Numerous materials feature unexplained phases with invisible or hidden order of electronic origin. A particularly mysterious case is that of ${\mathrm{Tb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$, which avoids magnetic order to the lowest temperatures, but nevertheless has an unexplained second-order phase transition near $T=0.5$ K. Our ultrasound measurements of ${\mathrm{Tb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ provide direct evidence of a huge softening followed by strong hardening of the structural lattice below $T=0.5$ K. In the absence of magnetic order at this temperature, our results provide conclusive evidence for the proposed quadrupolar order and emphasize the importance of higher-order multipolar interactions in rare-earth frustrated magnets.

Graphite has been used as a neutron moderator or reflector in many nuclear reactors. The irradiation of graphite in a nuclear reactor results in a complex population of defects. Heating of the irradiated graphite at high temperatures results in annihilation of the defects with release of an unusually large energy, called the Wigner energy. From various experiments on highly irradiated graphite samples from the CIRUS reactor at Trombay and ab initio simulations, we have identified various 2-, 3-, and 4-coordinated topological structures in defected graphite, and provided a microscopic mechanism of defect annihilation on heating and release of the Wigner energy. The annihilation process involves cascading cooperative movement of atoms in multiple steps involving an intermediate structure. Our work provides insights in understanding of the defect topologies and annihilation in graphite which is of considerable importance to wider areas of graphitic materials including graphene and carbon nanotubes.

Entanglement phase transitions in quantum chaotic systems subject to projective measurements and in random tensor networks have emerged as a new class of critical points separating phases with different entanglement scaling. We propose a mean-field theory of such transitions by studying the entanglement properties of random tree tensor networks. As a function of bond dimension, we find a phase transition separating area-law from logarithmic scaling of the entanglement entropy. Using a mapping onto a replica statistical mechanics model defined on a Cayley tree and the cavity method, we analyze the scaling properties of such transitions. Our approach provides a tractable, mean-field-like example of an entanglement transition. We verify our predictions numerically by computing directly the entanglement of random tree tensor network states.

Use of a spin-polarized current for the manipulation of magnetic domain walls in ferromagnetic nanowires has been the subject of intensive research for many years. Recently, due to technological advancements, creating nanocontacts with special characteristics is becoming more and more prevalent. We now present a full quantum investigation of the magnetoresistance and the spin transfer torque in a domain wall which is embedded in a nanocontact of ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$, where the size of the domain wall becomes a relevant tunable parameter. The dependence on the domain wall width as well as the spatial dependence of the torque along the domain wall can be analyzed in complete detail. The magnetoresistance drops with increasing domain wall width as expected but also shows characteristic modulations and points of resonant spin-flip transmission. The spin transfer torque has both significant in-plane and out-of-plane contributions even without considering relaxation. A closer inspection identifies contributions from the misalignment of the spin density for short domain walls as well as an effective gauge field for longer domain walls, both of which oscillate along the domain wall.

We report synthesis and excitonic emission properties of high-quality micrometer-thick $\mathrm{M}{\mathrm{g}}_x\mathrm{Z}{\mathrm{n}}_{1-x}\mathrm{O}$ films ($x=0.04$ and 0.10) grown on $a$-sapphire substrates by pulsed laser deposition. The band gap $E_g$ values of the $x=0.04$ and 0.10 samples have been shown to be larger than that of pure ZnO by ∼0.1 and ∼0.3 eV, respectively. From the investigations of the stimulated emission due to the exciton-exciton (ex-ex) scattering process at 3 K, the exciton binding energies $E_b$ of the $x=0.04$ and 0.10 samples are determined to be $80\pm 5$ and $120\pm 10\mathrm{meV}$, respectively, which are substantially larger than that of pure ZnO ($E_b=60\mathrm{meV}$). The present observation demonstrates that $E_b$ of ZnO-based materials can be increased to 100% by controlling the synthetic and alloying conditions. Purely excitonic stimulated emissions are observed in the temperature range from 3 to 300 K without showing any symptoms of electron-hole plasma emission. The mechanism of the optical gain changes from the ex-ex to ex-electron (ex-el) scattering process at 200 and 250 K for the $x=0.04$ and 0.10 samples, respectively, with increasing temperature. We argue that the enhancement of $E_b$ occurs as a consequence of a complex, combined effect of Mg addition on the effective electron/hole masses and the dielectric constant of the $\mathrm{M}{\mathrm{g}}_x\mathrm{Z}{\mathrm{n}}_{1-x}\mathrm{O}$ films.

