We study theoretically the interplay effect of Zeeman field and modulated spin-orbit coupling on the topological properties of a one-dimensional dimerized lattice, known as Su-Schrieffer-Heeger model. We find that in the weak (strong) modulated spin-orbit coupling regime, trivial regions or nontrivial ones with two pairs of zero-energy states can be turned into nontrivial regions by applying a uniform (staggered) perpendicular Zeeman field through a topological phase transition. Furthermore, the resulting nontrivial phase hosting a pair of zero-energy boundary states can survive within a certain range of the perpendicular Zeeman field magnitude. Due to the effective time-reversal, particle-hole, chiral, and inversion symmetries, in the presence of either a uniform or a staggered perpendicular Zeeman field, the topological class of the system is BDI, which can be characterized by $\mathbb{Z}$ index. We also examine the robustness of the nontrivial phase by breaking the underlying symmetries, which results in that inversion symmetry plays an important role.

We provide direct evidence to understand the origin of low thermal conductivity of SnSe using elastic measurements. Compared to state-of-the-art lead chalcogenides $\mathrm{Pb}Q(Q=\mathrm{Te}$, Se, S), SnSe exhibits low values of sound velocity $(\sim 1420\mathrm{m}/\mathrm{s})$, Young's modulus $(E\sim 27.7\mathrm{GPa})$, and shear modulus $(G\sim 9.6\mathrm{GPa})$, which are ascribed to the extremely weak Sn-Se atomic interactions (or bonds between layers); meanwhile, the deduced average Grüneisen parameter γ of SnSe is as large as ∼3.13, originating from the strong anharmonicity of the bonding arrangement. The calculated phonon mean free path (l ∼ 0.84 nm) at 300 K is comparable to the lattice parameters of SnSe, indicating little room is left for further reduction of the thermal conductivity through introducing nanoscale microstructures and microscale grain boundaries. The low elastic properties indicate that the weak chemical bonding stiffness of SnSe generally causes phonon modes softening which eventually slows down phonon propagation. This work provides insightful data to understand the low lattice thermal conductivity of SnSe.

We present a fully relativistic formulation of the energy loss of a charged particle traversing a conductive monoatomic layer and apply it to the case of graphene in a transmission electron microscope (TEM). We use two models of conductivity appropriate for different frequency regimes: (a) THz (terahertz) frequency range and (b) optical range. In each range we distinguish two types of contributions to the electron energy loss: the energy deposited in graphene in the form of electronic excitations (Ohm losses), and the energy that is emitted in the form of radiation. We find strong relativistic effects in the electron energy loss spectra, which are manifested, e.g., in the increased heights of the principal $\pi$ and $\sigma +\pi$ peaks that may be observed in TEM in the optical range. While the radiative energy losses are suppressed in the optical range in comparison to the Ohmic losses, we find that these two contributions are comparable in magnitude in the THz range, where the response of doped graphene is dominated by the Dirac plasmon polariton (DPP). In particular, relative contributions of the Ohmic and radiative energy losses are strongly affected by the damping of DPP. In the case of a clean graphene with low damping, the angular distribution of the radiated spectra at the sub-THz frequencies exhibit strong and possibly observable skewing towards graphene.

We theoretically study scattering process and superconducting triplet correlations in a graphene junction comprised of ferromagnet-RSO-superconductor in which RSO stands for a region with Rashba spin-orbit interaction. Our results reveal spin-polarized subgap transport through the system due to an anomalous equal-spin Andreev reflection in addition to conventional backscatterings. We calculate equal- and opposite-spin pair correlations near the F-RSO interface and demonstrate direct link of the anomalous Andreev reflection and equal-spin pairings arisen due to the proximity effect in the presence of RSO interaction. Moreover, we show that the amplitude of anomalous Andreev reflection, and thus the triplet pairings, are experimentally controllable when incorporating the influences of both tunable strain and Fermi level in the nonsuperconducting region. Our findings can be confirmed by a conductance spectroscopy experiment and may provide more insights into the proximity-induced RSO coupling in graphene layers reported recently in experiments [A. Avsar et al., Nat. Commun. 5, 4875 (2014); Z. Wang et al., Phys. Rev. Lett. 114, 016603 (2015); J. B. S. Mendes et al., Phys. Rev. Lett. 115, 226601 (2015); S. Dushenko et al., Phys. Rev. Lett. 116, 166102 (2016)].

