The electron density distributions in $\beta \text{−}\mathrm{Ge}$ have been experimentally determined at in situ high-pressure conditions using synchrotron diffraction techniques in a diamond anvil cell. Upon decompression, the electron density along the $c$ axis in tetragonal $\beta \text{−}\mathrm{Ge}$ displays a sudden drop at 10–11 GPa close to that of the structural α-β transition, while the $\beta \text{−}\mathrm{Ge}$ samples remain as single crystals until transforming to metastable Ge phases around 6.7–8.5 GPa. In contrast to the covalently bonded $\alpha \text{−}\mathrm{Ge}$ that displays only a weak participation of $d$ electrons in the valence band under compression above 7.7 GPa, our experimental results suggest that a large change in $d$-orbital participation can occur in the $\beta \text{−}\mathrm{Ge}$ lattice which has mixed covalent and metallic bonding. The $\beta \text{−}\mathrm{Ge}$ below 10 GPa may display large fluctuations in electronic properties, which sheds light for exploring novel materials with intriguing electronic and optical properties.

High-quality single crystals of ${\mathrm{NaYbSe}}_2$, which resembles a perfect triangular-lattice antiferromagnet without intrinsic disorder, are investigated by magnetization and specific heat, as well as the local probe techniques nuclear magnetic resonance (NMR) and electron spin resonance. The low-field measurements confirm the absence of any spin freezing or long-range magnetic order down to 50 mK, which suggests a quantum spin liquid ground (QSL) state with gapless excitations. Instability of the QSL state is observed upon applying magnetic fields. For the $H\perp c$ direction, a field-induced magnetic phase transition is observed above 2 T from the $C_{\mathrm{p}}\left(T\right)$ data, agreeing with a clear $\frac{M_s}{3}$ plateau of $M\left(H\right)$, which is associated with an up-up-down spin arrangement. For the $H\parallel c$ direction, a field-induced transition could be evidenced at a much higher field range (9–21 T). The ${}^{23}\mathrm{Na}$ NMR measurements provide microscopic evidence of field-induced ordering for both directions. A reentrant behavior of $T_{\mathrm{N}}$, originating from the thermal and quantum spin fluctuations, is observed for both directions. The anisotropic exchange interactions $J_{\perp }\simeq 4.7$ K and $J_z\simeq 2.33$ K are extracted from the modified bond-dependent XXZ model for the spin-$\frac{1}{2}$ triangular-lattice antiferromagnet. The absence of magnetic long-range order at zero fields is assigned to the effect of strong bond frustration, arising from the complex spin-orbit entangled $4f$ ground state. Finally, we derive the highly anisotropic magnetic phase diagram, which is discussed in comparison with the existing theoretical models for spin-$\frac{1}{2}$ triangular-lattice antiferromagnets.

We investigated the magnetic and structural transitions of $\mathrm{M}{\mathrm{n}}_{1-x}\mathrm{M}{\mathrm{g}}_x{\mathrm{V}}_2{\mathrm{O}}_4(x=0$, 0.05, 0.1, 0.15) single crystals with a normal spinel structure. These crystals all exhibit a cubic-to-tetragonal structural transition at $T_{\mathrm{S}}$ caused by the orbital ordering of ${\mathrm{V}}^{3+}$ ions, which is followed by a ferrimagnetic ordering transition at $T_{\mathrm{M}}$. We found that $T_{\mathrm{M}}$ is equal to $T_{\mathrm{S}}$ for $x\le 0.1$, but the ferrimagnetic transition is shifted to lower temperature by Mg doping and $T_{\mathrm{S}}$ and $T_{\mathrm{M}}$ can be separated clearly for $x=0.15$. Moreover, a certain degree of orbital fluctuations was brought about by the ferrimagnetic transition at $T_{\mathrm{M}}$, but suppressed below $T^{*}$, leading to another transition from weak orbital fluctuations to further orbital ordering at $T^{*}$. The orbital fluctuations were caused by the anomalous interchain interaction $J^{\mathrm{c}}$ and can be also enhanced by the substitution of Mg for Mn due to the decrease of the distance between ${\mathrm{V}}^{3+}$ ions along the $c$ direction ${}^c{R}_{\mathrm{v}\text{−}\mathrm{v}}$, which results in the increase of $J^{\mathrm{c}}$.

