By considering Majorana zero modes to laterally couple to the quantum dot, we evaluate the thermoelectric effect in one single-dot system. The calculation results show that if one Majorana zero mode couples to the dot, the thermoelectric effect will change but its efficiency cannot be improved. However, notable thermoelectric effect takes place when two Majorana zero modes are introduced simultaneously. We believe that the findings in this work provide an alternative method for the detection of the zero mode of Majorana bound state.

This work uncovers the thermal properties of colloidal CdSe/CdS quantum dots (QDs) associated with additional epitaxial strain at the core/shell interface. We investigate the temperature-dependent behavior of the longitudinal optical (LO) phonon frequencies of these QDs by Raman spectroscopy over a temperature range of $7–300\mathrm{K}$. For wurtzite and zinc-blende structured QDs, a characteristic redshift of the CdSe and CdS LO frequencies with increasing temperature is found. The extent of this redshift, however, depends on the interplay between the two materials in the heterostructure as well as on the temperature. We propose a simple method to separate the pure thermal properties of each material from characteristics determined by the interaction of core and shell. Thereby, it is possible to determine positive thermal expansion coefficients of CdSe and CdS in the temperature range of $7–300\mathrm{K}$ with high accuracy.

Periodically driven systems can host so-called anomalous topological phases in which protected boundary states coexist with topologically trivial Floquet bulk bands. We introduce an anomalous version of reflection symmetry protected topological crystalline insulators, obtained as a stack of weakly coupled two-dimensional layers. The system has tunable and robust surface Dirac cones even though the mirror Chern numbers of the Floquet bulk bands vanish. The number of surface Dirac cones is given by a new topological invariant determined from the scattering matrix of the system. Furthermore, we find that due to particle-hole symmetry, the positions of Dirac cones in the surface Brillouin zone are controlled by an additional invariant, counting the parity of modes present at high-symmetry points.

Electric and magnetic resonances of dielectric particles have recently uncovered a range of exciting applications in light steering at the nanoscale. Breaking of particle inversion symmetry further modifies its electromagnetic response, giving rise to bianisotropy known also as magnetoelectric coupling. Recent studies suggest the crucial role of magnetoelectric coupling in realization of photonic topological metamaterials. To further unmask this fundamental link, we design and test experimentally one-dimensional array composed of dielectric particles with overlapping electric and magnetic resonances and broken mirror symmetry. Flipping over half of the meta-atoms in the array, we observe the emergence of interface states providing photonic realization of the celebrated Jackiw-Rebbi model. We trace the origin of these states to the fact that local modification of particle bianisotropic response affects its effective coupling with the neighboring meta-atoms, which provides a promising avenue to engineer topological states of light.

An accurate ab initio theory of the H-bond structure of liquid water requires a high-level exchange correlation approximation from density functional theory. Based on the liquid structures modeled by ab initio molecular dynamics by using maximally localized Wannier functions as a basis, we study the infrared spectrum of water within the canonical ensemble. In particular, we employ both the Perdew-Burke-Ernzerhof (PBE) functional within the generalized gradient approximation (GGA) and the state-of-the-art meta-GGA level approximation provided by the strongly constrained and appropriately normed (SCAN) functional. We demonstrate that the SCAN functional improves not only the water structure but also the theoretical infrared spectrum of water. Our analyses show that the improvement in the stretching and bending bands can be mainly attributed to better descriptions of directional H bonding and the covalency at the inter- and intramolecular levels, respectively. On the other hand, better agreements in libration and hindered translation bands are due to the improved dynamics of the H-bond network enabled by a less structured liquid in the experimental direction. The spectrum predicted by SCAN shows much better agreement with experimental data than the conventionally widely adopted PBE functional at the GGA level.

