The electric dipole spin resonance (EDSR) combining strong spin-orbit coupling (SOC) and electric-dipole transitions facilitates fast spin control in a scalable way, which is the critical aspect of the rapid progress made recently in germanium (Ge) hole-spin qubits. However, a puzzle is raised because centrosymmetric Ge lacks the Dresselhaus SOC, a key element in the initial proposal of the hole-based EDSR. Here, we demonstrate that the recently uncovered finite $k$-linear Rashba SOC of 2D holes offers fast hole-spin control via EDSR with Rabi frequencies in excellent agreement with experimental results over a wide range of driving fields. We also suggest that the Rabi frequency can reach 500 MHz under a higher gate electric field or multiple GHz in a replacement by [110]-oriented quantum wells. These findings bring a deeper understanding for hole-spin qubit manipulation and offer design principles to boost the gate speed.

One remarkable feature of strongly correlated systems is the phenomenon of fractionalization where quasiparticles carry only a fraction of the charge or spin of the elementary constituents. Such quasiparticles often present anyonic statistics in two dimensions and lie at the heart of the fractional quantum Hall effect. We discuss the observation of fractionalization and anyonic statistics already in the integer quantum Hall effect coupled to a metallic island. A continuous fractional emitter is proposed, which sends dilute beams of noninteger charges, and its full counting statistics is obtained. The fractional charge is governed solely by the number of ballistic channels covered by the island and it is one half of the electron charge for a single ballistic channel. We further characterize the mixing of two such fractional beams through a quantum point contact beam splitter. We predict negative cross-correlations, in strong contrast with free electrons, that depend on the double exchange phase between electrons and the fractional charges emulating anyons. The result is similar to a genuine fractional edge state as recently measured at filling $\nu =1/3$. We revisit the physical interpretation of this experiment and point towards a direct braiding measurement rather than a deviation from fermionic antibunching.

A major challenge in valleytronics is to produce stable valley polarizations. Employing free electrons instead of excitons as valley-polarized carriers can provide long valley lifetimes. Using first-principles simulations, we have revealed the mechanism for the valley-selective injections of free electrons from nanoantennas to ${\mathrm{MoS}}_2$, through which long-lived valley polarizations can be achieved. Dynamically, electrons are injected into the $Q\left(Q^{\prime }\right)$ valleys instead of the most stable $K\left(K^{\prime }\right)$ valleys, due to the interfacial coupling. Under the $C_3$ symmetry and the time-reversal symmetry, the valley-selective electron injection is realized by the circularly polarized laser excitation, and can be tuned via the laser frequency. Because the electron injection is induced by the interfacial coupling, the valley selectivity holds as long as the interface locally has a $C_3$ symmetry. Our theoretical results pave the way for future valleytronic studies based on free carriers.

We analyze the electronic structure of the recently synthesized higher-order nickelate Ruddlesden-Popper phases ${\mathrm{La}}_{n+1}{\mathrm{Ni}}_n{\mathrm{O}}_{3n+1}$ $(n=4–6)$ using first-principles calculations. For all materials, our results show large holelike Fermi surfaces with $d_{x^2-y^2}$ character that closely resemble those of optimally hole-doped cuprates. For higher values of $n$, extra non-cuprate-like bands of $d_{z^2}$ orbital character appear. These aspects highlight that this Ruddlesden-Popper series can provide a means to modify the electronic ground states of nickelates by tuning their dimensionality. With their similarities and differences to the cuprates, this new family of materials can potentially shed light on the physics of copper-based oxides.

Laughlin states have recently been constructed on fractal lattices, and the charge and braiding statistics of the quasiholes were used to confirm that these states have Laughlin-type topology. Here we investigate density, correlation, and entanglement properties of the states on a fractal lattice derived from a Sierpinski triangle with the purpose of identifying similarities and differences compared to two-dimensional systems and with the purpose of investigating whether various probes of topology work for fractal lattices. Similarly to two-dimensional systems, we find that the connected particle-particle correlation function decays roughly exponentially with the distance between the lattice sites measured in the two-dimensional plane, but the values also depend on the local environment. Contrary to two-dimensional systems, we find that the entanglement entropy does not follow the area law if one defines the area to be the number of nearest-neighbor bonds that cross the edge of the selected subsystem. Considering bipartitions with two bonds crossing the edge, we find a close to logarithmic scaling of the entanglement entropy with the number of sites in the subsystem. This also means that the topological entanglement entropy cannot be extracted using the Kitaev-Preskill or the Levin-Wen methods. Studying the entanglement spectrum for different bipartitions, we find that the number of states below the entanglement gap is robust and the same as for Laughlin states on two-dimensional lattices.

