Recent experiments on underdoped high-temperature superconductors have shown that a biaxial charge density wave coexisting with antiferromagnetism is able to account for the hole-doping dependences of the Hall coefficient and the charge density wave ordering vector. Owing to the loss of long-range order over the range of hole dopings where charge density waves are observed, however, antiferromagnetism poses a serious challenge to the viability of Landau quantization and quantum oscillations. Here we make the surprising finding that the states along the antiferromagnetic Brillouin zone boundary, which are usually the most heavily broadened by antiferromagnetic fluctuations, become gapped once the biaxial charge density wave forms. This gapping along with the location of the electron pocket produced by Fermi surface reconstruction at a momentum-space point of high symmetry causes the single nodal electron pocket to be largely protected against antiferromagnetic fluctuations in strong magnetic fields, enabling resolved Landau levels and quantum oscillations to remain robust against antiferromagnetic correlation lengths as short as $\sim 10\AA$.

Recent studies discovered that the binary transition-metal compounds $A_x{B}_y$ demonstrate extremely large magnetoresistance (XMR) under magnetic field $B$, for example, ${10}^6\%$ in $\mathrm{PtB}{\mathrm{i}}_2$ and ${10}^5\%$ in $\mathrm{WT}{\mathrm{e}}_2$. The underlying physical origins, however, are quite diverse, such as electron-hole balance, backscattering forbidden of Dirac/Weyl fermions, and high mobility. Here we experimentally find an ideal compound ($\alpha -\mathrm{W}{\mathrm{P}}_2$) where the perfect electron-hole compensation can be sustained within a large temperature range (from 2 to 100 K). The XMR of $\alpha -\mathrm{W}{\mathrm{P}}_2$ is measured as high as $8.74\times {10}^5\%$ under 9 T $B$ at 2 K, but it is remarkably decreased from $8.74\times {10}^5\%$ to 18% when the temperature is raised from 2 to 100 K; simultaneously, the mobility is decreased by more than two orders of magnitude. Magnetotransport characterizations show that MR is proportional to $B^2$ and the pronounced dHvA quantum oscillations come from the conventional Schrödinger fermions in $\alpha -\mathrm{W}{\mathrm{P}}_2$, which rules out the possibility of Dirac fermions. These evidences strongly suggest that XMR observed in binary $A_x{B}_y$ semimetals is mainly attributed to high mobility, rather than Dirac/Weyl fermions, or resonant electron-hole compensation. This work elucidates the underlying physical origin of XMR in these compounds.

Macroscopic wave packets of spin-polarized exciton-polaritons in two-dimensional microcavities experience the zitterbewegung, the effect manifested by the appearance of the oscillatory motion of polaritons in the direction normal to the initial propagation direction. The oscillating trajectories of exciton-polaritons are adjustable by the control parameters: the splitting of the longitudinal and transverse exciton-polariton modes, the wave vector, and the width of the resonant cw pump. Our theoretical analysis supported by the numerical calculations allowed us to optimize values of the control parameters suitable for a direct experimental observation of the zitterbewegung effect.

We study theoretically the spin-flip relaxation processes for a single electron in a self-assembled InAs/GaAs quantum dot, and we show that the dominant channel is the spin admixture induced by symmetry-breaking shear strain. This mechanism, determined within the eight-band envelope-function $\mathit{k}·\mathit{p}$ theory, can be mapped onto two effective spin-phonon terms in a conduction-band (effective-mass) Hamiltonian that have a similar structure and interfere constructively. Unlike the Dresselhaus coupling, which dominates spin relaxation in larger, unstrained dots, the shear strain contribution cannot be modeled by a generic, standard term in the Hamiltonian but rather relies on the actual strain distribution in the quantum dot.

We study the effects of the nonlinear piezoelectricity and the In distribution on the exciton energy, the electron-hole electric dipole moment, and the fine-structure splitting in stress-tunable In(Ga)As/GaAs quantum dots integrated onto a piezoelectric actuator. In particular, we investigate in detail the contributions of various elements of the expansion of the electrical polarization in terms of externally induced elastic strain on the latter two important quantum dot properties. Based on the comparison of the effects of first- and second-order piezoelectricity we provide a simple relation to estimate the influence of applied anisotropic stress on the quantum dot dipole moment for quantum dots significantly lattice mismatched to the host crystal.