We investigated quantum Hall states in an inverted HgTe quantum well (QW) close to the critical thickness using transconductance fluctuation (TF) measurements. In the conduction band, several integer quantum Hall states were observed, corresponding to filling factors $\nu =1,2,3,4$. For magnetic fields above 2 T, quantum Hall states $\nu =0$ were observed in the normal gap. These observations agreed well with the previous studies of quantum Hall states on GaAs QWs and graphene. Interestingly, TFs corresponding to anomalous positive filling factor $\nu$ were clearly observed in the valence band. We attribute the emergence of those TFs to the localization and charging of the heavy holes located in the side maxima of the valence band.

In two-dimensional (2D) electron systems, an off-resonant high-frequency circularly polarized electromagnetic field can induce the quasistationary bound electron states of repulsive scatterers. As a consequence, the resonant scattering of conduction electrons through the quasistationary states and the capture of conduction electrons by the states appear. The present theory describes the transport properties of 2D electron gas irradiated by a circularly polarized light, which are modified by these processes. Particularly, it is demonstrated that irradiation of 2D electron systems by the off-resonant field results in the quantum correction to conductivity of resonant kind.

In this study, we investigated theoretically the Mott-insulating phase of a deficient spinel chalcogenide ${\mathrm{GaV}}_4{\mathrm{S}}_8$, which is known to form a tetrahedral ${\mathrm{V}}_4{\mathrm{S}}_4$ cluster unit that results in molecular orbitals (MOs) with a narrow bandwidth in the noninteracting limit. We used a cluster extension of charge self-consistent embedded dynamical mean-field theory to study the impact of strong intracluster correlations on the spectral properties as well as the structural degrees of freedom of the system. We found that the strong tetrahedral clustering renders the atomic Mott picture ineffective, and that the resulting MO picture is essential to describe the Mott phase. It was also found that, while the spectral properties can be qualitatively described by the truncation of the Hilbert space down to the lowest-energy MO, a proper description of the structural degrees of freedom requires the inclusion of multi-MO correlations that span a larger energy window. Specifically, we found that the lowest-energy MO description overemphasizes the clustering tendency, while the inclusion of the Hund's coupling between the lower- and higher-energy MOs corrects this tendency, bringing the theoretically predicted crystal structure into good agreement with the experiment.

We have performed angle-resolved photoemission spectroscopy of Bi(111) thin films grown on Si(111), and investigated the evolution of band structure with temperature. We revealed an unexpectedly large temperature variation of the energy dispersion for the Rashba-split surface state and the quantum-well states, as seen in the highly momentum-dependent energy shift as large as 0.1 eV. A comparison of the band dispersion between experiment and first-principles band-structure calculations suggests that the interlayer spacing at the topmost Bi bilayer expands upon temperature increase. The present study provides a pathway for investigating the interplay between lattice and electronic states through the temperature dependence of band structure.

The Smrcka-Streda version of Kubo's linear response formula is widely used in the literature to compute nonequilibrium transport properties of heterostructures. It is particularly useful for the evaluation of intrinsic transport properties associated with the Berry curvature of the Bloch states, such as anomalous and spin Hall currents as well as the dampinglike component of the spin-orbit torque. Here we demonstrate in a very general way that the widely used decomposition of the Kubo-Bastin formula introduced by Smrcka and Streda contains an overlap, which has lead to widespread confusion in the literature regarding the Fermi surface and Fermi sea contributions. To remedy this pathology, we propose a decomposition of the Kubo-Bastin formula based on the permutation properties of the correlation function and derive a set of formulas, without an overlap, that provides direct access to the transport effects of interest. We apply these formulas to selected cases and demonstrate that the Fermi sea and Fermi surface contributions can be uniquely addressed with our symmetrized approach.

