Lack Of Inversion Symmetry Research Articles

Phonon dynamics and anharmonicity in phase segregated structural domains of MgZnO film

Phonon decay channels and anharmonicities in embedded domains in MgZnO film were studied via selective resonant Raman scattering at the elevated temperature regime up to 870 K. The resonant conditions provided by excitation with different ultraviolet laser lines enabled the detection of longitudinal optical (LO) phonons from domains with the wurtzite structure, and domains with the cubic rocksalt structure which lack inversion symmetry due to alloying. The phonon behavior was modeled in terms of three- and four-phonon decay processes using Ridley and Klemens type decay processes. It is found that the wurtzite phase displays dominantly three-phonon decay with a small four-phonon component. In contrast, the cubic phase displays a higher degree of anharmonicity in which the four-phonon processes contribute significantly to the temperature dependent frequency shift. At the elevated temperature range, the LO frequency shift rate is ∼−2.6 × 10−2 cm−1/K for the wurtzite structure while that of the cubic exhibits a much larger shift rate of ∼−1.6 × 10−1 cm−1/K. The larger anharmonicity of the domains with the cubic structure is discussed in terms of strain and deformation effects.

Noncentrosymmetric superconductor with a bulk three-dimensional Dirac cone gapped by strong spin-orbit coupling

The layered, noncentrosymmetric heavy element ${\text{PbTaSe}}_{2}$ is found to be superconducting. We report its electronic properties accompanied by electronic-structure calculations. Specific heat, electrical resistivity, and magnetic-susceptibility measurements indicate that PbTaSe${}_{2}$ is a moderately coupled, type-II BCS superconductor (${T}_{c}=3.72$ K, Ginzburg--Landau parameter $\ensuremath{\kappa}=17$) with an electron-phonon coupling constant of ${\ensuremath{\lambda}}_{ep}=0.74$. Electronic-structure calculations reveal a single bulk three-dimensional Dirac cone at the $K$ point of the Brillouin zone derived exclusively from its hexagonal Pb layer; it is similar to the feature found in graphene except there is a 0.8 eV gap opened by spin-orbit coupling. The combination of large spin-orbit coupling and lack of inversion symmetry also results in large Rashba splitting on the order of tenths of an eV.

Superconductivity in non-centrosymmetric ThCoC2

Superconductivity in compounds whose crystal structure lacks inversion symmetry are known to display intriguing properties that deviate from conventional BCS superconducting behavior. Here we report magnetization, resistivity, and heat capacity measurements on polycrystalline samples of ThCoC2, which has been reported to crystallize in the non-centrosymmetric CeNiC2 prototype structure, and show clear evidence of bulk superconductivity in ThCoC2 with a critical temperature of Tc = 2.65 K. From the specific heat data we find a Sommerfeld coefficient of γ = 8.38 mJ mol−1 K−2 and a Debye temperature of ΘD = 449 K. Interestingly the Hc2 superconducting phase diagram displays positive curvature, and the specific heat at low temperature deviates from conventional exponential temperature dependence, which is suggestive of possible unconventional superconducting behavior in ThCoC2, similar to that seen in the isostructural and isoelectronic non-centrosymmetric superconductor LaNiC2.

Large spin splitting in the conduction band of transition metal dichalcogenide monolayers

We study the conduction band spin splitting that arises in transition metal dichalcogenide (TMD) semiconductor monolayers such as MoS$_2$, MoSe$_2$, WS$_2$ and WSe$_2$ due to the combination of spin-orbit coupling and lack of inversion symmetry. Two types of calculation are done. First, density functional theory (DFT) calculations based on plane waves and pseudo potentials that yield large splittings, between 3 and 30 meV. Second, we derive a tight-binding model, that permits to address the atomic origin of the splitting. The basis set of the model is provided by the maximally localized Wannier orbitals, obtained from the DFT calculation, and formed by 11 atomic-like orbitals corresponding to 5$d$ and 3$p$ orbitals of the transition metal (W,Mo) and chalcogenide (S,Se) atoms respectively. In the resulting Hamiltonian we can independently change the atomic spin-orbit coupling constant of the two atomic species at the unit cell, which permits to analyse their contribution to the spin splitting at the high symmetry points. We find that ---in contrast to the valence band--- both atoms give comparable contributions to the conduction band splittings. Given that these materials are most often $n-$doped, our findings are important for developments in TMD spintronics.