The Su-Schrieffer-Heeger (SSH) model was introduced to describe the electron-phonon interactions leading to Peierls distortion in polyacetylene (PA). The same model was recently predicted to give rise to unique polaronic effects, including sharp transitions in polaron properties, quasi-self-trapping due to polaron interactions, and the formation of strongly bound yet light bipolarons, at strong electron-phonon coupling in the single and two-polaron limit. This suggests that organic polyenes with strong electron-phonon coupling may exhibit qualitatively different conduction properties from those with weak coupling. In order to observe this, it is necessary to design materials with a range of SSH electron-phonon coupling. Here, we use gradient-corrected density-functional-theory calculations to predict the SSH model parameters for a variety of polyenes, derived from PA by the substitution of the hydrogen atoms with molecular groups. We show that even though the calculations do not accurately reproduce the band gaps, the derivatives of the band structure parameters giving the phonon-induced couplings can be computed with good accuracy. We show that the electron-phonon coupling in such polyenes correlates with the Hammett constant of the substituted molecular group and that some substitutions enhance the coupling strength by as much as a factor of 2. We also find that the electron-phonon coupling in conjugated systems with heavier atoms (such as Si) is much weaker than in PA despite the lower phonon frequencies.

The algebraic spin liquid is a long-sought-after phase of matter characterized by the absence of quasiparticle excitations, a low-energy description in terms of emergent Dirac fermions and gauge fields interacting according to $(2+1)$-dimensional quantum electrodynamics $\left({\mathrm{QED}}_3\right)$, and power-law correlations with universal exponents. The prototypical algebraic spin liquid is the Affleck-Marston $\pi$-flux phase, originally proposed as a possible ground state of the spin-$\frac{1}{2}$ Heisenberg model on the two-dimensional square lattice. While the latter model is now known to order antiferromagnetically at zero temperature, recent sign-problem-free quantum Monte Carlo simulations of spin-$\frac{1}{2}$ fermions coupled to a compact $\text{U}\left(1\right)$ gauge field on the square lattice have shown that quantum fluctuations can destroy Néel order and drive a direct quantum phase transition to the $\pi$-flux phase. We show this transition is in the universality class of the chiral Heisenberg ${\mathrm{QED}}_3$– Gross-Neveu-Yukawa model with a single $\text{SU}\left(2\right)$ doublet of four-component Dirac fermions (i.e., $N_f=1)$, pointing out important differences with the corresponding putative transition on the kagome lattice. Using an $\epsilon$ expansion below four space-time dimensions to four-loop order, and a large-$N_f$ expansion up to second order, we show the transition is continuous and compute various thermodynamic and susceptibility critical exponents at this transition, setting the stage for future numerical determinations of these quantities. As a by-product of our analysis, we also obtain charge-density-wave and valence-bond-solid susceptibility exponents at the semimetal-Néel transition for interacting fermions on the honeycomb lattice.

We study a Rashba quantum ring subjected to an in-plane static electric field. Due to the violation of the rotational symmetry, the system radiates at the Rabi frequency upon interaction with a strong electromagnetic field. With a proper choice of the driving amplitude, the radiation frequency is in the range of $0.1-10\mathrm{THz}$. Due to the Rashba spin-orbit interaction which splits the spin-up and spin-down states and affects the transitions between the ground and excited states, the system exhibits circular dichroism and optical activity. The static electric field and gate-voltage-dependent Rashba parameter strongly influence the system. In other words, the Rashba quantum ring may serve as an electrically tunable terahertz emitter, absorption modulator, dichroic element, and polarization rotator.

Two-dimensional (2D) materials have recently attracted significant interest in condensed-matter physics and materials science due to rich intriguing physics and potential applications. Here, by first-principles calculations, we discover a phase of transition-metal dichalcogenides $(\mathrm{Mo}{\mathrm{S}}_2$ and its family) named the $1H^{\prime }$ phase. We demonstrate that $1H^{\prime }\mathrm{Mo}{\mathrm{S}}_2$ monolayer has multiple topological phase transitions from a topologically trivial insulator state to a 2D type-II Dirac semimetal state, and then to a 2D ideal type-I semimetal state under a moderate tensile strain. The Dirac points are protected by the coexistence of space-inversion $\left(P\right)$ and time-reversal $\left(T\right)$ symmetry. Our results highlight a family of nonhoneycomb 2D Dirac semimetals with potential application in nanoelectronics.