Iron is a common trace impurity in the group-III nitrides. Iron is also intentionally introduced in III-nitride electronic devices to create semi-insulating substrates and in the context of spintronics and quantum information applications. Despite the wide-ranging consequences of iron's presence in III-nitrides, the properties of iron impurities in the nitrides are not fully established. We investigate the impact of iron impurities on the electrical and optical properties of GaN, AlN, and InN using first-principles calculations based on a hybrid functional. We report formation energies of substitutional and interstitial iron impurities as a function of the Fermi-level position. We also investigate complexes of Fe with substitutional oxygen on the nitrogen site, with nitrogen vacancies and with hydrogen interstitials. In GaN and AlN, iron on the cation site is amphoteric. We discuss the role of the Fe-induced acceptor level and its impact on nonradiative recombination in the context of loss mechanisms in light emitters, and current collapse in high-electron-mobility transistors. In InN, we find the iron interstitial to be the most favorable configuration, where it acts as a shallow double donor.

We analyze the electron spin relaxation rate $1/T_1$ of individual ion-implanted ${}^{31}\mathrm{P}$ donors in a large set of metal-oxide-semiconductor (MOS) silicon nanoscale devices, with the aim of identifying spin relaxation mechanisms peculiar to the environment of the spins. The measurements are conducted at low temperatures ($T\approx 100$ mK) as a function of external magnetic field $B_0$ and donor electrochemical potential ${\mu }_{\mathrm{D}}$. We observe a magnetic field dependence of the form $1/T_1\propto B_0^5$ for $B_0\gtrsim 3$ T, corresponding to the phonon-induced relaxation typical of donors in the bulk. However, the relaxation rate varies by up to two orders of magnitude between different devices. We attribute these differences to variations in lattice strain at the location of the donor. For $B_0\lesssim 3\mathrm{T}$, the relaxation rate changes to $1/T_1\propto B_0$ for two devices. This is consistent with relaxation induced by evanescent-wave Johnson noise created by the metal structures fabricated above the donors. At such low fields, where $T_1>1\mathrm{s}$, we also observe and quantify the spurious increase of $1/T_1$ when the electrochemical potential of the spin excited state $|\uparrow \rangle$ comes in proximity to empty states in the charge reservoir, leading to spin-dependent tunneling that resets the spin to $|\downarrow \rangle$. These results provide precious insights into the microscopic phenomena that affect spin relaxation in MOS nanoscale devices, and provide strategies for engineering spin qubits with improved spin lifetimes.

We develop a theory of the valley Hall effect in high-quality graphene samples, in which strain-fluctuation-induced random gauge potentials have been suggested as the dominant source of disorder. We find a near-quantized value of valley Hall conductivity in the band transport regime, which originates from an enhanced side jump of a Dirac electron when it scatters off the gauge potential. By assuming a small residue charge density our theory reproduces qualitatively the temperature and gap dependence of the observed valley Hall effect at the charge neutral point. Our study suggests that the valley Hall effect in graphene systems represents a new paradigm for the anomalous Hall physics where gauge disorder plays an important role.

We investigate the spectral profiles of time signals attributed to coherent phonon generation in an undoped Si crystal. Here, the retarded longitudinal-optical- (LO-) phonon Green function relevant to the temporal variance of induced charge density of ionic cores is calculated by employing the polaronic quasiparticle model developed by the authors [Y. Watanabe et al., Phys. Rev. B 95, 014301 (2017); Phys. Rev. B 96, 125204 (2017)]. A spectral asymmetry is revealed in the frequency domain of the signals under the condition that an LO-phonon mode stays almost energetically resonant with a plasmon mode in the early time region; this lasts for approximately 100 fs immediately after the irradiation of an ultrashort pump-laser pulse. It is understood that based on the adiabatic picture in time, this asymmetry is caused by the Rosen-Zener coupling between both modes. The associated experimental results are obtained by measuring time-dependent electro-optic reflectivity signals, and it is proved that these are in harmony with the calculated ones. The spectra become more symmetric, as the photoexcited carrier density further changes from that meeting the above condition to higher and lower sides of carrier densities. Moreover, the effect of optical nutation of carrier density on the coherent phonon signals is addressed, and the present results are compared with the asymmetry caused by transient Fano resonance and the spectral profiles observed in a GaAs crystal.

