We implemented a finite-difference algorithm for computing anomalous Hall and Nernst conductivity. Based on the expression to evaluate the Berry curvature in an insulating system [J. Phys. Soc. Jpn. 74, 1674 (2005)], we extended the methods to a metallic system. We calculated anomalous Hall conductivity and Nernst conductivity in a two-dimensional ferromagnetic material ${\mathrm{FeCl}}_2$ and three-dimensional ferromagnetic transition-metals bcc-Fe, hcp-Co, and fcc-Ni. Our results are comparable to previously reported results computed by the Kubo formula or Wannier representation. To evaluate anomalous Nernst coefficients, the detailed Fermi-energy dependence of the anomalous Hall conductivity is required. The present method will open an efficient thermoelectric material design based on the high-throughput first-principles screening.

Vortices in spin-0 superfluids generically carry magnetic fields inside their cores, so that even neutral superfluid vortices may be thought of as magnetic flux tubes. We give a systematic analysis of this ‘vortex magnetic effect’ using effective field theory, clarifying earlier literature on the subject. Our analysis shows that in superfluid helium-4 the vortex magnetic effect may be large enough to be experimentally detectable.

We study the phase diagram and phase transitions of the three dimensional multicomponent lattice Higgs model with noncompact Abelian discrete groups. The model with noncompact U(1) gauge group is known to undergo, for a sufficiently large number of scalar fields $N$, a continuous transition associated to the charged fixed point of the continuous Abelian Higgs field theory. We show that in the model with gauge group ${\mathbb{Z}}_q^{\left(nc\right)}\equiv 2\pi \mathbb{Z}/q$ only critical transitions in the orthogonal universality classes are present for small values of $N$, while a symmetry enlargement to the continuous Abelian Higgs universality class happens when $q\ge 5$ and $N$ is large enough.

The parameter-free computation of charge transport properties of semiconductors is now routine owing to advances in the ab initio description of the electron-phonon interaction. Many studies focus on the low-field regime in which the carrier temperature equals the lattice temperature and the current power spectral density (PSD) is proportional to the mobility. The calculation of high-field transport and noise properties offers a stricter test of the theory as these relations no longer hold, yet few such calculations have been reported. Here, we compute the high-field mobility and PSD of hot holes in silicon from first principles at temperatures of 77 and 300 K and electric fields up to 20 kV ${\mathrm{cm}}^{-1}$ along various crystallographic axes. We find that the calculations quantitatively reproduce experimental trends including the anisotropy and electric-field dependence of hole mobility and PSD. The experimentally observed rapid variation of energy relaxation time with electric field at cryogenic temperatures is also correctly predicted. However, as in low-field studies, absolute quantitative agreement is, in general, lacking, a discrepancy that has been attributed to inaccuracies in the calculated valence-band structure. Our paper highlights the use of high-field transport and noise properties as a rigorous test of the theory of electron-phonon interactions in semiconductors.

Achieving high-fidelity control of spin qubits with conventional electron dipole spin resonance (EDSR) requires large magnetic field gradients of about $1{\mathrm{mTnm}}^{-1}$, which also couple the qubit to charge noise, and large drive amplitudes of order 1 mV. The flopping mode is an alternative method to drive EDSR of an electron in a double quantum dot, where the large displacement between both dots increases the driving efficiency. We propose to operate the flopping mode in the strong-driving regime to use the full magnetic field difference between the two dots. In simulations, the reduced required magnetic field gradients suppress the infidelity contribution of charge noise by more than two orders of magnitude, while providing Rabi frequencies of up to $60\mathrm{M}\mathrm{Hz}$. However, the near degeneracy of the conduction band in silicon introduces a valley degree of freedom that can degrade the performance of the strong-driving mode. This necessitates a valley-dependent pulse optimization and makes operation to the strong-driving regime questionable.