By using an exact multiscattering electromagnetic Green's function method, we present rigorous calculations on the rate and efficiency of Förster resonance energy transfer (FRET) from a donor to an acceptor when they are located in the hotspots of nanoparticle clusters. A nonlocal effect has been considered by using a hydrodynamic model. It is found that the FRET rate and efficiency can be enhanced simultaneously by more than 9 and 3 orders of magnitude, respectively, due to the strong coupling plasmon resonances originating from collective excitations of nanoparticle clusters. The physical origins for these phenomena have been disclosed. Two opposite phenomena, the energy transfer rate being independent or dependent on the local density of optical states (LDOS), have been observed in the same system under different conditions. These findings not only help us to understand the unresolved debate on how the FRET rate depends on the LDOS, but also provide a way to realize ultrafast the energy transfer process with ultrahigh efficiency.

Two-dimensional (2D) ${\mathrm{BNSi}}_2$ has been recently proposed to be a viable candidate material with a graphenelike structure. We have carried out ab initio evolutionary ground state searches and uncovered a number of 2D structures in the $\left(\mathrm{BN}\right){}_x{\mathrm{Si}}_{1-x}$ pseudobinary system with considerably lower energies. Nevertheless, our formation energy analysis shows that none of these or previously proposed polymorphs are stable with respect to phase separation into 2D Si and BN. Examination of the related CSi, ${\mathrm{BNC}}_2$, and ${\mathrm{BNSi}}_2$ 2D compounds indicates that the lack of miscibility in the last two systems is due to limited involvement of nitrogen electronic states in the covalent bonding.

We explore topological transitions in parameter space in order to enable adiabatic passages between regions adiabatically disconnected within a given parameter manifold. To this end, we study the Hamiltonian of two coupled qubits interacting with external magnetic fields, and make use of the analogy between the Berry curvature and magnetic fields in parameter space, with spectrum degeneracies associated to magnetic charges. Symmetry-breaking terms induce sharp topological transitions on these charge distributions, and we show how one can exploit this effect to bypass crossing degeneracies. We also investigate the curl of the Berry curvature, an interesting but as of yet not fully explored object, which together with its divergence uniquely defines this field. Finally, we suggest a simple method for measuring the Berry curvature, thereby showing how one can experimentally verify our results.

Organolead halide perovskites are a family of hybrid organic-inorganic compounds whose remarkable optoelectronic properties have been under intensive scrutiny in recent years. Here we use inelastic x-ray scattering to study low-energy lattice excitations in single crystals of methylammonium lead iodide and bromide perovskites. Our findings confirm the displacive nature of the cubic-to-tetragonal phase transition, which is further shown, using neutron and x-ray diffraction, to be close to a tricritical point. Lastly, we detect quasistatic symmetry-breaking nanodomains persisting well into the high-temperature cubic phase, possibly stabilized by local defects. These findings reveal key structural properties of these materials, and also bear important implications for carrier dynamics across an extended temperature range relevant for photovoltaic applications.

Electronic nematicity plays an important role in iron-based superconductors. These materials have a layered structure and the theoretical description of their magnetic and nematic transitions has been well established in the two-dimensional approximation, i.e., when the layers can be treated independently. However, the interaction between iron layers mediated by electron tunneling may cause nontrivial three-dimensional behavior. Starting from the simplest model for orbital nematic in a single layer, we investigate the influence of interlayer tunneling on the bulk nematic order and a possible preemptive state where this order is only formed near the surface. We found that the interlayer tunneling suppresses the bulk nematicity, which makes favorable the formation of a surface nematic order above the bulk transition temperature. The purely electronic tunneling Hamiltonian, however, favors a nematic order parameter that alternates from layer to layer. The uniform bulk state typically observed experimentally may be stabilized by the coupling with the elastic lattice deformation. Depending on the strength of this coupling, we found three regimes: (i) surface nematic and alternating bulk order, (ii) surface nematic and uniform bulk order, and (iii) uniform bulk order without the intermediate surface phase. The intermediate surface-nematic state may resolve the current controversy about the existence of a weak nematic transition in the compound ${\mathrm{BaFe}}_2{\mathrm{As}}_{2-x}{\mathrm{P}}_x$.