Accurately described excitonic properties of transition metal dichalcogenide heterobilayers (HBLs) are crucial to comprehend the optical response and the charge carrier dynamics of them. Excitons in multilayer systems possess an inter- or intralayer character whose spectral positions depend on their binding energy and the band alignment of the constituent single layers. In this paper, we report the electronic structure and the absorption spectra of ${\mathrm{MoS}}_2/{\mathrm{WS}}_2$ and ${\mathrm{MoSe}}_2/{\mathrm{WSe}}_2$ HBLs from first-principles calculations. We explore the spectral positions, binding energies, and the origins of inter- and intralayer excitons and compare our results with experimental observations. The absorption spectra of the systems are obtained by solving the Bethe-Salpeter equation on top of a ${\mathrm{G}}_0{\mathrm{W}}_0$ calculation, which corrects the independent-particle eigenvalues obtained from density-functional theory. Our calculations reveal that the lowest energy exciton in both HBLs possess an interlayer character which is decisive regarding their possible device applications. Due to the spatially separated nature of the charge carriers, the binding energy of interlayer excitons might be expected to be considerably smaller than that of intralayer ones. However, according to our calculations, the binding energy of lowest energy interlayer excitons is only $\sim 20\%$ lower due to the weaker screening of the Coulomb interaction between layers of the HBLs. Therefore, it can be deduced that the spectral positions of the interlayer excitons with respect to intralayer ones are mostly determined by the band offset of the constituent single layers. By comparing oscillator strengths and thermal occupation factors, we show that in luminescence at low temperature, the interlayer exciton peak becomes dominant, while in absorption it is almost invisible.

We propose two schemes to coherently transfer arbitrary quantum states of the two-electron singlet-triplet qubit across a chain of three quantum dots. The schemes are based on electrical control over the detuning energy of the quantum dots. The first is a pulse-gated scheme, requiring dc pulses and engineering of inter- and intradot Coulomb energies. The second scheme is based on the adiabatic theorem, requiring time-dependent control of the detuning energy through avoided crossings at a rate that the system remains in the ground state. We simulate the transfer fidelity using typical experimental parameters for silicon quantum dots. Our results give state transfer fidelities between $94.3\%<\mathcal{F}<99.5\%$ at sub-ns gate times for the pulse-gated scheme and between $75.4\%<\mathcal{F}<99.0\%$ at tens of ns for the adiabatic scheme. Taking into account dephasing from charge noise, we obtain state transfer fidelities between $94.0\%<\mathcal{F}<99.2\%$ for the pulse-gated scheme and between $64.9\%<\mathcal{F}<93.6\%$ for the adiabatic scheme.

We study the magnetotransport properties of patterned 3D topological insulator nanostructures with several leads, such as kinks or Y-junctions, near the Dirac point with analytical as well as numerical techniques. The interplay of the nanostructure geometry, the external magnetic field, and the spin-momentum locking of the topological surface states lead to a richer magnetoconductance phenomenology as compared to straight nanowires. Similar to straight wires, a quantized conductance with perfect transmission across the nanostructure can be realized across a kink when the input and output channels are pierced by a half-integer magnetic flux quantum. Unlike for straight wires, there is an additional requirement depending on the orientation of the external magnetic field. A right-angle kink shows a unique $\pi$-periodic magnetoconductance signature as a function of the in-plane angle of the magnetic field. For a Y-junction, the transmission can be perfectly steered to either of the two possible output legs by a proper alignment of the external magnetic field. These magnetotransport signatures offer new ways to explore topological surface states and could be relevant for quantum transport experiments on nanostructures which can be realized with existing fabrication methods.

Floquet states of periodically driven systems could exhibit rich topological properties. Many of them are absent in their static counterparts. One such example is the chiral edge states in anomalous Floquet topological insulators, whose description requires a different topological invariant and type of bulk-edge correspondence. In this work, we propose a prototypical quenched lattice model, whose two Floquet bands could exchange their Chern numbers periodically and alternatively via touching at quasienergies 0 and $\pi$ under the change of a single system parameter. This process in principle allows the generation of as many Floquet chiral edge states as possible in a highly controllable manner. The quantized transmission of these edge states are extracted from the Floquet scattering matrix of the system. The flexibility in controlling the number of topological edge channels provided by our scheme could serve as a starting point for the engineering of robust Floquet transport devices.

Quantum-mechanical calculations are used to determine the temperature dependence of the Gibbs energy of vacancy formation in nickel. Existing data reveal a discrepancy between the high-temperature estimates from experiments and low-temperature approximations from density functional theory. Our finite-temperature calculations—which include the effects of magnetism and fully interacting phonon vibrations—demonstrate that this discrepancy is mostly caused by the previously neglected explicit anharmonic contribution.