In this study, we examine whether periodic driving of a model with a quasiperiodic potential can generate interesting Floquet phases that have no counterparts in the static model. Specifically, we consider the Aubry-André model, which is a one-dimensional time-independent model with an on-site quasiperiodic potential $V_0$ and a nearest-neighbor hopping amplitude that is taken to have a staggered form. We add a uniform hopping amplitude that varies in time either sinusoidally or as a square pulse with a frequency $\omega$. Unlike the static Aubry-André model, which has a simple phase diagram with only two phases (only extended or only localized states), we find that the driven model has four possible phases for the Floquet eigenstates: a phase with gapless quasienergy bands and only extended states, a phase with multiple mobility gaps separating different quasienergy bands, a mixed phase with coexisting extended, multifractal, and localized states, and a phase with only localized states. The multifractal states have generalized inverse participation ratios that scale with the system size with exponents that are different from the values for both extended and localized states. In addition, we observe intricate reentrant transitions between the different kinds of states when $\omega$ and $V_0$ are varied. The appearance of such transitions is confirmed by the behavior of the Shannon entropy. Many of our numerical results can be understood from an analytic Floquet Hamiltonian derived using a Floquet perturbation theory that uses the inverse of the driving amplitude as the perturbation parameter. In the limit of high frequency and large driving amplitude, we find that the Floquet quasienergies match the energies of the undriven system, but the Floquet eigenstates are much more extended. We also study the spreading of a one-particle wave packet, and we find that it is always ballistic but the ballistic velocity varies significantly with the system parameters, sometimes showing a nonmonotonic dependence on $V_0$ that does not occur in the static model. Finally, we compare the results for the driven model, which has a static staggered hopping amplitude, with a model that has a static uniform hopping amplitude, and we find some significant differences between the two cases. All of our results are found to be independent of the driving protocol, either sinusoidal or square pulse. We conclude that the interplay of the quasiperiodic potential and driving produces a rich phase diagram that does not appear in the static model.

Quasiparticle interference (QPI) imaging is a powerful tool for the study of the low-energy electronic structure of quantum materials. However, the measurement of QPI by scanning tunneling microscopy (STM) is restricted to surfaces and is thus inherently constrained to two dimensions. QPI has proved immensely successful for the study of materials that exhibit a quasi-two-dimensional electronic structure, yet it raises questions about how to interpret QPI in materials that have a highly three-dimensional electronic structure. In this paper, we address this question and establish the methodology required to simulate and understand QPI arising from three-dimensional systems as measured by STM. We calculate the continuum surface Green's function in the presence of a defect, which captures the role of the surface and the vacuum decay of the wave functions. We find that defects at different depths from the surface will produce unique sets of scattering vectors for three-dimensional systems, which nevertheless can be related to the three-dimensional electronic structure of the bulk material. We illustrate the consequences that the three-dimensionality of the electronic structure has on the measured QPI for a simple cubic nearest-neighbor tight-binding model, and then demonstrate application to a real material using a realistic model for PbS. Our method unlocks the use of QPI imaging for the study of quantum materials with three-dimensional electronic structures and introduces a framework to generically account for $k_z$ dispersions within QPI simulations.

Semipolar $\left(11\overline{2}2\right)$ AlInN/GaN heterostructures on GaN templates were studied using photoluminescence (PL) spectroscopy both at 15 K and at room temperature. The polarization-resolved PL measurements revealed a dominant polarization along the $\left[11\overline{2}\overline{3}\right] c^{\prime }$ and a weaker signal along the $\left[1\overline{1}00\right] m$-direction, i.e., the two in-plane directions of the semipolar $\left(11\overline{2}2\right)$ growth plane. We observed slightly different polarization degrees of $0.34\pm 0.01, 0.25\pm 0.02$, and $0.20\pm 0.01$ at room temperature, respectively, depending on the degree of strain relaxation in the $\left[1\overline{1}00\right]$ direction. From a theoretical model based on a $\mathbf{k}·\mathbf{p}$ calculation, we find that the transition from the conduction band (CB) to the uppermost valence band (VB) is C for AlInN similar to AlN, followed by the transition A from the CB to the second VB, for a wide range of compositions. Thus, the in-plane transition matrix elements from the CB to the two topmost VBs near the $\mathrm{\Gamma }$ point of the Brillouin zone are dominated by $M_{\left[11\overline{2}\overline{3}\right]}$ for the C transition and by $M_{\left[1\overline{1}00\right]}$ for the A transition. For the samples under consideration with an energy splitting of about $18\pm 1\mathrm{meV}$, there is sizable thermal occupation of the second VB at room temperature, reasonably explaining the experimental results. The results show that AlInN possesses a band structure similar to AlN, which might explain the strong Stokes shift and the large variation in the band-gap values reported previously.