The $p$-wave superconducting state and persistent supercurrent in mesoscopic symmetric and asymmetric loops are investigated by numerically solving the Bogoliubov–de Gennes equations self-consistently. For square loops, the spatial variations of the superconducting order parameters are sensitive to the arm width and the superconducting pairing strength as well as temperature. Finite circulating currents emerge even if the threaded magnetic flux is absent, arising from the broken equality of the ${\mathrm{\Delta }}_{+}$ and ${\mathrm{\Delta }}_{-}$ pairing components. When the flux turns on, novel sawtooth patterns of the current evolution show up due to the appearance of energy jumps in flux for the loop with a thick wall, which is consistent with the experimental detection of unconventional quantum oscillations in thick samples of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ [X. Cai et al., Phys. Rev. B 87, 081104(R) (2013)]. Several spatial patterns of the pairing orders are obtained, and the peaks of the zero-energy local density of states indicate the occurrence of the odd-frequency pairing for some appropriate flux. Particularly, with increasing flux, a peculiar intermediate state with asymmetric profiles is found in our present $p$-wave system. In the case of rectangular loops, the flux-dependent evolution feature behaves in a more complicated manner than in the square case. Nonzero currents can also occur at zero flux by tuning the ratio of the length to height of the loop. Interestingly, a stable state with asymmetric modulations similar to those of the intermediate state presented in the square loop can be easily realized at finite flux in the rectangular system due to the aspect-ratio effect.

In this work, a systematic investigation of the alloying behavior on the $M$ sublattice of $M_2\mathrm{AlC}$, where $M$ is Ti, V, Zr, and Hf with elements in the first transition metal row as well as Ca and Sc is carried out via a combination of alloy theoretic approaches and density functional theory for 41 alloy systems. The cluster expansion formalism is used to explore the configurational space in ternary MAX phases. On the basis of their solid-solution behavior, the alloys are classified into three regimes: phase separation, weak ordering, and strong ordering. Observed trends are investigated in terms of indicators at the electronic and structural levels. For the systems showing ordering, the ordered structures are identified and their structural and electronic properties are investigated. The likelihood of some of the systems to exist as solid solutions at finite temperature is discussed.

The role of quantum ion dynamics in the low melting temperatures of Li is investigated from first principles theory. Free energies of solid and liquid phases are obtained at the classical and quantum ion levels. The results are used to determine the Li melting curve in the 40–60 and 110–150 GPa pressure ranges and are in excellent agreement with experimental data around 50 GPa. They predict the resumption of a positive melting slope at higher pressure. Quantum corrections to individual energy terms are far more significant than their net effect on the melting temperatures near 50 GPa, even though lithium behaves as a quantum solid at this pressure. The scales of these corrections increase with compression. A case is made for the possibility for anomalous melting at much higher pressures, where quantum ion dynamics are expected to play a prominent role.

The exceptionally large piezoelectric response of the morphotropic-phase-boundary (MPB) composition of the lead-free piezoelectric system $(1-x\left)\mathrm{Ba}\right(\mathrm{T}{\mathrm{i}}_{0.8}\mathrm{Z}{\mathrm{r}}_{0.2}){\mathrm{O}}_3-x\left(\mathrm{B}{\mathrm{a}}_{0.7}\mathrm{C}{\mathrm{a}}_{0.3}\right)\mathrm{Ti}{\mathrm{O}}_3$ has attracted great attention in recent years. Here in this paper we report a detailed investigation of the structural phase transformation behavior of the MPB composition $(x=0.50)$ driven by electric field, stress, and temperature. We show that the system exhibits metastable phases in a wide temperature range, and that the large piezoresponse at room temperature has a significant contribution from the increased fraction of the metastable phases induced by the poling field. Using a “powder poling” technique we also demonstrate the equivalence of stress and electric field with regard to the nature of the structural transformation. The fundamental significance of this interesting observation is discussed.