Magnetoreflection (MR) and magnetophotoluminescence (MPL) are studied in the diluted magnetic semiconductor ${\mathrm{Zn}}_{1-x}{\mathrm{Mn}}_x\mathrm{Te}$. The use of both methods in parallel allows us to follow features resulting from the mean-field exchange behavior as well as individual characteristics of ${\mathrm{Mn}}^{2+}$ paramagnetic ions. Resonance linewidths observed in MR are analyzed and shown to depend on spin-dependent scattering of excitons. A striking feature of the MPL spectrum is an enhancement of all spin-polarized luminescence structures in an external magnetic field. Relations between the Zeeman energy shift of exciton transitions due to exchange interaction of Mn ions with mobile carriers and circular polarization of MPL due to internal recombination in ${\mathrm{Mn}}^{2+}$ are revealed. Mixing of left and right circular light polarizations is demonstrated in excitonic and donor-acceptor pair (DAP) structures of MPL peaks. An enhancement of binding energy with increasing magnetic field for excitons localized by potential fluctuations is demonstrated and explained. It is observed that the energy of MPL peaks related to DAPs is only weakly sensitive to magnetic field. This relative stability is caused by a suppression of the mean-field Zeeman shift due to the two-component form of impurity wave functions. The simultaneous presence of paramagnetic ions and residual nonmagnetic impurities in our ${\mathrm{Zn}}_{1-x}{\mathrm{Mn}}_x\mathrm{Te}$ samples provides for a rich physical picture.

We discuss the orbital angular momentum (OAM) and the edge-mass current in neutral fermionic superfluids with broken time-reversal symmetry. Recent mean field studies imply that total OAM of a uniform superfluid depends on boundary conditions and is not a thermodynamic quantity. We point out that this does not conflict with thermodynamics because there is no intensive external field conjugate to OAM with which a uniform superfluid is stable in the thermodynamic limit, in sharp contrast to the orbital magnetization in a nonsuperfluid system. We establish a simple physical picture for the sensitivity of OAM to boundaries by introducing the notion of “unpaired fermions” and “fermionic Landau criterion” within a mean field description. In order to go beyond the mean field approximation, we perform a density matrix renormalization group calculation and conclude that the mean field understanding is essentially correct.

Universal thermal data in conformal field theory (CFT) offer a valuable means for characterizing and classifying criticality. With improved tensor network techniques, we investigate the universal thermodynamics on a nonorientable minimal surface, the crosscapped disk (or real projective plane, ${\mathbb{RP}}^2$). Through a cut-and-sew process, ${\mathbb{RP}}^2$ is topologically equivalent to a cylinder with rainbow and crosscap boundaries. We uncover that the crosscap contributes a fractional topological term $\frac{1}{2}lnk$ related to a nonorientable genus, with $k$ a universal constant in two-dimensional CFT, while the rainbow boundary gives rise to a geometric term $\frac{c}{4}ln\beta$, with $\beta$ the manifold size and $c$ the central charge. We have also obtained analytically the logarithmic rainbow term by CFT calculations, and discuss its connection to the renowned Cardy-Peschel conical singularity.

Magnetic skyrmions with topologically protected spin textures have recently attracted much attention due to their fascinating properties and potential application in advanced spintronics devices. For ferromagnetic thin films, skyrmions usually appear only in the presence of magnetic field. The stabilization of skyrmions in the absence of magnetic field remains an important challenge. Here, using a real-space phase field model based on Ginzburg-Landau theory, we demonstrate that a nonuniform strain can stabilize skyrmions in the FeGa thin film without a magnetic field. The phase field simulations show that the FeGa thin film exhibits a metastable skyrmion phase in the absence of magnetic field when a nonuniform strain with a cosine profile is applied. It is found that the metastable skyrmions can be transformed into a helical phase if a localized magnetic field or pulse of spin-polarized current is applied, resulting in the coexistence of skyrmion and helical phases in the ferromagnetic thin films. Furthermore, the skyrmion and helical phases can remain dynamically stable during the motion driven by a spin-polarized current. The coexistence of skyrmion and helical phases in the ferromagnetic thin films without magnetic field has potential application in skyrmion-based spintronic devices.

The magnetic order in $\mathrm{CaK}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Ni}}_x\right)}_4{\mathrm{As}}_4$ (1144) single crystals ($x=0.051$ and 0.033) has been studied by neutron diffraction. We observe magnetic Bragg peaks associated with the same propagation vectors as found for the collinear stripe antiferromagnetic (AFM) order in the related ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ (122) compound. The AFM state in 1144 preserves tetragonal symmetry and only a commensurate, noncollinear structure with a hedgehog spin-vortex crystal (SVC) arrangement in the Fe plane and simple AFM stacking along the $\mathit{c}$ direction is consistent with our observations. The SVC order is promoted by the reduced symmetry in the FeAs layer in the 1144 structure. The long-range SVC order coexists with superconductivity, however, similar to the doped 122 compounds, the ordered magnetic moment is gradually suppressed with the developing superconducting order parameter. This supports the notion that both collinear and noncollinear magnetism and superconductivity are competing for the same electrons coupled by Fermi surface nesting in iron arsenide superconductors.