We propose a global optimization strategy for atomistic structure determination based on two new concepts: a few-atom complementary energy landscape and atomic role models. Global optimization of costly energy expressions may be aided by performing some of the optimization on model energy landscapes. These are often based on a sum-of-atomic-contributions form that accurately reproduces every local energy minimum of the true energy expression. However, we propose that, by not including all atomic contributions, the resulting energy landscapes may become more convex, making the search for the global optimum more facile. A role model is someone we aspire to be more like; in the same vein we define the role model of an atom to be another atom whose local environment the first atom seeks to obtain itself. Basing a complementary energy landscape on the distance of some atoms from their role models in a feature space, we arrive at a useful few-atom complementary energy landscape. We show that relaxation in this landscape is an effective mutation when employed in an evolutionary algorithm used to identify the bulk cristobalite structure of ${\mathrm{SiO}}_2$ and the $\left(1\times 4\right)$ surface reconstruction of anatase ${\mathrm{TiO}}_2$(001).

We study the magnetoresistance of an ultrahigh mobility GaAs/AlGaAs two-dimensional electron system (2DES) in a weak magnetic field under low-frequency $(f<20$ GHz) microwave (MW) irradiation. We observe that, with decreasing MW frequency, microwave induced resistance oscillations (MIRO) are damped and multiphoton processes become dominant. At very low MW frequency $(f<4$ GHz), MIRO disappear gradually and an SdH-like oscillation develops. Our analysis indicates that the oscillation may originate from alternating Hall-field induced resistance oscillations (ac-HIRO), due to the transition from the elastic scattering in real space. On the other hand, from the view of photon energy, the oscillations can be understood as a multiphoton process of MIRO in the low MW frequency limit. We show that the two different nonequilibrium mechanisms of MIRO and HIRO can be unified under low-frequency MW irradiation.

The effects of structural supermodulation with the period $\lambda \approx 26$ Å along the $b$ axis of ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ have been observed in photoemission studies from the early days as the presence of “diffraction replicas” of the intrinsic electronic structure. Although predicted to affect the electronic structure of the Cu-O plane, the influence of supermodulation potential on Cu-O electrons has never been observed in photoemission. In the present study, we clearly see the effects on the ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ electronic structure; we observe a hybridization of the intrinsic bands with the supermodulation replica bands in the form of avoided crossings and a corresponding reconstruction of the Fermi surface. We estimate the hybridization gap, $2{\mathrm{\Delta }}_h\sim 25$ meV, in the slightly underdoped samples. The hybridization weakens with doping and the anticrossing can no longer be resolved in strongly overdoped samples. In contrast, the “shadow” replica, shifted by $(\pi ,\pi )$, is found not to hybridize with the original bands within our detection limits.

By means of quantum Monte Carlo simulations, combined with a stochastic analytic continuation, we examine the spin dynamics of the spin-1/2 planar ($XY$) ferromagnet on the kagome lattice with additional four-site ring exchange terms. Such exchange processes were previously considered to lead into an extended $Z_2$ quantum spin liquid phase beyond a quantum critical point from the $XY$ ferromagnet. We examine the dynamical spin structure factor in the nonmagnetic regime and probe for signatures of spin fractionalization. Furthermore, we contrast our findings and the corresponding energy scales of the excitation gaps in the ring exchange model to those emerging in a related Balents-Fisher-Girvin model with a $Z_2$ quantum spin liquid phase, and monitor the softening of the magnon mode upon approaching the quantum critical point from the $XY$-ferromagnetic regime.

To investigate the relationship between the charge redistribution and ferromagnetism at the interface between perovskite transition-metal oxides $\mathrm{LaNi}{\mathrm{O}}_3$ (LNO) and $\mathrm{LaMn}{\mathrm{O}}_3$ (LMO), we performed x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) measurements on the sandwiched layer in the trilayer structures. In the LNO/LMO heterostructures, one electron (hole) per unit-cell area is transferred from Mn (Ni) to Ni (Mn) ions across the interface unless there is a spatial limitation in the layer accommodating the transferred charges. A detailed analysis of the $\mathrm{Ni}-L_{2,3}$ and $\mathrm{Mn}-L_{2,3}$ XMCD spectra reveals that the magnetization of Ni ions is induced only in $\mathrm{N}{\mathrm{i}}^{2+}$ ions at the interface, while that of Mn ions is induced in both $\mathrm{M}{\mathrm{n}}^{3+}$ and $\mathrm{M}{\mathrm{n}}^{4+}$ ions. Combining these results with the previous XAS results on bilayer structures revealing the asymmetric charge redistribution of the LNO/LMO interface [Kitamura et al., Appl. Phys. Lett. 108, 111603 (2016)] shows that Ni magnetization is induced only in the $\mathrm{N}{\mathrm{i}}^{2+}$ ions of the 1 ML LNO adjacent to the interface, while the magnetization of Mn ions is increased in the 3–4 ML LMO of the interfacial region. These results indicate a close relationship between the charge redistribution due to the interfacial charge transfer and the ferromagnetism of the LNO/LMO interface. Furthermore, the XMCD spectra clearly demonstrate that the vectors of induced magnetization of both ions are aligned ferromagnetically, suggesting that the delicate balance between the exchange interactions occurring inside each layer and across the interface may induce the canted antiferromagnetism of $\mathrm{N}{\mathrm{i}}^{2+}$ ions, resulting in weak magnetization in the 1 ML LNO adjacent to the interface.