We report a theoretical investigation of the magnetocaloric effect in paramagnetic intermetallic $\mathrm{Pr}{\mathrm{Ni}}_2$. We choose this particular compound because it presents high cubic symmetry, and two sets of crystalline electrical field parameters were reported, allowing a systematic study. In one of these sets, the magnetic entropy increases upon applied magnetic field in the low temperature region $(T<10\mathrm{K})$ for $H=5\mathrm{T}$. This anomaly was investigated through a model Hamiltonian which includes the magnetic, exchange, and crystalline electric field interactions. The simulated curves of the temperature dependence of isothermal entropy variation and adiabatic temperature change present the inverse magnetocaloric effect, which is not expected for a paramagnetic system.

The quantum metric is a central quantity of band theory but has so far not been related to many response coefficients due to its nonclassical origin. However, within a newly developed Kubo formalism for fast relaxation, the decomposition of the dc electrical conductivity into both classical (intraband) and quantum (interband) contributions recently revealed that the interband part is proportional to the quantum metric. Here, we show that interband effects due to the quantum metric can be significantly enhanced and even dominate the conductivity for semimetals at charge neutrality and for systems with highly quenched bandwidth. This is true in particular for topological flat-band materials of nonzero Chern number, where for intermediate relaxation rates an upper bound exists for the resistivity due to the common geometrical origin of quantum metric and Berry curvature. We suggest to search for these effects in highly tunable rhombohedral trilayer graphene flakes.

Iron-bearing carbonates play an important role in Earth's carbon cycle. Owing to their stability at mantle conditions, recently discovered iron carbonates with tetrahedrally coordinated carbon atoms are candidates for carbon storage in the deep Earth. The carbonates' iron oxidation and spin state at extreme pressure and temperature conditions contribute to the redox conditions and element partitioning in the deep mantle. By laser heating ${\mathrm{FeCO}}_3$ at pressures of about 83 GPa, ${\mathrm{Fe}}_4^{3+}{\mathrm{C}}_3{\mathrm{O}}_{12}$ and ${\mathrm{Fe}}_2^{2+}{\mathrm{Fe}}_2^{3+}{\mathrm{C}}_4{\mathrm{O}}_{13}$ were synthesized and then investigated by x-ray emission spectroscopy to elucidate their spin state, both in situ and temperature quenched. Our experimental results show both phases in a high-spin state at all pressures and over the entire temperature range investigated, i.e., up to 3000 K. The spin state is conserved after temperature quenching. A formation path is favored where ${\mathrm{Fe}}_4^{3+}{\mathrm{C}}_3{\mathrm{O}}_{12}$ forms first and then reacts to ${\mathrm{Fe}}_2^{2+}{\mathrm{Fe}}_2^{3+}{\mathrm{C}}_4{\mathrm{O}}_{13}$, most likely accompanied by the formation of oxides. Density functional theory calculations of ${\mathrm{Fe}}_2^{2+}{\mathrm{Fe}}_2^{3+}{\mathrm{C}}_4{\mathrm{O}}_{13}$ at 80 GPa confirm the experimental findings with both ferric and ferrous iron in high-spin state with antiferromagnetic order at 80 GPa. As the intercrystalline cation partitioning between the Fe-bearing carbonates and the surrounding perovskite and ferropericlase depends on the spin state of the iron, an understanding of the redox conditions prevalent in subducted slab regions in the lower mantle has to take the latter into account. Especially, ${\mathrm{Fe}}_2^{2+}{\mathrm{Fe}}_2^{3+}{\mathrm{C}}_4{\mathrm{O}}_{13}$ may play a key role in subducted material in the lower mantle, potentially with a similar role as silicate perovskite.

