We theoretically propose a nonreciprocal spin Seebeck effect, i.e., nonreciprocal spin transport generated by a temperature gradient, in bulk antiferromagnetic insulators with broken inversion symmetry. We find that nonreciprocity in antiferromagnets has rich properties not expected in ferromagnets or their interfaces. In particular, we show that polar antiferromagnets, in which the crystal lacks spatial-inversion symmetry, exhibit perfect nonreciprocity—one-way spin current flow irrespective of the direction of the temperature gradient. We also show that nonpolar centrosymmetric crystals can exhibit nonreciprocity when a magnetic order breaks the inversion symmetry, and in this case, the direction of the nonreciprocal flow can be controlled by reversing the magnetic domain. As their representatives, we calculate the nonreciprocal spin Seebeck voltages for the polar antiferromagnet $\alpha \text{−}{\mathrm{Cu}}_2{\mathrm{V}}_2{\mathrm{O}}_7$ and the honeycomb antiferromagnet ${\mathrm{MnPS}}_3$, while varying the temperature and magnetic field.

The effects of impurities in spin-1/2 Heisenberg chains have recently experienced a renewed interest due to experimental realizations in solid state systems and ultracold gases. Nonmagnetic impurities lead to effectively isolated finite chain segments with a discrete spectrum and characteristic correlations, which have a distinct effect on the dynamic structure factor. Using bosonization and the numerical density matrix renormalization group, we provide detailed quantitative predictions for the momentum- and energy-resolved structure factor in doped systems. Due to the impurities, spectral weight is shifted away from the antiferromagnetic wave vector $k=\pi$ into regions which normally have no spectral weight in the thermodynamic limit. The effect can be quantitatively described in terms of scaling functions, which are derived from a recurrence relation based on bosonization.

The recently advocated power-law scaling for fractal packing in amorphous metals is examined in a metallic glass under hydrostatic pressure. We show that the scaling relation $v_a\sim q_1^{-\zeta }$ between the sample molar volume $v_a$ and the first peak position $q_1$ in the scattering function exhibits a varying exponent ζ from 2.77 to 3.72 in different stages of compression, rather than a constant as the universal fractal dimensionality. Fractal packing of short- and medium-range icosahedral clusters is found to exist but undetectable with high angle scattering. Therefore, the substructure of the metallic glass does not contribute to the power-law exponent. Moreover, we show that the space filling contributed from different alloy components with varying atomic sizes is overlooked in the scaling relation that gives rise to the so-called fractal dimension. The amorphous packing of metallic glasses is actually compact with dimension 3.

We investigated the current-induced domain-wall (DW) depinning for various applied magnetic fields on a well-indentified single pinning site in epitaxial Co/Ni-based spin-valve wire of micronic width. The DW depinning process occurs with thermal activation involving a single energy barrier associated with a single pinning site. By measuring the DW depinning probability for various positive and negative applied fields $(H+,H-)$ and currents $(I+,I-)$, we built a map highlighting regions where spin-transfer torque (STT) effect, Joule heating, and Oersted field dominate. We then propose a method to quantify characteristic parameters of both adiabatic and nonadiabatic components of STT despite the presence of other effects due to current injection. The suitability of the method is validated by the fact the extracted values are close to those obtained previously on single [Co/Ni] layer where Oersted field and Joule effects were negligible.

We derive a necessary and sufficient criterion for when a two-dimensional gapped many-body system with Abelian anyons and a unitary ${\mathbb{Z}}_2$ symmetry has a protected gapless edge mode. Our criterion is phrased in terms of edge theories—or more specifically, chiral boson edge theories with ${\mathbb{Z}}_2$ symmetry—and it applies to any bosonic or fermionic system whose boundary can be described by such an edge theory. At an operational level, our criterion takes as input a chiral boson edge theory with ${\mathbb{Z}}_2$ symmetry, and then produces as output a prediction as to whether this edge theory can be gapped without breaking the symmetry. Like previous work, much of our derivation involves constructing explicit perturbations that gap chiral boson edge theories. Interestingly, however, we find that the standard class of gapping perturbations—namely, cosine terms constructed from null vectors—is not sufficient to gap some edge theories with ${\mathbb{Z}}_2$ symmetry, and thus we are forced to go beyond the usual null-vector analysis to establish our results.