Structurally chiral materials hosting multifold fermions with large topological number have attracted considerable attention because of their naturally long surface Fermi arcs and bulk quantized circular photogalvanic effect (CPGE). Multifold fermions only appear in metallic states, and therefore most studies so far have only focused on the semimetals in compounds with chiral crystal structures. In this work, we show that the structurally chiral topological trivial insulators are also exotic states, which is interesting from the application point of view, owing to their natural advantage to host a large bulk photovoltaic effect in the visible wavelength region. In recent decades, the shift current in the visible wavelength region was limited to be $10\mu {\mathrm{A}/\mathrm{V}}^2$ in all the experimentally measured reports. By scanning the insulators with chiral structure, we found a class of compounds with photoconductivity ranging from $\sim 20$ to $\sim 80\mu {\mathrm{A}/\mathrm{V}}^2$, which is comparable to the largest reported shift current. This work illustrates that the compounds with chiral structure can host both quantum CPGE and a strong shift current in the second-order optical response. Moreover, this work offers a good platform for the study of the shift current and its future application by putting the focus on insulators with chiral lattices, so far overlooked in photovoltaic technologies.

Topological phases of matter are distinguished by topological invariants, such as Chern numbers and topological spins, that quantize their response to electromagnetic currents and changes of ambient geometry. Intriguingly, in the $\nu =2/5$ fractional quantum Hall effect, prominent theoretical approaches—the composite fermion theory and conformal field theory—have constructed two distinct states, the Jain composite fermion (CF) state and the Gaffnian state, for which many of the topological indices coincide and even the microscopic ground-state wave functions have high overlap with each other in system sizes accessible to numerics. At the same time, some aspects of these states are expected to be very different; e.g., their elementary excitations should have either Abelian (CF) or non-Abelian (Gaffnian) statistics. In this paper we investigate the close relationship between these two states by considering not only their ground states, but also the low-energy charged excitations. We show that the low-energy physics of both phases is spanned by the same type of quasielectrons of the neighboring Laughlin phase. The main difference between the two states arises due to an implicit assumption of short-range interaction in the CF approach, which causes a large splitting of the variational energies of the Gaffnian excitations. We thus propose that the Jain phase emerges as an effective Abelian low-energy description of the Gaffnian phase when the Hamiltonian is dominated by two-body interactions of sufficiently short range.

We propose the metasurface-based cavities for splitting a linearly polarized (LP) electromagnetic wave into two symmetric circularly polarized (CP) beams with opposite helicities. More specifically, periodic strip slits and rectangular C-slits etched on substrate integrated waveguide cavities are shown to be capable of perfectly coupling the LP wave into the cavities from the strip slit array, and then releasing them with specific polarization directions according to the orientations of the C-slits. Such metasurface-based cavities with gradient oriented C-slit array would further create the opposite equivalent phase gradients for the right-hand and left-hand CP transmitting waves that are decomposed from the released LP wave with anomalous birefractions and polarization conversion simultaneously.

Comprehensive knowledge of Casimir forces and associated electrostatics from conductive SiC and Ru surfaces can be essential in diverse areas ranging from micro/nanodevice operation in harsh environments to multilayer coatings in advanced lithography technologies. Hence, the Casimir force was measured between an Au-coated microsphere and N-doped SiC samples with Si- and C-terminated faces, and the results were compared with the measurements using the same microsphere and a metallic Ruthenium surface. Electrostatic calibration showed that the Si- and C-faces behave differently with a nearly $\sim 0.6–0.7$ V difference in the contact potentials ${V_0}^{\mathrm{Si}/\mathrm{C}}$. We attribute this to a higher incorporation of N on the C-terminated face in the near surface region resulting in the formation of ${\mathrm{NO}}_{\mathrm{x}}$ and an increased work function compared to the Si-terminated surface, which is in agreement with x-ray photoelectron spectroscopy data. Notably, the contact potential of the SiC-C face ($V_0^{\mathrm{C}}\sim 0.1$ V) was closer to the metallic Ru-Au system ($V_0^{\mathrm{Ru}}\sim 0.05$ V). However, the measured optical properties of the SiC-Si/C terminated surfaces with ellipsometry did not show any substantial differences indicating that the effective depth of the Si/C terminating surface layers are significantly smaller than the photon penetration depth not leading to any differences in the calculated forces via Lifshitz theory. Nonetheless, the measured Casimir forces, after compensation of the electrostatics contributions, showed differences between the Si/C faces, whereas the comparison with the Lifshitz theory prediction shows better agreement for the SiC-Si face. Finally, comparison of the Casimir forces below 40 nm separations between the SiC-Si/C and Ru surfaces indicated that the short-range roughness effects on the Casimir force increase in magnitude with increasing metallic behavior of the plate surface. Therefore, not only the material optical properties but also the conductive state and roughness of the surface layers must be carefully taken into account in short range Casimir interactions between more complex dielectric materials.