Epitaxial semiconductor-superconductor (SM-SC) hybrid nanowires are potential candidates for implementing Majorana qubits. Recent experimental and theoretical works show that charged impurities in SM remain a major problem in all existing hybrid nanowires, in which the SM is either InAs or InSb while the SC is mainly Al. Here, we theoretically validate the recently proposed PbTe-Pb hybrid nanowire as a potential candidate for Majorana devices. By studying the electrostatic and electronic properties of PbTe nanowires, we demonstrate that the huge dielectric constant of PbTe endows itself a high tolerance of charged impurity, which is a potential advantage over InAs and InSb nanowires. Moreover, we find that the effective axial Landé $g$ factor and Rashba spin-orbit coupling strength of PbTe nanowires are comparable to those of InAs nanowires. The conceivable merits of using Pb as the SC are (i) Pb has a larger superconducting gap, higher critical temperature, and higher parallel critical magnetic field than those of Al; (ii) a superconducting gap comparable with those of InAs-Al and InSb-Al can be induced in PbTe-Pb even by a weak coupling between Pb and PbTe, which simultaneously relieves the adverse renormalization and induced disorder effects on SM from SC; and (iii) Pb film can be grown on PbTe with a thin buffer CdTe layer in between, solving the lattice mismatch problem as an important source of disorder. In the presence of a parallel magnetic field, we show that the typical BdG energy spectrum and tunneling spectroscopy of PbTe-Pb resemble those of InAs and InSb based hybrid nanowires exposed to a tilting magnetic field, as a result of the highly anisotropic Landé $g$ factors of PbTe nanowires. The calculated topological phase diagrams of PbTe-Pb indicate that the multivalley character of PbTe makes it easier than InAs and InSb to access topological superconducting phases. Our results could facilitate the experimental realization of PbTe-Pb hybrid nanowires and inspire further theoretical works.

The valley-polarized quantum anomalous Hall (VQAH) state, which forwards a strategy for combining valleytronics and spintronics with nontrivial topology, attracts intensive interest in condensed-matter physics. So far, the exploration of VQAH states has still been limited to magnetic systems. Here, using the low-energy effective model and Floquet theorem, we propose an alternative mechanism to realize the Floquet VQAH state in nonmagnetic heterobilayers under light irradiation. We then realize this proposal via first-principles calculations in transition metal dichalcogenide heterobilayers, which initially possess the time-reversal invariant valley quantum spin Hall (VQSH) state. By irradiating circularly polarized light, the time-reversal invariant VQSH state can evolve into the VQAH state, behaving as an optically switchable topological spin-valley filter. These findings not only offer a rational scheme to realize the VQAH state in periodically driven nonmagnetic systems, but also pave a fascinating path for designing topological spintronic and valleytronic devices with high tunability. Our proposal can also be extended to design the general QAH effect without magnetic orders.

The Haldane phase for antiferromagnetic spin-1 chains is a celebrated topological state of matter, featuring gapped excitations and fractional spin-1/2 edge states. Here, we provide numerical evidence that this phase can be realized with a Hubbard model at half filling, where each $s=1$ spin is stored in a four-site fermionic structure. We find that the noninteracting limit of our proposed model describes a one-dimensional topological insulator, and we conjecture it to be adiabatically connected to the Haldane phase. Our work suggests a route to build spin-1 networks using Hubbard model quantum simulators.

Weyl semimetals are arguably the most paradigmatic form of a gapless topological phase. While the stability of Weyl nodes, as quantified by their topological charge, has been extensively investigated, recent interest has shifted to the manipulation of the location of these Weyl nodes for non-Abelian braiding. To accomplish this braiding it is necessary to drive significant Weyl node motion using realistic experimental parameter changes. We show that a family of phase transitions characterized by certain symmetry constraints impose that the Weyl nodes have to reorganize by a large amount, shifting from one high-symmetry plane to another. Additionally, for a subset of pairs of nodes with nontrivial Euler class topology, this reorganization can only occur through a braiding process with adjacent nodes. As a result, the Weyl nodes are forced to move a large distance across the Brillouin zone and to braid, all driven by small temperature changes, a process we illustrate with ${\mathrm{Cd}}_2{\mathrm{Re}}_2{\mathrm{O}}_7$.