The detailed behavior of the in-plane infrared-active vibrational modes has been determined in A${\mathrm{Fe}}_2{\mathrm{As}}_2$ (A$=\mathrm{Ca}$, Sr, and Ba) above and below the structural and magnetic transitions at $T_N=172$, 195, and 138 K, respectively. Above $T_N$, two infrared-active $E_u$ modes are observed. In all three compounds, below $T_N$, the low-frequency $E_u$ mode is observed to split into upper and lower branches; with the exception of the Ba material, the oscillator strength across the transition is conserved. In the Ca and Sr materials, the high-frequency $E_u$ mode splits into an upper and a lower branch; however, the oscillator strengths are quite different. Surprisingly, in both the Sr and Ba materials, below $T_N$ the upper branch appears to be either very weak or totally absent, while the lower branch displays an anomalous increase in strength. The frequencies and atomic characters of the lattice modes at the center of the Brillouin zone have been calculated for the high-temperature phase for each of these materials. The high-frequency $E_u$ mode does not change in position or character across this series of compounds. Below $T_N$, the $E_u$ modes are predicted to split into features of roughly equal strength. We discuss the possibility that the anomalous increase in the strength of the lower branch of the high-frequency mode below $T_N$ in the Sr and Ba compounds, and the weak (silent) upper branch, may be related to the orbital ordering and a change in the bonding between the Fe and As atoms in the magnetically ordered state.

In quasi-1D $\pi$-conjugated polymers such as trans-polyacetylene and polyenes, electron correlation effects determine the “reversed” excited state ordering in which the lowest two-photon $2A_g$ state lies below the lowest one-photon $1B_u$ state. In this paper, we present conclusive theoretical evidence of reversed excited state ordering in fairly 2D $\pi$-conjugated systems, namely, diamond-shaped graphene quantum dots (DQDs). Our electron correlated calculations show that DQDs begin to exhibit reversed excited ordering with increasing size, in disagreement with independent-particle picture. This signals the onset of strong correlation effects which renders them nonluminescent. Further, we calculate and analyze the two-photon absorption (TPA) spectra as well as photoinduced absorption (PA) spectra of these systems and find excellent agreement with the available experimental results. Our investigations demonstrate that unlike a strictly 1D system like trans-polyacetylene, the nonlinear and excited state absorptions in DQDs are highly intricate, with several even parity states responsible for strong absorptions. Our results could play an important role in the design of graphene-based nonlinear optical devices.

The quadratic Jahn-Teller effect of ${\mathrm{C}}_{60}{}^{n-}$ ($n=1$–5) is investigated from the first principles. Employing the density functional theory calculations with hybrid functional, the quadratic vibronic coupling constants of ${\mathrm{C}}_{60}{}^{-}$ were derived. The warping of the adiabatic potential energy surface of ${\mathrm{C}}_{60}{}^{-}$ by the quadratic vibronic coupling is estimated to be about 2 meV, which is much smaller than the Jahn-Teller stabilization energy ($\approx 50\mathrm{meV}$). Because of the selection rule and the vibronic reduction, the quadratic coupling slightly modifies the vibronic states of ${\mathrm{C}}_{60}$ anions. Particularly, in the case of ${\mathrm{C}}_{60}{}^{3-}$, parity and symmetry selection rule significantly reduces the effect of quadratic coupling on vibronic states. The present results confirm that the low-energy vibronic dynamics of ${\mathrm{C}}_{60}{}^{n-}$ is of pseudorotational type.

Using the nonequilibrium Green function method, we study the Andreev reflection in a Y-shaped graphene-superconductor device by tight-binding model. Considering both the zigzag and armchair terminals, we confirm that the zigzag terminals are the better choice for detecting the Andreev reflection in the absence of external field. Due to scattering from the boundaries of the finite-size center region, the difference between Andreev retroreflection and specular reflection is hard to distinguish. Although adjusting the size of the device makes the difference visible, to distinguish them quantitatively is still impossible through the transport conductance of a three-terminal device. The problem is circumvented when applying a perpendicular magnetic field on the center region, which makes the incident electrons and the reflected holes propagate along the edge or the interface. In this case, the retroreflected and specular reflected holes from the different bands have opposite effective masses, therefore the moving direction of one is opposite to the other. Which external terminal the reflected holes flow into depends entirely on the kind of the Andreev reflection. Therefore, the specular Andreev reflection can be clearly distinguished from the retroreflected one in the presence of strong magnetic field, even for the device with finite size.