We propose and theoretically analyze a vibrational spectroscopy imaging technique, termed, termed electron- and light-induced stimulated Raman (ELISR) scattering, that combines the high spatial resolution of electron microscopy with the molecular sensitivity of surface-enhanced Raman spectroscopy. With ELISR, electron-beam excitation of plasmonic nanoparticles is utilized as a spectrally-broadband but spatially-confined Stokes beam in the presence of a diffraction-limited pump laser. To characterize this technique, we develop a numerical model and conduct full-field electromagnetic simulations to investigate two distinct nanoparticle geometries, nanorods, and nanospheres, coated with a Raman-active material. Our results show the significant $({10}^6–{10}^7)$ stimulated Raman enhancement that is achieved with dual electron and optical excitation of these nanoparticle geometries. Importantly, the spatial resolution of this vibrational spectroscopy for electron microscopy is solely determined by the nanoparticle geometry and the plasmon mode volume. Our results highlight the promise of ELISR for simultaneous high-resolution electron microscopy with sub-diffraction-limited Raman spectroscopy, complementing advances in super-resolution microscopy, correlated light and electron microscopy, and vibrational electron energy loss spectroscopy.

We theoretically investigate two spin configurations of a negative trion in monolayer tungstenides, called the intravalley spin singlet and intervalley spin triplet. The energy splitting between them is attributed to the trion fine structure, arising from the electron-hole exchange interaction. Using the Rytova-Keldysh potential, we show that the splitting which acts as an effective Zeeman effect is tuned by varying the dielectric environment. For the negative trion in the ${\mathrm{WSe}}_2$ monolayer encapsulated in hexagonal boron nitride, the calculated splitting is around 6 meV, which is in agreement with recent experiments. Owing to the strong exchange, we show that the negative intervalley trion acquires a giant Berry curvature. This valley pseudospin-dependent Berry curvature largely dominates the intrinsic Berry curvature arising from the Bloch bands. An in-plane applied electric field gives rise to an anomalous valley Hall transport. We investigate the dependence of the anomalous Hall conductivity and magnetization on several parameters such as dielectric screening length and doping. The trion Hall effect can be considered as the basis for valley-based electronics applications.

We employ synchrotron-based near-field infrared spectroscopy to reveal the vibrational properties of bulk, few-sheet, and single-sheet members of the $M{\mathrm{PS}}_3$ $(M=\mathrm{Mn}, \mathrm{Fe}, \mathrm{Ni})$ family of materials and compare our findings with complementary lattice dynamics calculations. ${\mathrm{MnPS}}_3$ and the Fe analog are similar in terms of their symmetry crossovers, from $C2/m$ to $P\overline{3}1m$, as the monolayer is approached. These states differ as to the presence of a $C_3$ rotation around the metal center. On the other hand, ${\mathrm{NiPS}}_3$ does not show a symmetry crossover, and the lack of a $B_u$ symmetry mode near 450 ${\mathrm{cm}}^{-1}$ suggests that $C_3$ rotational symmetry is already present, even in the bulk material. We discuss these findings in terms of local symmetry and temperature effects as well as the curious relationship between these symmetry transformations and those that take place under pressure.

The development of next-generation quaternary chalcogenides, such as $\mathrm{C}{\mathrm{u}}_2\mathrm{ZnSn}{\mathrm{S}}_4$ (CZTS) and $\mathrm{C}{\mathrm{u}}_2\mathrm{ZnGe}{\mathrm{S}}_4$ (CZGS), for solar energy and thermoelectric applications hinges upon both careful experimentation and accurate quantum mechanical modeling. To address the latter, many have turned to density functional theory (DFT), which offers several choices for the approximate treatment of electron exchange and correlation (XC). Popular XC functionals include the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) and the recently developed strongly constrained and appropriately normed (SCAN) meta-GGA. Extensions of DFT functionals, such as adding a Hubbard $U$ correction and introducing a fraction of the Fock exchange (hybrid functionals), have been used widely to model systems containing $3d$ metal ions. However, no studies yet have compared comprehensively PBE($+U$) and SCAN($+U$) in the quality of their predictions of the bulk and defect thermodynamics of quaternary chalcogenides, which play a critical role in device fabrication and performance. Hence, here we calculate the (i) 0 K formation energies of bulk Ge compounds and (ii) neutral defect formation energies including charge-balanced (e.g., $\mathrm{C}{\mathrm{u}}_{\mathrm{Zn}}+\mathrm{Z}{\mathrm{n}}_{\mathrm{Cu}}$) and charge-imbalanced (e.g., $\mathrm{C}{\mathrm{u}}_{\mathrm{Sn}}$) combinations of antisites and vacancies in CZTS and CZGS using the PBE, $\mathrm{PBE}+U$, SCAN, $\mathrm{SCAN}+U$, and the hybrid Heyd-Scuseria-Ernzerhof XC frameworks. We find that the formation energies of charge-imbalanced defects are more sensitive to the choice of the XC functional than those of charge-balanced defects, which can be explained by the differences in the extent of penalization of defect-generated delocalized electrons/holes by PBE, $\mathrm{PBE}+U$, SCAN, and $\mathrm{SCAN}+U$. Additionally, our results show that SCAN systematically underbinds Ge-containing compounds, thus highlighting the need for even further improvement of XC functionals. Based on our findings, we recommend the use of SCAN for modeling quaternary chalcogenides because its errors are systematic, and it has the firmest theoretical underpinning. Our work provides guidance for future modeling of quaternary chalcogenides.