Crystal growth, structural and physical properties of the 5d noncentrosymmetric LaOsSi3

LaOsSi3 single crystals are synthesized for the first time by an arc melting method. The crystal features a tetragonal BaNiSn3-type structure (space group I4mm) which lacks inversion symmetry along the crystallographic c axis and is isostructural with the intensively studied Rashba-type noncentrosymmetric superconductors LaRhSi3 and LaIrSi3. Unlike LaRhSi3 and LaIrSi3 displaying superconductivity, LaOsSi3 shows only metallic behavior over the measured temperature range of 2 K–300 K. The Sommerfeld coefficient γ derived from specific heat is 5.76 mJ·mol−1K−2, indicating that LaOsSi3 has a weak electronic correlation effect. The absence of superconductivity in LaOsSi3 may lie in the Os 5d bands in the electronic structure. If it is true, it would be useful to provide complementary knowledge in understanding the relation between conduction and the d bands of M in LaMSi3 compounds (M = transition metals).

Effect of spin-orbit couplings in graphene with and without potential modulation

We investigate the effect of Rashba and intrinsic spin-orbit couplings on the electronic properties and spin configurations of Dirac fermions confined in: (i) a flat graphene sheet, (ii) a graphene wire with $p$-$n$-$p$ structure, and (iii) a superlattice of graphene wires. The interplay between the spin-orbit interaction mechanisms breaks the electron-hole symmetry and the spin configuration induced by Rashba spin-orbit coupling lacks inversion symmetry in $k$ space. We show that the Rashba spin-orbit interaction doubles the Fabry-P\'erot resonant modes in the transmission spectrum of a graphene wire and opens new channels for the electron transmission. Moreover, it leads to the appearance of spin split extra Dirac cones in the energy spectrum of a graphene superlattice. It is shown that the spin of the electrons and holes confined in a flat graphene sheet is always perpendicular to their motion while this is not the case for the other nanostructures.

Spin splitting of two-dimensional states in the conduction band of asymmetric heterostructures: Contribution from the atomically sharp interface

The effect of an atomically sharp impenetrable interface on the spin splitting of the spectrum of two-dimensional electrons in heterostructures based on (001) III-V compounds has been analyzed. To this end, the single band Hamiltonian $\Gamma_{6c}$ for envelope functions is supplemented by a general boundary condition taking into account the possibility of the existence of Tamm states. This boundary condition also takes into account the spin-orbit interaction, the asymmetry of a quantum well, and the lack of inversion symmetry in the crystal and contains the single phenomenological length $R$ characterizing the structure of the interface at atomic scales. The model of a quasitriangular well created by the electric field $F$ has been considered. After the unitary transformation to zero boundary conditions, in the modified Hamiltonian interfacial contribution appears, from which the two-dimensional spin Hamiltonian is obtained through averaging over the fast motion along the normal. In the absence of magnetic field $\boldsymbol B$, this contribution is the sum of the Dresselhaus and the Bychkov-Rashba terms with the constants renormalized owing to the interfacial contribution. In the field $\boldsymbol B$ containing the quantizing component $B_z$, the off - diagonal (in cubic axes) components of the $g$-factor tensor are linear functions of $|B_z|$ and the number of the Landau level $N$. The results are in qualitative agreement with the experimental data.

Controlled Hydrolysis Synthesis and Luminescence Properties of Uniform TiO2 Spheres with Different Titanium Alkoxides

A series of uniform and well-dispersed TiO2 spheres have been successfully synthesized through a controlled hydrolysis route by using different titanium alkoxides as reactants. The types of titanium alkoxides and stirring time have an effect on the uniformity and dispersion of the TiO2 spherical particles. The addition of a small amount of salt also plays a crucial role for the formation of the monodisperse TiO2 spheres. Under ultraviolet excitation, the as-obtained Eu(3+)-doped TiO2 spheres exhibit red emission corresponding to the electric-dipole allowed 5D0-7F2 transition of Eu3+ ions, which is induced by the lack of inversion symmetry at the Eu3+ ions site. The Eu(3+)-doped TiO2 phosphors might find potential applications in the fields such as optical displays, photoelectric devices, and light-emitting diodes (LEDs).

