopping renormalization. However, the instability toward CDW formation always requires a nonzero threshold interaction strength; i.e., one cannot fine-tune $t^{\prime \prime }$ to recover the QBCPs in the interacting system. These perturbative arguments are supported by quantum Monte Carlo simulations for which we carefully compare the corresponding threshold scales at and beyond the QBCP fine-tuning point. From this analysis, we thus gain a quantitative microscopic understanding of the fragility of the QBCPs in this basic interacting fermion system.

We present here an atomistic theory of the electronic and optical properties of hexagonal InAsP quantum dots in InP nanowires in the wurtzite phase. These self-assembled quantum dots are unique in that their heights, shapes, and diameters are well known. Using a combined valence-force-field, tight-binding, and configuration-interaction approach we perform atomistic calculations of single-particle states and excitonic, biexcitonic, and trion complexes as well as emission spectra as a function of the quantum dot height, diameter, and As versus P concentration. The atomistic tight-binding parameters for InAs and InP in the wurtzite crystal phase were obtained by ab initio methods corrected by empirical band gaps. The low energy electron and hole states form electronic shells similar to parabolic or cylindrical quantum confinement, only weakly affected by hexagonal symmetry and As fluctuations. The relative alignment of the emission lines from excitons, trions, and biexcitons agrees with that for InAs/InP dots in the zincblende phase in that biexcitons and positive trions are only weakly bound. The random distribution of As atoms leads to dot-to-dot fluctuations of a few meV for the single-particle states and the spectral lines. Due to the high symmetry of hexagonal InAsP nanowire quantum dots the exciton fine structure splitting is found to be small, of the order a few $\mu \mathrm{eV}$ with significant random fluctuations in accordance with experiments.

Alkali-doped fullerides exhibit a wealth of unusual phases that remain controversial by nature. Here we report a cryogenic scanning tunneling microscopy study of the submolecular structural and electronic properties of expanded fullerene ${\mathrm{C}}_{60}^{n-}$ films with various cesium dopings. By varying the discrete charge states and film thicknesses, we reveal a large tunability of orientational ordering of ${\mathrm{C}}_{60}^{n-}$ anions, yet the tunneling conductance spectra are all robustly characteristic of energy gaps, hallmarks of Jahn-Teller instability and electronic correlations. The Fermi level lies halfway within the insulating gap for stoichiometric Cs doping level of $n=1$, 2, 3, and 4, apart from which it moves toward band edges with concomitant electronic states within the energy gap. Our findings establish the universality of Jahn-Teller instability and clarify the relationship between the doping and structural and electronic properties in ${\mathrm{Cs}}_n{\mathrm{C}}_{60}$ fullerides.

Attenuated total reflection (ATR) by surface plasmon polariton (SPP) is a method for evaluating the dispersion relation of SPP from the position of a dip in the reflection spectrum. However, recent studies have shown that the dips are displaced from SPP resonance because they are produced by a type of Fano resonance, i.e., the interference between the resonant reflection process accompanied by resonant excitation of SPP and the direct reflection process without resonant excitation. This result suggests that the system properties difficult to be achieved in the dispersion relation of SPP can be characterized using the ATR method. In this study, we investigate the effect of surface roughness due to nanosized dimples created in the initial stage of pitting corrosion on the ATR spectrum, from the viewpoint of Fano resonance. Using the temporal coupled-mode method, it is shown that the Fano resonance in ATR is caused by the phase change of direct reflection because of the absorption on the metal surface, and the spectral shape is determined by this phase, along with the ratio of the external (radiative) decay rate to the total decay rate of the resonant mode. Moreover, it is clarified that the internal and external decay rates extracted from the ATR spectrum provide information on corrosion, such as the effective thickness of the metal film and the randomness in dimple distribution.

We present a density functional theory study of the carrier-density and strain dependence of magnetic order in two-dimensional (2D) ${MAX}_3(M$ = V, Cr, Mn, Fe, Co, Ni; $A$ = Si, Ge, Sn; and $X$ = S, Se, Te) transition metal trichalcogenides. Our ab initio calculations show that this class of compounds includes wide and narrow gap semiconductors, metals, and half-metals, and that most of these compounds are magnetic. Although antiferromagnetic order is most common, ferromagnetism is predicted in $M{\mathrm{SiSe}}_3$ for $M$ = Mn and Ni; in $M{\mathrm{SiTe}}_3$ for $M$ = V and Ni; in ${\mathrm{MnGeSe}}_3$; $M{\mathrm{GeTe}}_3$ for $M$ = Cr, Mn, and Ni; in ${\mathrm{FeSnS}}_3$; and in $M{\mathrm{SnTe}}_3$ for $M$ = V, Mn, and Fe. Among these compounds ${\mathrm{CrGeTe}}_3,{\mathrm{VSnTe}}_3$, and ${\mathrm{CrSnTe}}_3$ are ferromagnetic semiconductors. Our calculations suggest that the competition between antiferromagnetic and ferromagnetic order can be substantially altered by strain engineering, and in the semiconductor case also by gating. The associated critical temperatures can be enhanced by means of carrier doping and strains.