The amount of heat an integer quantum Hall edge state can carry in equilibrium is quantized in universal units of the heat flux quantum $J_q=\frac{\pi k_B^2}{12\hslash }{T}^2$ per edge state. We address the question of how heat transport in realistic one-dimensional devices can differ from the usual chiral Luttinger liquid theory. We show that a local measurement can reveal a nonquantized amount of heat carried by the edge states, despite a globally equilibrium situation. More specifically, we report a heat enhancement effect in edge states interacting with Ohmic reservoirs in the presence of nonlocal interactions or chirality-breaking diffusive currents. In contrast to a nonequilibrium, nonlinear drag effect, we report an equilibrium, linear phenomenon. The chirality of the edge states creates additional correlations between the reservoirs, reflected in a higher-than-quantum heat flux in the chiral channel. We show that for different types of coupling the enhancement can be understood as static or dynamical back action of the reservoirs on the chiral channel. We show that our results qualitatively hold by replacing the dissipative Ohmic reservoirs by an energy-conserving mesoscopic capacitor and consider the respective transmission lines for different types of interaction.

The chirality-induced spin selectivity (CISS) effect gives rise to strongly spin-dependent transport through many organic molecules and structures. Its discovery raises fascinating fundamental questions as well as the prospect of possible applications. The basic phenomenology, a strongly asymmetric magnetoresistance despite the absence of magnetism, is now understood to result from the combination of spin-orbit coupling and chiral geometry. However, experimental signatures of electronic helicity were observed at room temperature, i.e., at an energy scale that exceeds the typical spin-orbit coupling in organic systems by several orders of magnitude. This paper shows that a much larger energy scale for CISS emerges for currents carried by polarons, i.e., in the presence of strong electron-phonon coupling. In particular, we found that polaron fluctuations play a crucial role in the two manifestations of CISS in transport measurements—the spin-dependent transmission probability through the system and asymmetric magnetoresistance.

The long-wavelength physics of monolayer graphene in the presence of periodic strain fields has a natural chiral scattering network description. When the strain field varies slowly compared to the graphene lattice and the effective magnetic length of the induced valley pseudomagnetic field, the low-energy physics can be understood in terms of valley-polarized percolating domain-wall modes. Inspired by a recent experiment, we consider a strain field with threefold rotation and mirror symmetries but without twofold rotation symmetry, resulting in a system with the connectivity of the oriented kagome network. Scattering processes in this network are captured by a symmetry-constrained phenomenological $S$ matrix. We analyze the phase diagram of the kagome network and show that the bulk physics of the strained graphene can be qualitatively captured by the network when we account for a percolation transition at charge neutrality. We also discuss the limitations of this approach to properly account for boundary physics.

${\mathrm{Nd}}_2{\mathrm{Fe}}_{14}\mathrm{B}$'s unsurpassed, hard magnetic properties for a wide range of temperatures result from a combination of a large volume magnetization from Fe and a strong single-ion anisotropy from Nd. Here, using finite temperature first-principles calculations, we focus on the other crucial roles played by the Fe atoms in maintaining the magnetic order on the Nd sublattices, and hence the large magnetic anisotropy, and directly generating significant uniaxial anisotropy at high temperatures. We identify effective spins for atomistic modeling from the material's interacting electrons and quantify pairwise and higher order, nonpairwise magnetic interactions among them. We find the Nd spins couple most strongly to spins on sites belonging to two specific Fe sublattices: $8j_1$ and $8j_2$. Moreover, the Fe $8j_1$ sublattice also provides the electronic origin of the unusual, nonmonotonic temperature dependence of the anisotropy of ${\mathrm{Y}}_2{\mathrm{Fe}}_{14}\mathrm{B}$. Our work provides atomic-level resolution of the properties of this fascinating magnetic material.