We study annihilation mechanisms of small first- and second-order skyrmions and antiskyrmions on the frustrated $J_1\text{−}{J}_2\text{−}{J}_3$ square lattice with broken inversion symmetry (Dzyaloshinskii-Moriya interaction). We find that annihilation happens via the injection of the opposite topological charge in the form of meron or antimeron nucleation. Overall, the exchange frustration generates a complex energy landscape with not only many (meta)stable and unstable local energy solutions but also many possible paths connecting them. Whenever possible, we compute the activation energy and attempt frequency for the annihilation of isolated topological defects. In particular, we compare the average lifetime of the antiskyrmion calculated with transition state theory with direct Langevin simulations, for which excellent agreement is obtained.

Heusler compounds with heavy elements often display noncollinear magnetic structures, which can lead to phenomena such as the topological Hall effect. In this study, we report the structural, magnetic, electronic, and transport properties of IrMnGa. X-ray and neutron diffraction experiments reveal that the compound crystallizes in the cubic half-Heusler space group 216 with $Y$-type disorder. Manganese occupies two nonequivalent positions, causing frustration and preventing long-range magnetic order. As a consequence, a spin glass state is observed below 74 K. The spin glass state exhibits a pronounced bifurcation between field-cooled and zero-field-cooled magnetization curves, shifted hysteresis loops after field cooling, magnetic relaxation, the memory effect, absence of magnetic-ordering peaks in neutron diffraction, and a sharp cusp of the ac susceptibility. The shift of the freezing temperature as a function of ac frequency is well described by the Vogel-Fulcher law and by a critical-scaling approach indicating that IrMnGa is a canonical spin glass. Magnetotransport (including the anomalous Hall effect) and heat capacity were also investigated.

The superconductivity in the Bi-II phase of elemental bismuth (transition temperature $T_{\mathrm{c}}\simeq 3.92$ K at pressure $p\simeq 2.80$ GPa) was studied experimentally by means of the muon-spin rotation as well as theoretically by using the Eliashberg theory in combination with density functional theory calculations. Experiments reveal that Bi-II is a type-I superconductor with a zero temperature value of the thermodynamic critical field $B_{\mathrm{c}}\left(0\right)\simeq 31.97$ mT. The Eliashberg theory approach provides a good agreement with the experimental $T_{\mathrm{c}}$ and the temperature evolution of $B_{\mathrm{c}}$. The estimated value for the retardation (coupling) parameter $k_{\mathrm{B}}{T}_{\mathrm{c}}/{\omega }_{\ln }\approx 0.07$ (${\omega }_{\ln }$ is the logarithmically averaged phonon frequency) suggests that Bi-II is an intermediately coupled superconductor.

We have investigated the magnetic properties of the highly anisotropic layered ferromagnetic semiconductor ${\mathrm{CrI}}_3$ under hydrostatic pressure with the magnetic field along the easy axis of magnetization. At ambient pressure, ${\mathrm{CrI}}_3$ undergoes a second-order paramagnetic-to-ferromagnetic phase transition at $T_C=60.4$ K. $T_C$ is found to increase sublinearly from 60.4 to 64.9 K as pressure increases from 0 to 1.0 GPa. With the increase in pressure, the transition becomes sharper while magnetization at low field decreases monotonically due to a decrease in magnetocrystalline anisotropy. The weak low-field anomaly at around 48 K, resulting from two-step magnetic ordering, also shifts toward higher temperatures with increasing pressure. The observed increase in $T_C$ and the decrease in magnetization could originate from a change in coupling between the layers and the Cr-I-Cr bond angle with pressure.