We investigate the ground state and quantum dynamics of an interacting bosonic chain with nonreciprocal hopping. In sharp contrast to its Hermitian counterpart, the ground state can support Mott insulators in systems with noninteger filling due to the competition between nonreciprocal hopping and the on-site interaction. For the quantum dynamics, conservation laws for non-Hermitian systems manifest a stark difference from their Hermitian counterpart. In particular, for any Hermitian operator that commutes with the Hamiltonian operator, its expectation value is guaranteed to be nonconserved in the non-Hermitian quantum dynamics. To systematically identify the non-Hermitian conservation law, we establish a generic approach for constructing the conserved quantities in non-Hermitian many-body quantum systems with completely real spectra, and illustrate it concretely by the system under study. The direct experimental observation of Mott insulators in systems with noninteger filling and non-Hermitian conservation laws can be performed by ultracold atoms in optical lattices with the engineered nonreciprocal hopping.

We study the impact of the interlevel energy constraints imposed by Haldane exclusion statistics on energy relaxation processes in one-dimensional systems coupled to a bosonic bath. By formulating a second-quantized description of the relevant Fock space, we identify certain universal features of this relaxation dynamics, and show that it is generically slower than that of spinless fermions. Our study focuses on the Calogero-Sutherland model, which realizes Haldane Exclusion statistics exactly in one dimension; however, our results apply to any system that has the associated pattern of interlevel occupancy constraints in Fock space.

The electromagnetic properties of ultrathin epitaxial ruthenate films have long been the subject of debate. Here we combine experimental with theoretical investigations of ${\left(\mathrm{SrTi}{\mathrm{O}}_3\right)}^5\text{−}{\left(\mathrm{SrRu}{\mathrm{O}}_3\right)}^n\text{−}{\left(\mathrm{SrTi}{\mathrm{O}}_3\right)}^5 (\mathrm{ST}{\mathrm{O}}^5\text{−}\mathrm{SR}{\mathrm{O}}^n\text{−}\mathrm{ST}{\mathrm{O}}^5$) heterostructures with $n=1$ and 2 unit cells, including extensive atomic-resolution scanning-transmission electron-microscopy imaging, electron-energy-loss spectroscopy chemical mapping, as well as transport and magnetotransport measurements. The experimental data demonstrate that the $\mathrm{ST}{\mathrm{O}}^5\text{−}\mathrm{SR}{\mathrm{O}}^2\text{−}\mathrm{ST}{\mathrm{O}}^5$ heterostructure is nearly stoichiometric, metallic, and ferromagnetic with $T_{\mathrm{C}}\sim 128\mathrm{K}$, even though it lacks the characteristic bulk-SRO octahedral tilts and matches the cubic STO structure. In contrast, the $\mathrm{ST}{\mathrm{O}}^5\text{−}\mathrm{SR}{\mathrm{O}}^1\text{−}\mathrm{ST}{\mathrm{O}}^5$ heterostructure features Ru-Ti intermixing in the ${\mathrm{RuO}}_2$ layer, also without octahedral tilts, but is accompanied by a loss of metallicity and ferromagnetism. Density-functional theory calculations show that stoichiometric $n=1$ and $n=2$ heterostructures are metallic and ferromagnetic with no octahedral tilts, while nonstoichiometry in the Ru sublattice in the $n=1$ case opens an energy gap and induces antiferromagnetic ordering. Thus, the results indicate that the observed nonstoichiometry is the cause of the observed loss of metallicity and ferromagnetism in the $n=1$ case.

Microwave-assisted switching (MAS) has gathered fundamental interest and has been explored for magnetic recording since the switching field can be reduced by exciting magnetization with a microwave field in the GHz range. We theoretically study the effect of thermal activation in MAS and apply the developed analytical method to examine the result of MAS experiments on CoCrPt-${\mathrm{SiO}}_2$ granular media. The effective energy barrier height for magnetization switching under a microwave field is calculated from the Landau-Lifshitz-Gilbert equation. The calculated energy barrier shows that the thermal activation process plays a crucial role in MAS and modifies the dependences of the switching field on the microwave frequency and amplitude. The switching field decreases with increasing the microwave frequency up to the critical frequency, at which point the switching field abruptly increases. Thermal activation significantly increases the critical frequency and makes the slope of the frequency dependence of the switching field gradual. The maximum assistance effect increases with increasing the microwave field amplitude and is almost independent of the thermal activation. The analytical method quantitatively explains the gradual microwave frequency dependence and high critical frequency observed in MAS experiments on granular media. These results indicate the importance of thermal activation to optimize MAS behavior for practical use.

