It is shown that the phase shift between an applied weak alternating magnetic field and the magnetocaloric response signal of the magnetic material is drastically sensitive to the order of phase transition. Namely, at the second-order phase transition, the phase shift does not depend on the magnetic field magnitude, while in the first-order phase transition this one depends significantly on the field strength. We have shown that this effect follows from the general critical dynamics theory.

At and near charge neutrality, monolayer graphene in a perpendicular magnetic field is a quantum Hall ferromagnet. In addition to the highly symmetric Coulomb interaction, residual lattice-scale interactions, Zeeman, and sublattice couplings determine the fate of the ground state. Going beyond the simplest model with ultra-short-range residual couplings to more generic couplings, one finds integer phases that show the coexistence of magnetic and lattice order parameters. Here, we show that fractional quantum Hall states in the vicinity of charge neutrality have even richer phase diagrams, with a plethora of phases with simultaneous magnetic and lattice symmetry breaking.

Vanadium metal is extensively used in modern technology, especially in the alloy and steel industry; it exhibits anomalous thermal expansion behavior across the entire temperature regime. Here, we extensively investigate the phonon anharmonicity contributed by volume change (implicit anharmonicity) and thermal amplitude (explicit anharmonicity) and their impact on thermal expansion and thermal transport in vanadium up to 2000 K, close to the melting temperature. We compared the different methods to evaluate the phonon anharmonicity, namely, the quasiharmonic approximation (QHA), temperature-dependent effective potential (TDEP) method, and machine-learned force-field-based molecular dynamics (MLMD) simulations. At 300 K, QHA overestimates the thermal expansion coefficient by $\sim 20\%$, while TDEP provides an excellent description of the experimental data. This reveals a significant explicit anharmonicity at room temperature. At higher temperatures, the experimental thermal expansion coefficient continues to rise up to twice the QHA estimates, indicating significant anharmonicity. However, TDEP underestimates the experimental observations, as it only includes low orders of anharmonicity. MLMD, which includes all the anharmonic effects, successfully explains the anomalous expansion behavior over 500–2000 K. It is expected that the electronic entropy and the electron-phonon interaction would influence the thermal expansion, but their effect appears to be small. We used MLMD to calculate the spectral energy density of phonons up to 2000 K, which revealed small phonon shifts but large broadening. Above $\sim 2000\mathrm{K}$, MLMD captures the melting and reproduces the experimental volume increase on melting. We have also calculated the lattice thermal conductivity using the TDEP-based third-order-perturbation method and the MLMD-based Green-Kubo method, over 300–1500 K, which includes a higher order of anharmonicity. This brings out the important contribution from the four and higher orders of phonon anharmonicity to the lattice thermal conductivity.

The lack of both time-reversal and spatial inversion symmetry in polar magnets is a prerequisite for the occurrence of optical magnetoelectric effects such as nonreciprocal directional dichroism, i.e., a difference in refractive index and absorption for two counter-propagating electromagnetic waves, which has the potential for the realization of optical diodes. In particular, antiferromagnetic materials with magnetic excitations in the THz range such as ${\mathrm{Fe}}_2{\mathrm{Mo}}_3{\mathrm{O}}_8$ are promising candidates for next-generation spintronic applications. In a combined experimental and theoretical effort we investigated the THz excitations of the polar honeycomb antiferromagnet ${\mathrm{Fe}}_2{\mathrm{Mo}}_3{\mathrm{O}}_8$ in external magnetic fields and their nonreciprocal directional dichroism, together with the temperature dependence of the electronic transitions in the mid- and near-infrared frequency range. Using an advanced single-ion approach for the Fe ions, we are able to describe optical excitations from the THz to the near-infrared frequency range quantitatively and model the observed nonreciprocal directional dichroism in the THz regime successfully.