We report an in situ scanning tunneling microscopy study of atomically thin films of ${\mathrm{CeCoIn}}_5$, a $d$-wave heavy-fermion superconductor. Both hybridization and superconducting gaps are observed even in monolayer ${\mathrm{CeCoIn}}_5$, providing direct evidence of superconductivity of heavy quasiparticles mediated by purely two-dimensional bosonic excitations. In these atomically thin films, $T_c$ is suppressed to nearly half of the bulk, but is similar to ${\mathrm{CeCoIn}}_5/{\mathrm{YbCoIn}}_5$ superlattices containing ${\mathrm{CeCoIn}}_5$ layers with the same thickness as the thin films. Remarkably, the out-of-plane upper critical field ${\mu }_0{H}_{c2\perp }$ at zero temperature is largely enhanced from those of bulk and superlattices. The enhanced $H_{c2\perp }$ well exceeds the Pauli and bulk orbital limits, suggesting the possible emergence of unusual superconductivity with parity mixing caused by the inversion symmetry breaking.

The electronic structure changes of SmS surfaces under potassium (K) doping are elucidated using synchrotron-based core-level photoelectron spectroscopy and angle-resolved photoelectron spectroscopy (ARPES). The Sm core-level and ARPES spectra indicate that the Sm mean valence of the surface increased from the nearly divalent to trivalent states, with increasing K deposition. Carrier-induced valence transition (CIVT) from ${\mathrm{Sm}}^{2+}$ to ${\mathrm{Sm}}^{3+}$ exhibits a behavior opposite to that under conventional electron doping. Excess electrons from K atoms are transferred to S sites and the liberated electrons from ${\mathrm{Sm}}^{3+}$ ions due to CIVT at the surface are trapped like local excitons around the ${\mathrm{Sm}}^{3+}$ ions, which is inconsistent with the phase transition from the black insulator with ${\mathrm{Sm}}^{2+}$ to the gold metal with ${\mathrm{Sm}}^{3+}$ under pressure. This CIVT helps to clarify the pressure-induced black-to-golden phase transition in this material, which originates from the Mott transition of excitons.

Explicit anharmonicity, defined as the vibrational contribution beyond the quasiharmonic approximation, is qualitatively different between the group V and group VI bcc refractory elements. Group V elements show a small and mostly negative anharmonic entropy, whereas group VI elements have a large positive anharmonic entropy, strongly increasing with temperature. Here, we explain this difference utilizing highly accurate anharmonic free energies and entropies from ab initio calculations for Nb and Ta (group V), and Mo and W (group VI). The numerically calculated entropies are in agreement with prior experimental data. The difference in behavior between the two sets of elements arises not from their high-temperature behavior but rather from the $0\mathrm{K}$ quasiharmonic reference state. We understand this by analyzing the $0\mathrm{K}$ and the high-temperature phonon density of states and the electronic density of states. The qualitative difference disappears when the anharmonicity is instead referenced with a high-temperature effective harmonic potential. However, even for an optimized effective harmonic reference, the remaining effective anharmonicity is significant. The reason is that the anharmonicity in the bcc systems—carried by asymmetric distributions in the nearest neighbors—can never be accounted for by a harmonically restricted potential.

The electron states in the quasicrystal (QC) are hot topics recently. While previous attentions were focused on such intrinsic QCs as the Penrose lattice, the recent “twistronics” provides us with a new type of QC, i.e., the extrinsic QC, including the ${30}^{\circ }$-twisted bilayer graphene and ${45}^{\circ }$-twisted bilayer cuprates as two synthesized examples, unifiedly dubbed as TB-QC. Here we build an efficient microscopic framework to study electron-electron interaction driven superconductivities (SCs) in these extrinsic QCs, and find that their nature sits in between those of crystals and intrinsic QCs. Remarkably, our microscopic calculations on the three exemplar TB-QCs reveal various novel topological SCs carrying high angular momenta and high Chern numbers protected by their unique QC symmetries, absent in conventional crystalline materials. The nature of SCs in these extrinsic QCs is also fundamentally different from those in intrinsic QCs in the aspect of pairing-symmetry classifications and topological properties.