The magnetism of the double perovskite compounds ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{FeCoO}}_6$ $\left(x=0,1,2\right)$ is contrasted using magnetization, neutron diffraction, and electron paramagnetic resonance with the support from density functional theory calculations. ${\mathrm{La}}_2{\mathrm{FeCoO}}_6$ is identified as a long-range ordered antiferromagnet displaying a near-room temperature transition at $T_N=270$ K, accompanied by a low temperature structural phase transition at $T_S=200$ K. The structural phase transformation at $T_S$ occurs from $R\overline{3}c$ at 300 K to $Pnma$ at 200 K. The density functional theory calculations support an insulating noncompensated antiferromagnetic structure. The long-range ordered magnetism of ${\mathrm{La}}_2{\mathrm{FeCoO}}_6$ transforms to short-range glassy magnetism as La is replaced with Sr in the other two compounds. The magnetism of ${\mathrm{La}}_2{\mathrm{FeCoO}}_6$ is differentiated from the nonequilibrium glassy features of ${\mathrm{Sr}}_2{\mathrm{FeCoO}}_6$ and ${\mathrm{SrLaFeCoO}}_6$ using the cooling-and-heating-in-unequal-fields magnetization protocols. This contrasting magnetism in the ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{FeCoO}}_6$ series is evidenced in electron paramegnetic resonance studies. The electronic density of states estimated using the density functional theory calculations contrast the insulating feature of ${\mathrm{La}}_2{\mathrm{FeCoO}}_6$ from the metallic nature of ${\mathrm{Sr}}_2{\mathrm{FeCoO}}_6$. From the present suite of experimental and computational results on ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{FeCoO}}_6$, it emerges that the electronic degrees of freedom, along with antisite disorder, play an important role in controlling the magnetism observed in double perovskites.

It is well established that the spin-orbit interaction in heavy metal/ferromagnet heterostructures leads to a significant interfacial Dzyaloshinskii-Moriya interaction (DMI), which modifies the internal structure of magnetic domain walls (DWs) to favor Néel- over Bloch-type configurations. However, the impact of such a transition on the structure and stability of internal DW defects (e.g., vertical Bloch lines) needs to be explored. We present a combination of analytical and micromagnetic calculations to describe a type of topological excitation called a DW skyrmion characterized by a ${360}^{\circ }$ rotation of the internal magnetization in a Dzyaloshinskii DW. We further propose a method to identify DW skyrmions experimentally using Fresnel-mode Lorentz transmission electron microscopy; simulated images of DW skyrmions using this technique are presented based on the micromagnetic results.

The dynamics of magnetic moments in iron-chromium alloys with different levels of Cr clustering show unusual features resulting from the fact that even in a perfect body-centered-cubic structure, magnetic moments exhibit geometric magnetic frustration resembling that of a spin glass. Due to the long range exchange coupling and configuration randomness, magnetic moments of Cr solutes remain noncollinear at all temperatures. To characterize magnetic properties of Fe-Cr alloys, we explore the temperature dependence of magnetization, susceptibility, Curie temperature, and spin-spin correlations with spatial resolution. The static and dynamic magnetic properties are correlated with the microstructure of Fe-Cr, where magnetization and susceptibility are determined by the size of Cr precipitates at nominal Cr concentrations. The Curie temperature is always maximized when the solute concentration of Cr in the $\alpha$ phase is close to 5 to 6 at. %, and the susceptibility of Fe atoms is always enhanced at the boundary between a precipitate and solid solution. Interaction between Cr and Fe stimulates magnetic disorder, lowering the effective Curie temperature. Dynamic simulation of evolution of magnetic correlations shows that the spin-spin relaxation time in Fe-Cr alloys is in the 20 to 40 ps range.

We study the correlation functions of quantum spin-$1/2$ two-leg ladders at finite temperature, under a magnetic field, in the gapless phase at various relevant temperatures $T\ne 0$, momenta $q$, and frequencies $\omega$. We compute those quantities using the time-dependent density-matrix renormalization group (T-DMRG) in an optimal numerical scheme. We compare these correlations with the ones of dimerized quantum spin chains and simple spin chains, that we compute by a similar technique. We analyze the intermediate energy modes and show that the effect of temperature leads to the formation of an essentially dispersive mode corresponding to the propagation of a triplet mode in an incoherent background, with a dispersion quite different from the one occurring at very low temperatures. We compare the low-energy part of the spectrum with the predictions of the Tomonaga-Luttinger liquid field theory at finite temperature. We show that the field theory describes in a remarkably robust way the low-energy correlations for frequencies or temperatures up to the natural cutoff (the effective dispersion) of the system. We discuss how our results could be tested in, e.g., neutron-scattering experiments.