Employing a first-principles and model Hamiltonian approach, we work out the electronic properties of polar-polar ${\mathrm{LaVO}}_3/{\mathrm{KTaO}}_3$ (LVO/KTO, 001) heterostructures, with up to six layers of KTO and five layers of LVO. Our analyses indicate the existence of multiple Lifshitz transitions (LTs) within the $t_{2g}$ bands, which can be fine-tuned by adjusting the number of LVO layers or applying gate voltage. Contrary to the experimental report, spin-orbit coupling is found to be negligible, originating solely from the Ta $5d_{xy}$-derived band of KTO, while the $5d_{xz}$ and $5d_{yz}$ bands are considerably away from the Fermi level while LVO overlayers having no role in it. Magnetic properties of the heterostructures, due to vanadium ions, exhibit a pronounced sensitivity to the number of LVO and KTO layers. Our calculations indicate that the interlayer AFM, (so called A-AFM), is energetically most favorable. This is further supported by ground state energy calculations on extended $\sqrt{2}\times \sqrt{2}$ supercells. Moreover, we find that an insulator to metal transition at the interface requires four LVO layers, corroborating the experimental observation. The interfaces featuring ferromagnetic (FM) ground states turn out to be half-metallic after the critical thickness is reached. Considerations of the magnetic interactions appear crucial for the experimentally observed critical thickness for metallicity.

We demonstrate the formation of orbital and spin Hall effects (OHE/SHE) in the 1T phase of nonmagnetic transition metal dichalcogenides. With the aid of density functional theory calculations and model Hamiltonian studies on ${MX}_2$ ($M$ = Pt, Pd; and $X$ = S, Se, and Te), we show an intrinsic orbital Hall conductivity ($\sim {10}^3\hslash /e{\mathrm{\Omega }}^{-1}{\text{cm}}^{-1}$), which primarily emerges due to the orbital texture around the valleys in the momentum space. The robust spin-orbit coupling in these systems induces a sizable SHE out of OHE. Furthermore, to resemble the typical experimental setups, where the magnetic overlayers produce a proximity magnetic field, we examine the effect of magnetic field on OHE and SHE and showed that the latter can be doubled in these class of compounds. With a giant OHE and tunable SHE, the 1T-TMDs are promising candidates for spin and orbital driven quantum devices such as SOT-MRAM, spin nano-oscillators, spin logic devices, etc., and to carry out spin-charge conversion experiments for fundamental research.

Impurities exist inevitably in two-dimensional materials as they spontaneously adsorb onto the surface during fabrication, usually exerting detrimental effects on electronic transport. Here, we focus on a special type of impurities that preferentially adsorb onto the hollow regions of graphene nanoribbons (GNRs), and study how they affect the quantum transport in GNRs. Contrary to previous knowledge that random adatoms should localize electrons, the so-called Anderson localization, noteworthy quantized conductance peaks (QCPs) are observed at specific electron energies. These QCPs are remarkably robust against variations in system size, GNR edge, and adatom properties, and they can reappear at identical energies following an arithmetic sequence of device width. Further investigation of the wave function reveals a unique transport mode at each QCP energy which transmits through disordered GNRs reflectionlessly, while all the others become fully Anderson localized, indicating the survival of quantum ballistic transport in the localized regime. Our findings highlight the potential utility of hollow adatoms as a powerful tool to manipulate the conductivity of GNRs, and deepen the understanding of the interplay between impurities and graphene.

The effects of different forms of weak measurements on the nature of the measurement induced phase transition are theoretically studied in hybrid random quantum circuits of qubits. We use a combination of entanglement measures, ancilla purification dynamics, and a transfer matrix approach to compute the critical exponents, the effective central charge, and the multifractal spectrum of the measurement induced transitions. We compare weak measurements with an infinite number of discrete outcomes to a protocol with only a pair of outcomes and find that to within our numerical accuracy the universal critical properties are unaffected by the weak measurement protocols and are consistent with the universality class found for strong projective measurements.