In the presence of a finite interlayer displacement field, bilayer graphene has an energy gap that is dependent on stacking and largest for the stable AB and BA stacking arrangements. When the relative orientations between layers are twisted through a small angle to form a moiré pattern, the local stacking arrangement changes slowly. We show that for nonzero displacement fields the low-energy physics of twisted bilayers is captured by a phenomenological helical network model that describes electrons localized on domain walls separating regions with approximate AB and BA stacking. The network band structure is gapless and has of a series of two-dimensional bands with Dirac band-touching points and a density of states that is periodic in energy with one zero and one divergence per period.

Recently it was demonstrated that atomic oxygen can cause the extraction of substrate atoms off metal surfaces thus generating chemically different active sites. For Ag(110) this process occurs when ${\mathrm{O}}_2$ is dosed at 175 K leading, at low coverage, to the formation of single Ag vacancies. Vacancy creation proceeds thereby via the formation of O-Ag-O complexes, which involve a local reconstruction of the surface and ignite the disruption of the Ag substrate. In this paper, we report on the details of such processes and on the isolated structures formed by the O adatoms in the limit of very low coverage. We employ scanning tunneling microscopy and density functional theory to unravel the complex structures of O/Ag(110) which are transiently present under specific reaction conditions. A variety of features such as isolated gray dots, sombreros, shallow gray and white structures oriented along [001] and $\left[1\overline{1}0\right]$, gray stripes, and lozenges were identified and assigned to O adatoms in different configurations. The zigzag chains interact strongly with the STM tip and are easily disrupted, giving rise to highly mobile, sombrero-shaped, isolated O adatoms also far away from the scanned area, i.e., from the current injection spot. Around 200 K, not only Ag vacancies, which are mobile with anisotropic migration, can merge together into rather complex features, but also the mobile Ag atoms are trapped by O adatoms, thus facilitating the formation of an oxygen-decorated Ag chain along [001] which ultimately induces the well-known added-row reconstruction.

Precise low-temperature thermometry is a key requirement for virtually any quantum technological application. Unfortunately, as the temperature $T$ decreases, the errors in its estimation diverge very quickly. In this paper, we determine exactly how quickly this may be. We rigorously prove that the “conventional wisdom” of low-$T$ thermometry being exponentially inefficient is limited to local thermometry on translationally invariant systems with short-range interactions, featuring a nonzero gap above the ground state. This result applies very generally to spin and harmonic lattices. On the other hand, we show that a power-law-like scaling is the hallmark of local thermometry on gapless systems. Focusing on thermometry on one node of a harmonic lattice, we obtain valuable physical insight into the switching between the two types of scaling. In particular, we map the problem to an equivalent setup, consisting of a Brownian thermometer coupled to an equilibrium reservoir. This mapping allows us to prove that, surprisingly, the relative error of local thermometry on gapless harmonic lattices does not diverge as $T\to 0$; rather, it saturates to a constant. As a useful by-product, we prove that the low-$T$ sensitivity of a harmonic probe arbitrarily strongly coupled to a bosonic reservoir by means of a generic Ohmic interaction, always scales as $T^2$ for $T\to 0$. Our results thus identify the energy gap between the ground and first excited states of the global system as the key parameter in local thermometry, and ultimately provide clues to devising practical thermometric strategies deep in the ultracold regime.

We determine the information scrambling rate ${\lambda }_L$ due to electron-electron Coulomb interaction in graphene. ${\lambda }_L$ characterizes the growth of chaos and has been argued to give information about the thermalization and hydrodynamic transport coefficients of a many-body system. We demonstrate that ${\lambda }_L$ behaves for strong coupling similar to transport and energy relaxation rates. A weak coupling analysis, however, reveals that scrambling is related to dephasing or single-particle relaxation. Furthermore, ${\lambda }_L$ is found to be parametrically larger than the collision rate relevant for hydrodynamic processes, such as electrical conduction or viscous flow, and the rate of energy relaxation, relevant for thermalization. Thus, while scrambling is obviously necessary for thermalization and quantum transport, it does generically not set the timescale for these processes. In addition, we derive a quantum kinetic theory for information scrambling that resembles the celebrated Boltzmann equation and offers a physically transparent insight into quantum chaos in many-body systems.