Thermally activated domain-wall (DW) motion in magnetic insulators has been considered theoretically, with a particular focus on the role of Dzyaloshinskii-Moriya interaction (DMI) and thermomagnonic torques. The thermally assisted DW motion is a consequence of the magnonic spin current due to the applied thermal bias. In addition to the exchange magnonic spin current and the exchange adiabatic and the entropic spin transfer torques, we also consider the DMI-induced magnonic spin current, thermomagnonic DMI fieldlike torque, and the DMI entropic torque. Analytical estimations are supported by numerical calculations. We found that the DMI has a substantial influence on the size and the geometry of DWs, and that the DWs become oriented parallel to the long axis of the nanostrip. Increasing the temperature smoothes the DWs. Moreover, the thermally induced magnonic current generates a torque on the DWs, which is responsible for their motion. From our analysis it follows that for a large enough DMI the influence of DMI-induced fieldlike torque is much stronger than that of the DMI and the exchange entropic torques. By manipulating the strength of the DMI constant, one can control the speed of the DW motion, and the direction of the DW motion can be switched, as well. We also found that DMI not only contributes to the total magnonic current, but also it modifies the exchange magnonic spin current, and this modification depends on the orientation of the steady-state magnetization. The observed phenomenon can be utilized in spin caloritronics devices, for example in the DMI based thermal diodes. By switching the magnetization direction, one can rectify the total magnonic spin current.

Magnetic phenomena in itinerant electron systems have been at the forefront of materials science. Here we show that the Weyl spin-orbit coupling (SOC) in three-dimensional repulsively interacting itinerant Fermi systems opens up a platform to host new itinerant magnetic phases, excitations, and phase transitions. A putative ferromagnetic state (FM) is always unstable against a stripe spiral spin density wave (S-SDW) or a stripe longitudinal SDW (LSDW) at small or large SOC strengths, respectively. The stripe-ordering wave vector is given by the nesting momentum of the two SOC-split Fermi surfaces with the same or opposite helicities at small or large SOC strengths, respectively. The LSDW is accompanied by a charge density wave (CDW) with half of its pitch. The transition from the paramagnet to the SSDW or LSDW+CDW is described by quantum Lifshitz-type actions, in sharp contrast to the Hertz-Millis types for itinerant electron systems without SOC. The collective excitations and Fermi surface reconstructions inside the SSDW and LSDW+CDW are also studied. The effects of a harmonic trap in cold-atom experiments are briefly discussed. In view of recent ground-breaking experimental advances in generating two-dimensional SOC in cold atoms, these phenomena can be observed in current or near-future cold-atom experiments even at very weak interactions. They may also be relevant to some itinerant magnetic materials with a strong SOC.

Recently, the switching between the different charge-ordered phases of $1T-{\mathrm{TaS}}_2$ has been probed by ultrafast techniques, revealing unexpected phenomena such as “hidden” metastable states and peculiar photoexcited charge patterns. Here, we apply broadband pump-probe spectroscopy with varying excitation energy to study the ultrafast optical properties of $1T-{\mathrm{TaS}}_2$ in the visible regime. By scanning the excitation energy in the near-IR region we unravel the coupling between different charge excitations and the low-lying charge-density wave state. We find that the amplitude mode of the charge-density wave exhibits strong coupling to a long-lived doublon state that is photoinduced in the center of the star-shaped charge-ordered Ta clusters by the near-IR optical excitation.

It is inferred from bulk-sensitive muon Knight shift measurement for a ${\mathrm{Bi}}_{1.76}{\mathrm{Pb}}_{0.35}{\mathrm{Sr}}_{1.89}{\mathrm{CuO}}_{6+\delta }$ single-layer cuprate that the metal-insulator (MI) transition (in the low-temperature limit, $T\to 0$) occurs at the critical hole concentration $p=p_{\mathrm{MI}}=0.09\left(1\right)$, where the electronic density of states (DOS) at the Fermi level is reduced to zero by the pseudogap irrespective of the Néel order or spin glass magnetism. Superconductivity also appears for $p>p_{\mathrm{MI}}$, suggesting that this feature is controlled by the MI transition. More interestingly, the magnitude of the DOS reduction induced by the pseudogap remains unchanged over a wide doping range ($0.1\le p\le 0.2$), indicating that the pseudogap remains as a hallmark of the MI transition for $p>p_{\mathrm{MI}}$.