We report electrical resistivity, magnetization, and specific heat measurements on the correlated electron system ${\mathrm{Ce}}_{1-x}{\mathrm{Sm}}_x{\mathrm{CoIn}}_5$ ($0\le x\le 1$). Superconductivity (SC) in the heavy-fermion compound ${\mathrm{CeCoIn}}_5$, which is suppressed with increasing Sm concentration $x$, and antiferromagnetic (AFM) order of ${\mathrm{SmCoIn}}_5$, which is suppressed with decreasing $x$, converge near a quantum critical point at $x_{\mathrm{QCP}}\approx 0.15$, with no indication of coexistence of SC and AFM in the vicinity of the QCP. Non-Fermi-liquid (NFL) behavior is observed in the normal-state electrical resistivity, $\rho \left(T\right)$, and specific heat, $C\left(T\right)$, in the vicinity of the QCP; e.g., the coefficient and the exponent of the power-law $T$ dependence of $\rho \left(T\right)$ exhibit pronounced maxima and minima, respectively, at $x_{\mathrm{QCP}}$, while $C\left(T\right)/T$ exhibits a logarithmic divergence in $T$ at $x_{\mathrm{QCP}}$. A low-temperature upturn in $\rho \left(T\right)$ develops in the range $0.70\le x\le 0.85$ which is reminiscent of a single impurity Kondo effect, suggesting that Sm substitution tunes the relative strength of competing Kondo and Ruderman-Kittel-Kasuya-Yosida energy scales. The suppression of SC with increasing $x$ is probably associated with the exchange interaction between the Ce quasiparticles involved in the superconductivity and the magnetic moments of the Sm ions.

Topological metals and semimetals are new states of matter which are attracting great interest in current research. Here, based on first-principles calculations and symmetry analysis, we propose that the family of titanium-based compounds ${\mathrm{Ti}}_{3}X$ ($X$=Al, Ga, Sn, Pb) are unexplored topological metals. These materials feature the coexistence of a nodal loop and a nodal surface in their low-energy band structures. Taking ${\mathrm{Ti}}_3\mathrm{Al}$ as an example, we show that the material has an almost ideal nodal loop in the sense that the loop is close to the Fermi level and it is nearly flat in energy with energy variation $<0.25$ meV. The loop is protected by one of the two independent symmetries: the combined space-time-inversion symmetry and the mirror-reflection symmetry. The nodal surface at the $k_z=\pi$ plane is guaranteed by the nonsymmorphic screw rotational symmetry and the time-reversal symmetry. We discuss the effect of spin-orbit coupling and construct an effective model for describing the nodal loop. Our findings indicate that the ${\mathrm{Ti}}_3\mathrm{Al}$ family of compounds can serve as an excellent material platform for studying new topological phases and, particularly, the interplay between nodal loop and nodal surface fermions.

The interplay between the Kondo effect and magnetic ordering driven by the Ruderman-Kittel-Kasuya-Yosida interaction is studied within the two-dimensional Hubbard-Kondo lattice model. In addition to the antiferromagnetic exchange interaction $J_{\perp }$ between the localized spins and the conduction electrons, this model also contains the local repulsion $U$ between the conduction electrons. We use variational cluster approximation to investigate the competition between the antiferromagnetic phase, the Kondo singlet phase, and a ferrimagnetic phase on square lattice. At half-filling, the Néel antiferromagnetic phase dominates from small to moderate $J_{\perp }$ and $U{J}_{\perp }$, and the Kondo singlet elsewhere. Sufficiently away from half-filling, the antiferromagnetic phase first gives way to a ferrimagnetic phase (in which the localized spins order ferromagnetically, and the conduction electrons do likewise, but the two mutually align antiferromagnetically), and then to the Kondo singlet phase.

We use the determinant quantum Monte Carlo method (DQMC) to study the interaction-driven semimetal to antiferromagnetic insulator transition in a $\pi$-flux Hamiltonian with modulated hoppings, a model which has two species of Dirac fermions. It is found that the critical interaction strength $U_c$ is decreased by reducing the velocity of the outer Dirac cone, while the inner cone velocity fixes the bandwidth. Although $U_c$ is monotonic, at fixed inverse temperature $\beta$ the antiferromagnetic (AF) structure factor has a maximum as a function of the hopping modulation. We also study the corresponding strong coupling (Heisenberg model) limit, where the sublattice magnetization is enhanced by the alternation of the exchange couplings. The AF order is shown to be nonmonotonic, and maximal at an intermediate degree of anisotropy, in qualitative agreement with the Hubbard model. These results quantify the effect of the velocities on the critical interaction strength in Dirac fermion systems and enable an additional degree of control which will be useful in studying strong correlation physics in ultracold atoms in optical lattices.