All-optical spin switching (AOS) represents a new frontier in magnetic storage technology—spin manipulation without a magnetic field—but its underlying working principle is not well understood. Many AOS ferrimagnets such as GdFeCo are amorphous and renders the high-level first-principles study unfeasible. The crystalline half-metallic Heusler ${\mathrm{Mn}}_2\mathrm{RuGa}$ presents an opportunity. Here we carry out hitherto the comprehensive density functional investigation into the material properties of ${\mathrm{Mn}}_2\mathrm{RuGa}$, and introduce two concepts—the spin anchor site and the optical active site—as two pillars for AOS in ferrimagnets. In ${\mathrm{Mn}}_2\mathrm{RuGa}, \mathrm{Mn}\left(4a\right)$ serves as the spin anchor site, whose band is below the Fermi level and has a strong spin moment, while $\mathrm{Mn}\left(4c\right)$ is the optical active site whose band crosses the Fermi level. Our magneto-optical Kerr spectrum and band structure calculation jointly reveal that the delicate competition between the Ru-$4d$ and Ga-$4p$ states is responsible for the creation of these two sites. These two sites found here not only present a unified picture for both ${\mathrm{Mn}}_2\mathrm{RuGa}$ and GdFeCo, but also open the door for future applications. Specifically, we propose a ${\mathrm{Mn}}_2{\mathrm{Ru}}_x\text{Ga-based}$ magnetic tunnel junction where a single laser pulse can control magnetoresistance.

The newly synthesized strontium doped $R{\mathrm{NiO}}_2$ ($R$=Nd, Pr, and La) superconductors have stimulated extensive interests in understanding their pairing mechanism and pairing nature. Here we study the pairing mechanism in this family from a two-orbital model comprising the Ni- $3d_{x^2-y^2}$ and $3d_{xy}$ orbitals, equipped with extended Hubbard interactions and induced low-energy effective superexchange interactions. We then study the pairing symmetry in this system by using large scale variational Monte Carlo approach. Our results yield the intraorbital $d_{x^2-y^2}$-wave singlet pairing as the leading pairing symmetry in the nickelates, which is analogous to the cuprates. However, there exist two important differences between the physical properties of the two families due to the fact that at the low Sr-doping regime, while the Ni-$3d_{x^2-y^2}$ orbitals remain half-filled and singlyins occupied to form a Mott-insulating background, the Ni-$3d_{xy}$ orbitals accommodate nearly all the extra doped holes, which move freely on this background. The first difference lies in the single-particle aspect: while the $3d_{x^2-y^2}$ degree of freedom remains Mott insulating with spectra weight pinned down at zero at low dopings, the $3d_{xy}$ one behaves as Fermi liquid with spectra weight near 1. The second difference lies in the pairing aspect: while the huge intra-$3d_{x^2-y^2}$-orbital pairing gap is actually a pseudogap which has nothing to do with the SC, the small intra-$3d_{xy}$-orbital pairing gap serves as the true superconducting pairing gap, which is related to the $T_c$ via the BCS relation. Both differences can be verified by the angle-resolved photoemission spectrum.

We use high-pressure resistivity and single crystal x-ray diffraction at ambient and low temperature to investigate the charge ordering phase transition of ${\mathrm{CaMn}}_7{\mathrm{O}}_{12}$. We have found that at ambient temperature the Jahn-Teller distortion of the ${\mathrm{Mn}}^{3+}{\mathrm{O}}_6$ octahedra rapidly decreases above 20 GPa, and vanishes at 28 GPa, when two Mn octahedral sites initially occupied by ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Mn}}^{4+}$ become regular and equivalent as the result of a charge delocalization. Such a change correlates with a two orders of magnitude drop in the resistivity and a symmetry increase from the low-pressure rhombohedral R$\overline{3}$ phase to the cubic Im$\overline{3}$ structure, the same as one found at ambient pressure above 440 K. This yields the slope of the charge ordering phase boundary of $d{T}_c/dp\approx -6$ K/GPa. This result is further supported by the lack of a structural phase transition up to the maximum measured pressure of 30 GPa when the experiment is performed at 70 K. The satellite reflections of the structural modulation of the multiferroic phase of ${\mathrm{CaMn}}_7{\mathrm{O}}_{12}$ observed at 70 K were found to hold up to 25 GPa with the structure keeping a constant modulation vector $k=\left(000.925\right)$ with pressure. The average structure at 70 K does not show other indications of further phase transition.