We report on the first example of epitaxial ${\mathrm{Mn}}_{3+\delta }\mathrm{Ge}$ thin films with a cubic $L{1}_2$ structure. The films are found to exhibit frustrated ferromagnetism with an average magnetization corresponding to $0.98\pm 0.06{\mu }_B$/Mn, far larger than the parasitic ferromagnetism in hexagonal ${\mathrm{Mn}}_3\mathrm{Ge}$ and the partially compensated ferrimagnetism in tetragonal ${\mathrm{Mn}}_3\mathrm{Ge}$. The Hall conductivity is the largest reported for the kagome magnets with a low-temperature value of ${\sigma }_{xy}=1587$ S/cm. Density functional calculations predict that a chiral antiferromagnetic structure is lower in energy than a ferromagnetic configuration in an ordered stoichiometric crystal. However, chemical disorder driven by the excess Mn in our films explains why a frustrated ${120}^{\circ }$ spin structure is not observed. Comparisons between the magnetization and the Hall resistivity indicate that a noncoplanar spin structure contributes the Hall signal. Anisotropic magnetoresistance and planar Hall effect with hysteresis up to 14 T provides further insights into this material.

Quantum simulations are quickly becoming an indispensable tool for studying particle transport in correlated lattice models. One of the central topics in the study of transport is bad-metal behavior, characterized by the direct-current resistivity linear in temperature. In the fermionic Hubbard model, optical conductivity has been studied extensively, and a recent optical lattice experiment demonstrated bad-metal behavior in qualitative agreement with theory. Far less is known about transport in the bosonic Hubbard model. We investigate the conductivity in the Bose-Hubbard model and focus on the regime of strong interactions and high temperatures. We use numerically exact calculations for small lattice sizes. At weak tunneling, we find multiple peaks in the optical conductivity that stem from the Hubbard bands present in the many-body spectrum. This feature slowly washes out as the tunneling rate gets stronger. At high temperature, we identify a regime of $T$-linear resistivity, as expected. When the interactions are very strong, the leading inverse-temperature coefficient in conductivity is proportional to the tunneling amplitude. As the tunneling becomes stronger, this dependence takes quadratic form. At very strong coupling and half filling, we identify a separate linear resistivity regime at lower temperature, corresponding to the hard-core boson regime. Additionally, we unexpectedly observe that at half filling, in a big part of the phase diagram, conductivity is an increasing function of the coupling constant before it saturates at the hard-core-boson result. We explain this feature based on the analysis of the many-body energy spectrum and the contributions to conductivity of individual eigenstates of the system.

Pair-breaking edges of $d$-wave superconductors feature Andreev bound states at the Fermi energy. Since these states are energetically highly unfavorable they are susceptible to effects that shift them to finite energy. We investigate the free energy of two different mechanisms: spontaneous phase gradients in the superconducting order parameter and surface ferromagnetism caused by Fermi liquid interaction effects. We find that the surface magnetization appears at lower temperatures than the spontaneous current flow of the phase-crystal state. The magnetic state can, however, be energetically favorable at lower temperatures for sufficiently strong Fermi liquid effects. Thus, first-order transitions between the two states as function of system temperature are possible, suggesting a rich low-temperature phase diagram in $d$-wave superconductors.

We theoretically explore the possibility of realizing the symmetry-protected topological Haldane phase of spin-1 chains in a tunable hybrid platform of superconducting islands (SIs) and quantum dots (QDs). Inspired by recent findings suggesting that an appropriately tuned QD-SI-QD block may behave as a robust spin-1 unit, we study the behavior of many such units tunnel-coupled into linear chains. Our efficient and fully microscopic modeling of long chains with several tens of units is enabled by the use of the surrogate model solver [V. V. Baran et al., Phys. Rev. B 108, L220506 (2023); Phys. Rev. B 109, 224501 (2024)]. Our numerical findings indicate that the QD-SI-QD chains exhibit emblematic features of the Haldane phase, such as fractional spin-1/2 edge states and nonvanishing string order parameters, and that these persist over a sizable region of parameter space.