We study the thermodynamics of the delta-chain with competing ferro- and antiferromagnetic interactions in an external magnetic field which generalizes the field-free case studied previously. This model plays an important role for the recently synthesized compound ${\mathrm{Fe}}_{10}{\mathrm{Gd}}_{10}$, which is nearly quantum critical, as well as for the new kagome fluoride ${\mathrm{Cs}}_2{\mathrm{LiTi}}_3{\mathrm{F}}_{12}$. The classical version of the model is solved exactly and explicit analytical results for the low-temperature thermodynamics are obtained. The $s$-spin quantum model is studied using exact diagonalization and finite-temperature Lanczos techniques. Particular attention is focused on the magnetization and the susceptibility. The magnetization of the classical model in the ferromagnetic part of the phase diagram defines the universal scaling function which is valid for the quantum model. The dependence of the susceptibility on the spin quantum number $s$ at the critical point between the ferro- and ferrimagnetic phases is studied and the relation to ${\mathrm{Fe}}_{10}{\mathrm{Gd}}_{10}$ is discussed.

One of the defining features of spontaneously broken time-reversal symmetry (BTRS) is the existence of domain walls, the detection of which would be strong evidence for such systems. There is keen interest in BTRS currently, in part, due to recent muon spin rotation experiments, which have pointed towards ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ exhibiting a remarkable case of $s$-wave superconductivity with spontaneously broken time-reversal symmetry. A key question, however, is how to differentiate between the different theoretical models which describe such a state. Two particularly popular choices of model are $s+is$ and $s+id$ superconducting states. In this paper, we obtain solutions for domain walls in $s+is$ and $s+id$ systems, including the effects of lattice anisotropies. We show that, in general, both models exhibit spontaneous magnetic fields that extend along the entire length of the domain wall. We demonstrate the qualitative difference between the magnetic signatures of $s+is$ and $s+id$ domain walls and propose a procedure to extract the superconducting pairing symmetry from the magnetic-field response of domain walls.

We report a robust anomalous Nernst effect in ${\mathrm{Co}}_2\mathrm{MnGa}$ thin films in the thickness regime between 20 and 50 nm. The anomalous Nernst coefficient varied in the range of −2.0 to −3.0 $\mu \mathrm{V}/\mathrm{K}$ at 300 K. We demonstrate that the anomalous Hall and Nernst coefficients exhibit similar behavior and fulfill the Mott relation. We simultaneously measure all four transport coefficients of the longitudinal resistivity, transversal resistivity, Seebeck coefficient, and anomalous Nernst coefficient. We connect the values of the measured and calculated Nernst conductivity by using the remaining three magnetothermal transport coefficients, where the Mott relation is still valid. The intrinsic Berry curvature dominates the transport due to the relation between the longitudinal and transversal transport. Therefore, we conclude that the Mott relationship is applicable to describe the magnetothermoelectric transport in Weyl semimetal ${\mathrm{Co}}_2\mathrm{MnGa}$ as a function of film thickness.

The spin Hall conductivity is usually calculated for periodic solids, while the knowledge of local contributions from inhomogeneous regions like interfaces or surfaces would be desirable. In this work we derive a local expression for the spin Hall conductivity of two-dimensional systems in analogy to the local anomalous Hall conductivity discussed recently. Our approach is applicable to heterogeneous fully bounded systems and nanoribbons with both open and periodic boundary conditions.

We present in full detail a newly developed formalism enabling density functional perturbation theory (DFPT) calculations from a $\mathrm{DFT}+U$ ground state. The implementation includes ultrasoft pseudopotentials and is valid for both insulating and metallic systems. It aims at fully exploiting the versatility of DFPT combined with the low-cost $\mathrm{DFT}+U$ functional. This allows us to avoid computationally intensive frozen-phonon calculations when $\mathrm{DFT}+U$ is used to eliminate the residual electronic self-interaction from approximate functionals and to capture the localization of valence electrons, e.g., on $d$ or $f$ states. In this way, the effects of electronic localization (possibly due to correlations) are consistently taken into account in the calculation of specific phonon modes, Born effective charges, dielectric tensors, and in quantities requiring well converged sums over many phonon frequencies, as phonon density of states and free energies. The new computational tool is applied to two representative systems, namely CoO, a prototypical transition metal monoxide and ${\mathrm{LiCoO}}_2$, a material employed for the cathode of Li-ion batteries. The results show the effectiveness of our formalism to capture in a quantitatively reliable way the vibrational properties of systems with localized valence electrons.