The interaction with time-dependent external fields, especially the interplay between time-dependent driving and quantum correlations, changes the familiar picture of electron transport through nanoscale systems. Although the exact solution of the problem of AC quantum transport of noninteracting electrons has been known for more than two decades, the treatment of correlated particles presents a significant theoretical challenge. In this paper, using the perturbative separation of fast electron tunneling and slow driving timescales, we developed a practical approach for time-dependent quantum transport with nonequilibrium Green's functions. The fast electronic dynamics is associated with relative time while the slow driving is related to the central time in the Green's functions. The ratio of characteristic electron tunneling time over the period of harmonic driving is used as a small parameter in the theory to obtain convergent time-derivative expansions of the Green's functions. This enables the algebraic solution of the Kadanoff-Baym equations in Wigner space. Consequently, we produced analytical expressions for dynamical corrections to advanced, retarded, and lesser Green's functions, as well as an improved expression for AC electric current. The method developed is applicable to the general case of multichannel electron transport through a correlated central region. The theory is applied to different transport scenarios: time-dependent transport through a driven single-resonant level is compared to exact results; and electron transport through a molecular junction described by the Holstein model with a time-oscillating voltage bias is also investigated.

Spin injection is a powerful experimental probe into a wealth of nonequilibrium spin-dependent phenomena displayed by materials with sizable spin-orbit interactions. Here, we present a theory of coupled spin-charge diffusive transport in spin-valve devices built from two-dimensional materials. The formalism takes into account realistic spin-orbit effects with both spatially uniform and random components in van der Waals materials arising from the interfacial breaking of inversion symmetry. The various charge-to-spin conversion mechanisms known to be present in diffusive metals, including the spin Hall effect and several mechanisms contributing to current-induced spin polarization are accounted for. Our analysis shows that the dominant conversion mechanisms can be discerned by analyzing the nonlocal resistance of the spin valve for different polarizations of the injected spins and as a function of the applied in-plane magnetic field.

We report structural studies of the spin-orbit Mott insulator family ${\mathrm{K}}_x{\mathrm{Ir}}_y{\mathrm{O}}_2$, with triangular layers of edge-sharing ${\mathrm{IrO}}_6$ octahedra bonded by potassium ions. The potassium content acts as a chemical tuning parameter to control the amount of charge in the Ir-O layers. Unlike the isostructural families with Ir replaced by Co or Rh ($y=1$), which are metallic over a range of potassium compositions $x$, we instead find insulating behavior with charge neutrality achieved via iridium vacancies, which order in a honeycomb supercell above a critical composition $x_c$. By performing density functional theory calculations we attribute the observed behavior to a subtle interplay of crystal-field environment, local electronic correlations, and strong spin-orbit interaction at the ${\mathrm{Ir}}^{4+}$ sites, making this structural family a candidate to display Kitaev magnetism in the experimentally unexplored regime that interpolates between triangular and honeycomb structures.

Bond-order potentials have been developed for the ${\mathrm{Ti}}_3{\mathrm{AlC}}_2$ and ${\mathrm{Ti}}_3{\mathrm{SiC}}_2$ MAX phases within the Tersoff formalism. Parameters were determined by independently considering each interatomic interaction present in the system and fitting them to the relevant structural, elastic, and defect properties for a number of unary, binary, and ternary structures. A number of material properties, including those not used in the fitting procedure, are reproduced with a high degree of accuracy when compared to experiment and ab initio calculations. Additionally, well-documented MAX phase behaviors such as plastic anisotropy and kinking nonlinear elasticity are demonstrated to be captured by the potentials. As a first highly accurate atomistic model for MAX phases, these potentials provide the opportunity to study some of the fundamental mechanisms behind unique MAX phase properties. Additionally, the fitting procedure employed is highly transferable and should be applicable to numerous other MAX phases.