Charge density wave (CDW) induces periodic spatial modulation of the charge density that is commensurate or incommensurate with the host lattice periodicity, and leads to partial or complete electronic band-gap opening at the Fermi level ($E_{\mathrm{F}}$). The recent finding of unconventional hysteresis within the CDW phase of ${\mathrm{EuTe}}_4$, not observable in other rare-earth tellurides $R{\mathrm{Te}}_n$ ($n=2$, 3), has highlighted the role of the relative phase of CDW distortion in weakly coupled Te layers. However, detailed structural and dynamical characterization of CDW distortion on the hysteretic transition is lacking. Here we report on the static CDW order, dynamics of the amplitude mode, and their evolution on the hysteretic transition using meV resolution elastic and inelastic x-ray scattering. We discover previously unidentified multiple commensurate and incommensurate CDW wave vectors ${\mathbf{q}}_{\mathrm{CDW}}$ along all three crystallographic axes. Importantly, we find that the previously reported $b$-axis CDW peak is coupled with the interlayer CDW phase and consequently co-occurs with the doubling of the unit cell along the $c$ axis. We confirm the presence of the competing $a$-axis CDW order but found it to be four orders of magnitude weaker than the $b$-axis CDW. Furthermore, we observe multiple Kohn anomalies at ${\mathbf{q}}_{\mathrm{CDW}}$ driven by Fermi surface nesting and hidden nesting, confirming earlier reports based on electronic and lattice susceptibility simulations. The amplitude mode and Kohn anomalies are found to suppress on unconventional hysteretic transition, suggesting the presence of nondegenerate metastable states, which we identify from the x-ray scattering measurements and simulations.

While spin parity effects, physics crucially depending on whether the spin quantum number $S$ is half-odd integral or integral, have for decades been a source of new developments for the quantum physics of antiferromagnetic spin chains, the investigation into their possible ferromagnetic counterparts has remained largely unchartered, especially in the fully quantum (as opposed to the semiclassical) regime. Here we present such studies for monoaxial chiral ferromagnetic spin chains. We start by examining magnetization curves for finite-sized systems, where a magnetic field is applied perpendicular to the helical axis. For half-odd integer $S$, the curves feature discontinuous jumps identified as a series of level crossings, each accompanied by a shift of the crystal momentum $k$ by an amount of $\pi$. The corresponding curves for integer valued $S$ are continuous and exhibit crossover processes. For the latter case, $k=0$ throughout. These characteristics are observed numerically when the strength of the Dzyaloshinskii-Moriya interaction (DMI) $D$ is comparable to or larger than that of the ferromagnetic exchange interaction $J$. Solitons are known to be responsible for stepwise changes seen in magnetization curves in the classical limit. These findings therefore prompt us to revise the notion of a soliton, for arbitrary $S$, into a quantum mechanical entity. To unravel this phenomenon at the fully quantum level as is appropriate to spin chains with small $S$, we examine in detail special limiting Hamiltonians amenable to rigorous analysis, consisting of only the DMI and the Zeeman energy. Dubbed the $DH$ model (for $S=\frac{1}{2}$) and the projected $DH$ ($\mathrm{p}DH$) model (for general $S$), they have a set of $2S$ conserved quantities, each of which is the number of solitons of a specific integer-valued height (as measured in the $S_z$ basis), which ranges from 1 to $2S$. We discuss how to determine the exact crystal momentum of the lowest-energy state belonging to a sector with a given set of the $2S$ soliton numbers. Combined with energetic considerations, this information enables us to reproduce the spin parity effect in the magnetization curves. Finally, we show that the ground states of the special models have substantial numerical overlap with those for generic systems with a finite exchange interaction, suggesting the same physics to be valid there as well.

The structural diversity of hydrogen under high pressure is crucial for understanding its physical properties, especially for the superconductivity. Here, a combination of crystal structure prediction and first-principle calculations has predicted a metastable ${\mathrm{H}}_2\mathrm{He}$ at ultrahigh pressure of 800 GPa, where H atoms are arranged in graphenelike structure. ${\mathrm{H}}_2\mathrm{He}$ shows weak superconductivity with critical temperature of 4 K, which significantly increases to 155 and 201 K at electron doping of 0.6 and 1.2 $e$, respectively. Analysis suggests that the enhanced superconductivity is closely associated with the graphenelike H sublattice. The doped electrons almost transfer to H sublattice, inducing the increased H-derived density of states at the Fermi level, as well as their coupling with the phonon modes of graphenelike H layers. The current results could stimulate the experiment to search for more diverse hydrogen species at high pressure and provide a feasible way to design high-$T_{\text{c}}$ superconductors as well.