Superconductivity in the noncentrosymmetric half-Heusler compound LuPtBi: A candidate for topological superconductivity

We report superconductivity in the ternary half-Heusler compound LuPtBi, with Tc = 1.0 K and Hc2 = 1.6 T. The crystal structure of LuPtBi lacks inversion symmetry, hence the material is a noncentrosymmetric superconductor. Magnetotransport data show semimetallic behavior in the normal state, which is evidence for the importance of spin-orbit interaction. Theoretical calculations indicate that the strong spin-orbit interaction in LuPtBi should cause strong band inversion, making this material a promising candidate for 3D topological superconductivity.

Impact of electron–phonon interaction on dynamic conductivity of gapped Dirac fermions: Application to single layer MoS2

Single layer MoS2 is a two valleys semiconductor with a direct band gap in the visible. Because it lacks inversion symmetry, circular polarized light can be used to excite charge carriers almost exclusively from a single valley. Here we study how the electron–phonon interaction manifests in this material with gapped Dirac fermions and strong spin–orbit scattering. We find that, not only is the quasi-particle dynamics modified through the usual self-energy term but the gap itself becomes renormalized. Both its real and imaginary part acquire an energy dependence which reflects phonon structure and leads directly, in the longitudinal and transverse (Hall) optical conductivity, to a new peak at twice the phonon energy above the main absorption edge. Phonon assisted Holstein absorption side bands appear in the Hall conductivity even when there is no Drude contribution because of the gap changes and consequently the Berry curvature is modified. While we use parameters specific to the case of MoS2 for illustrative purposes, the theory presented here also applies to other gapped Dirac fermion systems such as those found in topological insulators and silicene.

Second harmonic microscopy of monolayer MoS2

We show that the lack of inversion symmetry in monolayer MoS${}_{2}$ allows strong optical second harmonic generation. The second harmonic of an 810-nm pulse is generated in a mechanically exfoliated monolayer, with a nonlinear susceptibility on the order of 10${}^{\ensuremath{-}7}$ m/V. The susceptibility reduces by a factor of seven in trilayers, and by about two orders of magnitude in even layers. A proof-of-principle second harmonic microscopy measurement is performed on samples grown by chemical vapor deposition, which illustrates potential applications of this effect in the fast and noninvasive detection of crystalline orientation, thickness uniformity, layer stacking, and single-crystal domain size of atomically thin films of MoS${}_{2}$ and similar materials.