Chiral carbon nanostructures have been found to display unexpected magnetic behaviors. Several theoretical calculations performed in the macroscopic limit $\mathbf{q}=0$ have addressed or predicted some of these findings. To gain more insight into the magnetism of these systems at finite $\mathbf{q}$, here we use linear response theory to calculate the wave-vector-dependent spin susceptibility $\chi (\mathbf{q},0)$ in a half-filling tight-binding model of a helix of carbon atoms with intrinsic spin-orbit coupling (SOC). We find that at the nesting wave number $q=2k_F$ the paramagnetic state of the system is unstable with respect to the formation of a spin-density-wave type state. Chirality has a small effect on the paramagnetic phase but has no impact on the spin-density-wave type state.

An electron-phonon system at commensurate filling often displays charge order (CO) in the ground state. Such a system subject to a laser pulse shows a wide variety of behavior. A weak pulse sets up low-amplitude oscillations in the order parameter, with slow decay to a slightly suppressed value. A strong pulse leads to destruction of the charge order with the order parameter showing rapid, oscillatory, decay to zero. The regime in-between, separating the weak pulse CO sustained state from the strong pulse CO destroyed state, shows an initial rapid decay of the order parameter, followed by a low-amplitude quiescent state, and the rise to a finite steady-state value over a timescale ${\tau }_{\text{cr}}$. The steady-state value goes to zero and ${\tau }_{\text{cr}}$ diverges as the pulse amplitude is increased towards a critical strength. We provide a complete characterization of the dynamics in this nonequilibrium problem for varying electron-phonon coupling and pulse strength, highlight the multiple gap-closing and -opening transitions that occur, and suggest a simple dynamical model incorporating the pulse-induced nonequilibrium electron population.

In double perovskite oxides ($A_2{BB}^{\prime }{\mathrm{O}}_6$), magnetism often arises from diluted magnetic lattices, created by combining a perovskite structure with localized $3d$ magnetic elements ($B$) alongside another perovskite lattice containing nearly nonmagnetic delocalized $4d/5d$ elements ($B^{\prime }$). Alternatively, the magnetic lattice can consist entirely of $3d$ elements, with one being completely nonmagnetic with a $d^0$ state. ${\mathrm{La}}_2{\mathrm{CoTiO}}_6$ (LCTO), a representative double perovskite oxide, contains Ti in a nonmagnetic state with a $d^0$ electron configuration due to its $4^{+}$ oxidation state. Experimental evidence shows that LCTO possesses a monoclinic structure (space group $P{2}_1/n$) and behaves as an antiferromagnet with a Néel temperature of 14.6 K. Through first-principles electronic structure calculations, we uncover that adjusting external hydrostatic pressure induces a sequence of phase transitions: from antiferromagnetic insulator (AFM-I) to antiferromagnetic metal (AFM-M), and ultimately to itinerant nonmagnetic metal (NM-M). The transition from AFM-I to AFM-M at $\sim 42$ GPa pressure coincides with a shift in spin states, moving from a high-spin (HS) state to a low-spin (LS) state, while Co retains a $d^7$ configuration. Distortion within the monoclinic structure under pressure plays a pivotal role in the spin-state transition. At the AFM-I to AFM-M transition, we observe a sharp decrease in the ratio of the octahedral volumes occupied by Co and Ti. Such change in ratio is linked to variations in octahedral volumes, akin to a breathing mode distortion. We explore the impact of the breathing mode distortion by examining a highly symmetric theoretical structure (space group $I4/mmm$), achieved by optimizing the structure with all $\angle$ Co-O-Ti angles set to ${180}^{\circ }$. Remarkably, the LS state in the LCTO theoretical structure persists under ambient pressure conditions, underscoring the unique role of breathing mode distortion in the monoclinic phase, facilitating the HS to LS transition under pressure. In the LS state, LCTO displays metallic behavior, even with a substantial local correlation (Hubbard $U$) on Co, as large as $U$ = 6 eV. The spin-state transition is further elucidated through an energy level diagram, illustrating a significant modification in the crystal field splitting between Co-$t_{2g}$ and Co-$e_g$ levels, driven by the robust hybridization of Co-$d$ and O-$p$ orbitals. Finally, with a further increase in pressure, the system attains the NM-M phase at $\sim 130$ GPa, leading to the complete suppression of the magnetic moment on Co.