Compression-induced solidification has been observed in cerium on nanosecond timescales. A series of experiments was conducted in Sandia National Laboratories’ Z Facility in which cerium was shock melted and subsequently shocklessly, or ramp, loaded across the melt line inducing solidification. The signature of solidification manifested in the recovery of material strength and the propagation of waves at the local elastic sound velocity. Density functional theory simulations of cerium along the experimental phase-space path exhibit spontaneous freezing to a tetragonal phase at the same pressure and closely predict the observed physical properties of solid and liquid cerium near melt.

In this study, the magnetic field effect was investigated on the magnetic and electric properties of a chiral-polar-ordered corundum, ${\mathrm{Ni}}_2\mathrm{InSb}{\mathrm{O}}_6$. Single-crystal soft x-ray and neutron diffraction measurements were used to verify the magnetic modulations with long wavelengths. The modulation direction tends to align along the magnetic field when it is applied perpendicular to the polar axis, suggesting that the nearly proper-screw-type helicoid should be formed below 77 K owing to the dominant activation of chirality-induced uniform Dzyaloshinskii-Moriya interactions despite the existence of both chirality and polarity. When a high magnetic field is applied perpendicular to the polar axis, a helix-to-canted antiferromagnetic transition is observed through the intermediate soliton lattice type state. In contrast, a magnetic field applied along the polar axis induces a first-order metamagnetic transition. These metamagnetic transitions accompany a change in the electric polarization along the polar axis.

We report the measurement of spin current-induced charge accumulation, the inverse Edelstein effect (IEE), on the surface of single-crystal candidate topological Kondo insulator $\mathrm{Sm}{\mathrm{B}}_6$. The robust surface conduction channel of $\mathrm{Sm}{\mathrm{B}}_6$ has been shown to exhibit a large degree of spin-momentum locking, and the spin-polarized current through an external ferromagnetic contact induces spin-dependent charge accumulation on the surface of $\mathrm{Sm}{\mathrm{B}}_6$. The dependence of the IEE signal on the bias current, an external magnetic field direction, and temperature are consistent with an anticlockwise spin texture of the $\mathrm{Sm}{\mathrm{B}}_6$ surface band in momentum space. The direction and magnitude of this effect, compared with the normal Edelstein signal, are clearly explained by the Onsager reciprocal relation. Furthermore, we estimate the spin-to-charge conversion efficiency, i.e., the IEE length, to be 4.46 nm, which is an order of magnitude larger than the efficiency found in other typical Rashba interfaces, implying that the Rashba contribution to the IEE signal may be small. Building upon existing reports on the surface charge and spin conduction nature of this material, our results provide additional evidence that the surface of $\mathrm{Sm}{\mathrm{B}}_6$ supports a spin-polarized conduction channel.

Following the recent excitement in two-dimensional (2D) magnetism, we propose a yet-unexplored class of 2D ferromagnets derived out of Cr-based organic-inorganic layered compounds. Our density functional study in conjunction with model calculation establishes that assisted by the finite magnetocrystalline anisotropy, the ferromagnetic long-range order survives at the atomically thin limit consisting of monolayer of Cr spins, with calculated magnetic transition temperatures of $\sim 10–20\text{−}\mathrm{K}$. Upon application of modest magnetic field, the magnetic transition temperature is found to increase substantially. The calculated exfoliation energy suggests that synthesis of these 2D compounds through mechanical exfoliation is possible, which should encourage future experiments on this proposed class of 2D magnets.