We study by dynamical mean-field theory the ground state of a quarter-filled Hubbard model of two bands with different bandwidths. At half-filling, this model is known to display an orbital selective Mott transition, with the narrower band undergoing Mott localization while the wider one being still itinerant. At quarter-filling, the physical behavior is different and to some extent reversed. The interaction generates an effective crystal field splitting, absent in the Hamiltonian, that tends to empty the narrower band in favor of the wider one, which also become more correlated than the former at odds with the orbital selective paradigm. Upon increasing the interaction, the depletion of the narrower band can continue till it empties completely and the system undergoes a topological Lifshitz transition into a half-filled single-band metal that eventually turns insulating. Alternatively, when the two bandwidths are not too different, a first order Mott transition intervenes before the Lifshitz's one. The properties of the Mott insulator are significantly affected by the interplay between spin and orbital degrees of freedom.

We show that when a monopolar current is passed through an $n\text{−}i\text{−}n$ structure, superlinear photocurrent response occurs when there is a polariton condensate. This is in sharp contrast to the previously observed behavior for a standard semiconductor laser. Theoretical modeling shows that this is due to a stimulated exciton-exciton scattering process in which one exciton relaxes into the condensate, while another one dissociates into an electron-hole pair.

We studied the spin depolarization of ensembles of nitrogen-vacancy (NV) centers in nitrogen-rich single-crystal diamonds. We found a strong dependence of the evolution of the polarized state in the dark on the concentration of NV centers. At low excitation power, we observed a simple exponential decay profile in the low-density regime and a paradoxical inverted exponential profile in the high-density regime. At higher excitation power, we observed complex behavior, with an initial sharp rise in the luminescence signal after the preparation pulse followed by a slower exponential decay. Magnetic field and excitation laser power-dependent measurements suggest that the rapid initial increase of the luminescence signal is related to recharging of the nitrogen-vacancy centers (from neutral to negatively charged) in the dark. The slow relaxing component corresponds to the longitudinal spin relaxation of the NV ensemble. The shape of the decay profile reflects the interplay between two mechanisms: the NV charge-state conversion in the dark and the longitudinal spin relaxation. These mechanisms, in turn, are influenced by ionization, recharging, and polarization dynamics during excitation. Interestingly, we found that charge dynamics are dominant in NV-dense samples even at very feeble excitation power. These observations may be important for the use of ensembles of NV centers in precession magnetometry and sensing applications.

We demonstrate that identical impurities immersed in a two-dimensional many-particle bath can be viewed as flux-tube-charged-particle composites described by fractional statistics. In particular, we find that the bath manifests itself as an external magnetic flux tube with respect to the impurities, and hence the time-reversal symmetry is broken for the effective Hamiltonian describing the impurities. The emerging flux tube acts as a statistical gauge field after a certain critical coupling. This critical coupling corresponds to the intersection point between the quasiparticle state and the phonon wing, where the angular momentum is transferred from the impurity to the bath. This amounts to a novel configuration with emerging anyons. The proposed setup paves the way to realizing anyons using electrons interacting with superfluid helium or lattice phonons, as well as using atomic impurities in ultracold gases.

We have incorporated spin-orbit coupling into the Aubry-André model of tight-binding electron motion in the presence of periodic potential with a period incommensurate with lattice constant. This model is known to exhibit an insulator-metal transition upon increasing the hopping amplitude. Without external magnetic field, spin-orbit coupling leads to a simple renormalization of the hopping amplitude. However, when the degeneracy of the on-site energies is lifted by an external magnetic field, the interplay of Zeeman splitting and spin-orbit coupling has a strong effect on the localization length. We studied this interplay numerically by calculating the energy dependence of the Lyapunov exponent in the insulating regime. In the limit of large periods, our numerical results can be interpreted in the language of the phase-space trajectories. As a first step, we have derived analytically the energy dependence of the localization observed in numerical simulations of the original Aubry-André model with large periods. Our main finding is that a very weak spin-orbit coupling leads to delocalization of states with energies smaller than the Zeeman shift. The origin of the effect is the spin-orbit-induced opening of new transport channels. We have also found that restructuring of the phase-space trajectories, which takes place at certain energies in the insulating regime, causes a singularity in the energy dependence of the localization length.