We report here the observation of the inverse exchange bias phenomenon in the ferrimagnetic double-perovskite ${\mathrm{Gd}}_2{\mathrm{CoRuO}}_6$. The compound crystallizes in a monoclinic structure (space group $P{2}_1/n$), and it undergoes long-range magnetic ordering below $T_C=60$ K. The ferrimagnetic state of the sample is manifested through magnetization reversal at low temperature. In order to corroborate the role of local structure on the magnetic properties of ${\mathrm{Gd}}_2{\mathrm{CoRuO}}_6$, we have performed a combined magnetization and x-ray absorption spectroscopy study on two different samples prepared through different cooling protocols (slow cooling and furnace cooling). Extended x-ray absorption fine-structure analysis indicates the presence of strong antisite disorder between Co and Ru in both samples. However, the slow-cooled sample shows completely random antisite disorder, whereas, small Co- and Ru-rich patches appear in the furnace-cooled sample. In both samples, a significant amount of mixed valency in Co and Ru ions is evident from our x-ray absorption near-edge structure. The antiparallel ordering along with different temperature dependence of the three magnetic sublattices, Gd, Co, and Ru, can be responsible for negative magnetization, whereas the inverse exchange bias effect can be attributed to the mixed interfacial coupling between adjacent magnetic nanoclusters. The difference in local structure can nicely be accounted for by the observed variations of the magnetic properties between two samples of ${\mathrm{Gd}}_2{\mathrm{CoRuO}}_6$.

We investigate the effect of strain and film thickness on the orbital and magnetic properties of ${\mathrm{LaSrCrO}}_3\left(\mathrm{LSCO}\right)/{\mathrm{LaSrMnO}}_3$ (LSMO) heterostructures using bulk magnetometry, soft x-ray magnetic spectroscopy, first-principles density-functional theory, high-resolution electron microscopy, and x-ray diffraction. We observe an antiparallel ordering of the magnetic moments between the ferromagnetic LSMO layers and the LSCO spacers, leading to a strain-independent ferromagnetic ground state of the LSCO/LSMO heterostructures for LSMO layers as thin as two unit cells. As the LSMO thickness is increased, a net ferromagnetic state is maintained, however, the average magnetic moment per Mn is found to be dependent on the magnitude of the substrate-induced strain. The differences in the magnetic responses are related to preferential occupation of the Mn $x^2-y^2$ (in-plane) $d$ orbitals for tensile strain and $3z^2-r^2$ (out-of-plane) orbitals under compressive strain, leading to competing ferromagnetic and antiferromagnetic exchange interactions within the LSMO layers. These results underscore the relative contributions of orbital, structural, and spin degrees of freedom and their tunability in atomically thin crystalline complex oxide layers.

The isotropic, nonmagnetic doped $\mathrm{BaBi}{\mathrm{O}}_3$ superconductors maintain some similarities to high-$T_{\mathrm{c}}$ cuprates, while also providing a cleaner system for isolating charge density wave (CDW) physics that commonly competes with superconductivity. Artificial layered superlattices offer the possibility of engineering the interaction between superconductivity and CDW. Here we stabilize a low-temperature, fluctuating short-range CDW order by using artificially layered epitaxial ${\left(\mathrm{BaPb}{\mathrm{O}}_3\right)}_{3m}/{\left(\mathrm{BaBi}{\mathrm{O}}_3\right)}_m$ ($m=1–10$ unit cells) superlattices that are not present in the optimally doped $\mathrm{BaP}{\mathrm{b}}_{0.75}\mathrm{B}{\mathrm{i}}_{0.25}{\mathrm{O}}_3$ alloy with the same overall chemical formula. Charge transfer from $\mathrm{BaBi}{\mathrm{O}}_3$ to $\mathrm{BaPb}{\mathrm{O}}_3$ effectively dopes the former and suppresses the long-range CDW; however, as the short-range CDW fluctuations strengthen at low temperatures charge appears to localize and superconductivity is weakened. The monolayer structural control demonstrated here provides compelling implications to access controllable, local density wave orders absent in bulk alloys and manipulate phase competition in unconventional superconductors.