We study one-dimensional topological superconductivity in the presence of time-reversal symmetry. This phase is characterized by having a bulk gap, while supporting a Kramers' pair of zero-energy Majorana bound states at each of its ends. We present a general simple model which is driven into this topological phase in the presence of repulsive electron-electron interactions. We further propose two experimental setups and show that they realize this model at low energies. The first setup is a narrow two-dimensional topological insulator partially covered by a conventional $s$-wave superconductor, and the second is a semiconductor wire in proximity to an $s$-wave superconductor. These systems can therefore be used to realize and probe the time-reversal invariant topological superconducting phase. The effect of interactions is studied using both a mean-field approach and a renormalization group analysis.

The critical two-dimensional classical Ising model on the square lattice has two topological conformal defects: the ${\mathbb{Z}}_2$ symmetry defect $D_{\epsilon }$ and the Kramers-Wannier duality defect $D_{\sigma }$. These two defects implement antiperiodic boundary conditions and a more exotic form of twisted boundary conditions, respectively. On the torus, the partition function $Z_D$ of the critical Ising model in the presence of a topological conformal defect $D$ is expressed in terms of the scaling dimensions ${\mathrm{\Delta }}_{\alpha }$ and conformal spins $s_{\alpha }$ of a distinct set of primary fields (and their descendants, or conformal towers) of the Ising conformal field theory. This characteristic conformal data ${\{{\mathrm{\Delta }}_{\alpha },s_{\alpha }\}}_D$ can be extracted from the eigenvalue spectrum of a transfer matrix $M_D$ for the partition function $Z_D$. In this paper, we investigate the use of tensor network techniques to both represent and coarse grain the partition functions $Z_{D_{\epsilon }}$ and $Z_{D_{\sigma }}$ of the critical Ising model with either a symmetry defect $D_{\epsilon }$ or a duality defect $D_{\sigma }$. We also explain how to coarse grain the corresponding transfer matrices $M_{D_{\epsilon }}$ and $M_{D_{\sigma }}$, from which we can extract accurate numerical estimates of ${\{{\mathrm{\Delta }}_{\alpha },s_{\alpha }\}}_{D_{\epsilon }}$ and ${\{{\mathrm{\Delta }}_{\alpha },s_{\alpha }\}}_{D_{\sigma }}$. Two key ingredients of our approach are (i) coarse graining of the defect $D$, which applies to any (i.e., not just topological) conformal defect and yields a set of associated scaling dimensions ${\mathrm{\Delta }}_{\alpha }$, and (ii) construction and coarse graining of a generalized translation operator using a local unitary transformation that moves the defect, which only exist for topological conformal defects and yields the corresponding conformal spins $s_{\alpha }$.

In self-assembled III-V semiconductor quantum dots, valence holes have longer spin coherence times than the conduction electrons, due to their weaker coupling to nuclear spin bath fluctuations. Prolonging hole spin stability relies on a better understanding of the hole to nuclear spin hyperfine coupling which we address both in experiment and theory in the symmetric (111) GaAs/AlGaAs droplet dots. In magnetic fields applied along the growth axis, we create a strong nuclear spin polarization detected through the positively charged trion $X^{+}$ Zeeman and Overhauser splittings. The observation of four clearly resolved photoluminescence lines—a unique property of the (111) nanosystems—allows us to measure separately the electron and hole contribution to the Overhauser shift. The hyperfine interaction for holes is found to be about five times weaker than that for electrons. Our theory shows that this ratio depends not only on intrinsic material properties but also on the dot shape and carrier confinement through the heavy-hole mixing, an opportunity for engineering the hole–nuclear spin interaction by tuning dot size and shape.