Surfaces of intermetallic compounds: Anab initioDFT study for B20-type AlPd

The low-index surfaces of the AlPd compound crystallizing in the B20 (FeSi-type) structure have been investigated using ab initio density functional methods. The space group of the B20 structure is $P{2}_{1}3$, with four threefold rotational axes along the $\ensuremath{\langle}111\ensuremath{\rangle}$ directions and three twofold screw axes along the $\ensuremath{\langle}100\ensuremath{\rangle}$ directions, but no inversion symmetry. The B20 structure exists in two enantiomorphic forms related by inversion. The termination of the structure perpendicular to the twofold screw axes is uniquely defined: The corrugated {100} surfaces are formed by the characteristic zigzag Al-Pd chains separated by shallow troughs. Perpendicular to the $\ensuremath{\langle}210\ensuremath{\rangle}$ directions the structure consists of slightly puckered planes containing Al and Pd in equal numbers. The {210} surfaces shows pseudo-fivefold symmetry. Their structure is shown to be closely related to that of the {110} surfaces of the B2 structure. Both the {100} and {210} surfaces undergo some structural relaxation, but no reconstruction changing their $(1\ifmmode\times\else\texttimes\fi{}1)$ periodicity. While perpendicular to the $\ensuremath{\langle}$100$\ensuremath{\rangle}$ and $\ensuremath{\langle}$210$\ensuremath{\rangle}$ directions only one surface termination is possible, perpendicular to the threefold $\ensuremath{\langle}$111$\ensuremath{\rangle}$ directions there are several possible surface terminations differing in structure and chemical composition. Because of the lack of inversion symmetry the threefold {111} surfaces have polar character. The (111) and $(\overline{1}\overline{1}\overline{1})$ surfaces are not equivalent; the (111) surfaces of one enantiomorph are identical to the $(\overline{1}\overline{1}\overline{1})$ surfaces of the other form (and vice versa). In both the (111) and $(\overline{1}\overline{1}\overline{1})$ directions several surface terminations are possible. The formation of threefold surfaces has been studied by simulated cleavage experiments and by calculations of the surface energies of all possible terminations. Perpendicular to the $\ensuremath{\langle}111\ensuremath{\rangle}$ direction the lowest energy has been found for a bilayer with three Al atoms per surface cell in the upper layer and one Al and one Pd in the lower part. The preferable termination perpendicular to the $\ensuremath{\langle}\overline{1}\overline{1}\overline{1}\ensuremath{\rangle}$ direction is more symmetric; it consists again of a bilayer with three Al atoms in the upper and three Pd atoms in the lower part. The surface energy of this termination further decreases if the Pd triplet is covered by additional Al atom. The calculated surface energies permit us to perform a Wulff construction of the equilibrium shape of AlPd crystallites. {100} and {210} facets together occupy 77$%$ of the surface area in about equal proportion. The high anisotropy of the energy of the {111} surfaces results in the substantial difference of the surface areas of the opposite threefold facets.

Cooling molecules the optoelectric way

In a newly implemented technique, the complex rotational and vibrational motions of molecules are not a hindrance but a help.

Anisotropic Formation and Distribution of Stacking Faults in II–VI Semiconductor Nanorods

Nanocrystals of cadmium selenide exhibit a form of polytypism with stable forms in both the wurtzite and zinc blende crystal structures. As a result, wurtzite nanorods of cadmium selenide tend to form stacking faults of zinc blende along the c-axis. These faults were found to preferentially form during the growth of the (001) face, which accounts for 40% of the rod's total length. Since II-VI semiconductor nanorods lack inversion symmetry along the c-axis of the particle, the two ends of the nanorod may be identified by this anisotropic distribution of faults.

GATE-CONTROLLED RASHBA SPIN–ORBIT INTERACTION INTENSITY IN SEMICONDUCTOR HETEROSTRUCTURES

In order to realize the spin-polarized field-effect-transistor, a controllable spin–orbit interaction is necessary. Two kinds of spin–orbit interaction, Dresselhaus and Rashba spin–orbit interaction, in semiconductor heterostructures have been widely discussed and investigated in terms of both theories and experiments. Dresselhaus and Rashba spin–orbit interaction mainly comes from the lack of inversion symmetry and effective electric field inside the quantum well, respectively. Many experimental investigations show that external voltages affect the carrier concentration of reservoirs, wavefunction distribution in the quantum well and the conduction band profile of the heterostructures. The details of the mechanisms and the efficiency of different effects on the spin–orbit interaction intensity are discussed through different structures and materials. The results show that an increase in carrier concentration or a decrease in gate voltage enhances the Rashba spin–orbit interaction intensity. On the other hand, the wavefunction penetration is the other important mechanism that affects the Rashba spin–orbit interaction intensity. The carrier concentration asymmetry factor strongly affects the efficiency of the external gate voltage on the Rashba spin–orbit interaction intensity.

Anomalous Electro-Optic Effect in Polar Liquid Films

We present an anomalous electro-optic effect in polar liquid films: liquids, usually considered to be isotropic, possess the linear electro-optic effect that occurs only in materials lacking inversion symmetry. Due to the observed large effect in the low-frequency range and slow response speed, this strange effect was thought to come from the field-induced orientation of large mass. Therefore, we brought forward a physical model that contributed to the interpretation of this phenomenon: field-induced pre-oriented, short-range orderly dipole clusters in liquid films break the macroscopic symmetry and results in this asymmetric effect. Finally, combined with spectral analysis, the formation of clusters induced by an electric pulse was proved.