Nanostructures with one-dimensional periodicity, such as multilayered structures, are currently in the focus of active research in the context of hyperbolic metamaterials and photonic topological structures. An efficient way to describe the materials with subwavelength periodicity is based on the concept of effective material parameters, which can be rigorously derived incorporating both local and nonlocal responses. However, to provide any predictions relevant for applications, effective material parameters have to be supplemented by appropriate boundary conditions. In this work, we provide a comprehensive treatment of spatially dispersive bulk properties of multilayered metamaterials as well as derive boundary conditions for the averaged fields. We demonstrate that local bianisotropic model does not capture all the features related to second-order nonlocal effects in the bulk of metamaterial. As we prove, while the bulk response of multilayers does not depend on the unit-cell choice, effective boundary conditions are strongly sensitive to the sequence of layers and multilayer termination. The developed theory provides a clear interpretation of the recent experiments on the reflectance of all-dielectric deeply subwavelength multilayers suggesting further avenues to experimentally probe electromagnetic nonlocality in metamaterials.

We study the motion of a pair of electrons along two separate parallel chains of quantum dots. The electrons that are released from the central dot of each chain tend to accompany and not avoid each other. The correlated electron motion involves entanglement of the wave functions which is generated in time upon release of the initial confinement. Observation of the simultaneous presence of electrons at the same side of the chain can provide a fingerprint of the paired electron motion.

The possibility of breaking structural symmetry with realization of Janus monolayers offers new possibilities in the field of two-dimensional (2D) materials, and various ternary systems including the class of group-III monochalcogenides have been suggested. However, interaction of oxygen was shown to modify optoelectronic properties of gallium monochalcogenides, and design of ternary systems with oxygen as a third component has not been considered yet. In this paper, we design and investigate 2D ${\mathrm{Ga}}_{2}X\text{O}$ ($X=\mathrm{S},\mathrm{Se},\mathrm{Te}$) systems by using first-principles calculations. Phonon spectra analysis and molecular dynamics simulations indicate that while ${\mathrm{Ga}}_2\mathrm{SO}$ and ${\mathrm{Ga}}_2\mathrm{SeO}$ are stable even at high temperatures ${\mathrm{Ga}}_2\mathrm{TeO}$ is dynamically unstable. Inclusion of oxygen makes ${\mathrm{Ga}}_2\mathrm{SO}$ and ${\mathrm{Ga}}_2\mathrm{SeO}$ less brittle when compared to their binary constituents. While $\mathrm{Ga}X$ monolayers have indirect band gaps, ${\mathrm{Ga}}_2\mathrm{SO}$ and ${\mathrm{Ga}}_2\mathrm{SeO}$ become direct band-gap semiconductors and the band gap can be further tuned by tensile/compressive strain. Additionally, depending on the type of the system, strong optical absorption within the infrared, visible, and/or ultraviolet region is also predicted. Finally, structural and electronic properties of bilayers of ${\mathrm{Ga}}_{2}X\text{O}$ are examined and compared with monolayers. Our results not only predict stable 2D ternary ${\mathrm{Ga}}_{2}X\text{O}$ structures but also suggest them as promising materials for optoelectronic applications.

We report on the effect of laser illumination with circularly polarized light on the electronic structure of $AB$-stacked graphite samples. By using Floquet theory in combination with Green's function techniques, we find that the polarized light induces band-gap openings at the Floquet zone edge $\hslash \mathrm{\Omega }/2$, bridged by chiral boundary states. These states propagate mainly along the borders of the constituting layers as evidenced by the time-averaged local density of states and the probability current density in several geometries. Semianalytic calculations of the Chern number suggest that these states are of topological nature, similar to those found in illuminated 2D samples like monolayer and bilayer graphene. These states are promising candidates for the realization of a three-dimensional version of the quantum Hall effect for Floquet systems.

The layered material ZrSiTe is currently extensively investigated as a nodal-line semimetal with Dirac-like band crossings protected by nonsymmorphic symmetry close to the Fermi energy. A recent infrared spectroscopy study on ZrSiTe under external pressure found anomalies in the optical response, providing hints for pressure-induced phase transitions at $\approx 4.1$ and $\approx 6.5$ GPa. By pressure-dependent Raman spectroscopy and x-ray diffraction measurements combined with electronic band structure calculations we find indications for two pressure-induced Lifshitz transitions with major changes in the Fermi surface topology in the absence of lattice symmetry changes. These electronic phase transitions can be attributed to the enhanced interlayer interaction induced by external pressure. Our findings demonstrate the crucial role of the interlayer distance for the electronic properties of layered van der Waals topological materials.