The electronic band structure of ${\mathrm{V}}_2{\mathrm{O}}_5$ is calculated using an all-electron quasiparticle self-consistent (QS) $GW$ method, including electron-hole ladder diagrams in the screening of $W$, named $\mathrm{QS}G\widehat{W}$ and using a full-potential linearized muffin-tin-orbital basis set. The optical dielectric function calculated with the Bethe-Salpeter equation (BSE) exhibits excitons with large binding energy, consistent with spectroscopic ellipsometry data and other recent calculations using a pseudopotential plane-wave-based implementation of the many-body-perturbation theory approaches. Convergence issues are discussed. Sharp peaks in the direction perpendicular to the layers at high energy are found to be an artifact of the truncation of the numbers of bands included in the BSE calculation of the macroscopic dielectric function. The static (electronic screening only) dielectric constant ${\varepsilon }_1(\omega =0)$ gives indices of refraction in good agreement with experiment. The exciton wave functions are analyzed in various ways. They correspond to charge transfer excitons with the hole primarily on oxygen and electrons on vanadium, but depending on which exciton, the distribution over different oxygens changes. The dark exciton at 2.6 eV is the most localized and has the highest weight on the bridge oxygen, while the lowest bright excitons for in-plane polarizations at 3.1 eV for $\mathbf{E}\parallel \mathbf{a}$ and 3.2 eV for $\mathbf{E}\parallel \mathbf{b}$ have their higher weight on the chain and vanadyl oxygens. The exciton wave functions have a spread of about 5–15Å, with asymmetric character for the electron distribution around the hole depending on which oxygen the hole is fixed at. The same method applied first to bulk layered ${\mathrm{V}}_2{\mathrm{O}}_5$ is here applied to monolayer ${\mathrm{V}}_2{\mathrm{O}}_5$. The monolayer quasiparticle gap increases inversely proportional to interlayer distance once the initial interlayer covalent couplings are removed which is thanks to the long-range nature of the self-energy and the reduced screening in a two-dimensional system. The optical gap on the other hand is relatively independent of interlayer spacing because of the compensation between the self-energy gap shift and the exciton binding energy, both of which are proportional to the screened Coulomb interaction $\widehat{W}$. Recent experimental results on very thin layer ${\mathrm{V}}_2{\mathrm{O}}_5$ obtained by chemical exfoliation provide experimental support for an increase in gap.

Disorder and doping have profound effects on the intrinsic physical mechanisms of superconductivity. In this paper, we employed the determinant quantum Monte Carlo method to investigate the symmetry-allowed superconducting orders on the two-dimensional honeycomb lattice within the Hubbard model, using doped graphene as the carrier, focusing their response to bond disorder. Specifically, we calculated the pairing susceptibility and effective pairing interactions for the $d+id$ wave and extended $s$-wave pairings for different electron densities and disorder strengths. Our calculations show that at high electron densities, increased disorder strength may lead to a transform from $d+id$ wave dominance to extended $s$ wave dominance. However, at lower electron densities, neither of the two superconducting pairings appears under larger disorder strength. Our calculations may contribute to a further understanding of the superconducting behavior in doped materials affected by disorder.

Intertwined charge and magnetic fluctuations in high-$T_{\text{c}}$ copper oxide superconductors (cuprates) are hypothesized to be a consequence of their correlated electronic nature. Among other observables, this is apparent in the doping dependence of incommensurate magnetic order, known as the Yamada relation (YR). We analyze the Hubbard model to challenge the universality of YR as a function of interaction strength $U$ through Kotliar-Ruckenstein slave-boson (SB) mean-field theory and truncated-unity functional renormalization group (TUFRG). While TUFRG tends to lock in to a doping dependence of the incommensurate magnetic ordering vector obtained for the perturbative weak-coupling limit, SB not only exhibits an enhanced sensitivity upon a variation of $U$ from weak to strong coupling, but also shows good agreement with experimental data. It supports the placement of weakly hole-doped cuprates in the intermediate- to strong-coupling regime.