We present a detailed microscopic investigation of fractional quantum Hall states with gapped boundaries in a coupled bilayer lattice model featuring holes whose counterpropagating chiral edge states are hybridized and gapped out. We focus on an lattice limit for cold-atom experiments, in which each hole just consists of a single removed site. Although the holes distort the original band structure and lead to in-gap remnants of the continuum edge modes, we find that the lowest nearly flat band representing a higher-genus system may naturally form by controlling the local hopping terms that gap out the boundaries. Remarkably, local interactions in this new flat band lead to various Abelian and non-Abelian fractional quantum Hall states with gapped boundaries residing on emergent higher-genus surfaces, which we identify by extracting the nontrivial topological ground-state degeneracies and the fractional statistics of quasiparticles. These results demonstrate the feasibility of realizing novel fractional quantum Hall states with gapped boundaries even in the extreme lattice limit, thus enabling a possible new route towards universal topological quantum computation.

Realization of intrinsic surface dominant transport in a wide temperature region for topological insulators (TIs) is an important frontier research to promote the progress of TIs toward future electronics. We report here systematic measurements of longitudinal electrical transport, Shubnikov–de Haas (SdH) quantum oscillations, the Hall coefficient ($R_{\mathrm{H}}^{2\mathrm{D}}$), and the Seebeck coefficient as a function of film thickness ($d$) and temperature using high-quality $\mathrm{B}{\mathrm{i}}_{2-x}\mathrm{S}{\mathrm{b}}_x\mathrm{T}{\mathrm{e}}_{3-y}\mathrm{S}{\mathrm{e}}_y$ single-crystal thin films grown by physical vapor-phase deposition. The thickness dependence of sheet conductance and the Seebeck coefficient clearly shows the suppression of semiconducting hole carriers of bulk states by reducing film thickness, reaching to the surface dominant transport at below $d_{\mathrm{c}}=14\mathrm{nm}$. Quantitative arguments are made as to how the contribution of itinerant carrier number ($n$) can be suppressed, using both $R_{\mathrm{H}}^{2\mathrm{D}}$ ($n_{\mathrm{Hall}}^{2\mathrm{D}}$) and SdH ($n_{\mathrm{SdH}}$). Intriguingly, the value of $n_{\mathrm{Hall}}^{2\mathrm{D}}$ approaches being twice that of $n_{\mathrm{SdH}}$ below $d_{\mathrm{c}}$. While $R_{\mathrm{H}}^{2\mathrm{D}}$ shows a negative sign in the whole temperature region, a change from negative to positive polarity is clearly observed for $S$ at high temperatures when $d$ is thick. We point out that this inconsistency observed between $R_{\mathrm{H}}^{2\mathrm{D}}$ and $S$ is intrinsic in three-dimensional (3D) TIs and its origin is the large difference in carrier mobility between the bulk and the topological surface. We propose that the Seebeck coefficient can become a convenient and effective tool to evaluate the intrinsic topological surface transport of 3D TIs in the absence of magnetic field.

We study the cross-section shape and size effects on the valley splitting in PbSe nanowires within the framework of empirical $s{p}^3{d}^5{s}^{*}$ tight-binding method. We consider idealized prismatic nanowires, grown along [110], with the cross-section shape varying from rectangular (terminated by $\left\{001\right\}$ and $\left\{110\right\}$ facets) to rhombic (terminated mostly by $\left\{111\right\}$ facets). The valley splitting energies have the maximal value (up to hundreds of meV) in rectangular nanowires, while in rhombic ones they are almost absent. The shape dependence is shown to be similar in nanowires with different point symmetry in a wide range of cross-section sizes.