Magneto-orbital effect without spin-orbit interactions in a noncentrosymmetric zeolite-templated carbon structure

A peculiar manifestation of orbital angular momentum is proposed for a zeolite-templated carbon system, C${}_{36}$H${}_{9}$. The structure, being a network of nanoflakes in the shape of a ``pinwheel,'' lacks inversion symmetry. While the unit cell is large, the electronic structure obtained with a first-principles density-functional theory and captured as an effective tight-binding model in terms of maximally localized Wannier functions, exhibits an unusual feature that the valence band top comes from two chiral states having orbital magnetic momenta of $\ifmmode\pm\else\textpm\fi{}1$. The noncentrosymmetric lattice structure then makes the band dispersion asymmetric, as reminiscent of, but totally different from, spin-orbit systems. The unusual feature is predicted to imply a current-induced orbital magnetism when holes are doped.

Atomic and itinerant effects at the transition-metal x-ray absorptionKpre-edge exemplified in the case of V2O3

X-ray absorption spectroscopy is a well-established tool for obtaining information about orbital and spin degrees of freedom in transition-metal and rare-earth compounds. For this purpose usually the dipole transitions of the $L$ ($2p$ to $3d$) and $M$ ($3d$ to $4f$) edges are employed, whereas higher order transitions such as quadrupolar $1s$ to $3d$ in the $K$ edge are rarely studied in that respect. This is due to the fact that usually such quadrupolar transitions are overshadowed by dipole-allowed $1s$ to $4p$ transitions and, hence, are visible only as minor features in the pre-edge region. Nonetheless, these features carry a lot of valuable information, similar to the dipole $L$-edge transition, which is not accessible in experiments under pressure due to the absorption of the diamond anvil pressure cell. We recently performed a theoretical and experimental analysis of such a situation for the metal-insulator transition of (V${}_{(1\ensuremath{-}x)}$Cr${}_{x}$)${}_{2}$O${}_{3}$. Since the importance of the orbital degrees of freedom in this transition is widely accepted, a thorough understanding of quadrupole transitions of the vanadium $K$ pre-edge provides crucial information about the underlying physics. Moreover, the lack of inversion symmetry at the vanadium site leads to on-site mixing of vanadium $3d$ and $4p$ states and related quantum mechanical interferences between dipole and quadrupole transitions. Here we present a theoretical analysis of experimental high-resolution x-ray absorption spectroscopy at the V $K$ pre-edge measured in partial fluorescence yield mode for single crystals. We carried out density functional as well as configuration interaction calculations in order to capture effects coming from both itinerant and atomic limits.

Chiral magnetization configurations in magnetic nanostructures in the presence of Dzyaloshinskii-Moriya interactions

Many low-dimensional systems, such as nanoscale islands, thin films, and multilayers, as well as bulk systems, such as multiferroics, are characterized by the lack of inversion symmetry, a fact that may give rise to a Dzyaloshinskii-Moriya (DM) interaction. For sufficient strength, the DM interaction will favor spiral spin configurations of definite chirality. In order to harness such systems for applications, it is important to understand the conditions under which these spiral spin configurations form and how they can be controlled via an external field. Here, we present exact solutions of the 1D magnetization profiles in such systems for arbitrary material parameters in closed form. Determining the energy per unit length exactly, we are able to present the critical strength of the DM interaction, at which spiral solutions are energetically favorable. These magnetization profiles, in general, take the form of a domain wall or soliton lattice, with all solitons having the same chirality, whose sign is dictated by DM interaction. Conversely, given an energetically favorable spiral solution, we determine quantitatively how the magnetization profile changes as a function of the applied field.

Second Harmonic Generation Correlation Spectroscopy for Nanocrystal Characterization

Many early trials at protein crystallization produce large amounts of sub-diffraction limited crystals. These nanocrystalline showers are challenging to quantitatively characterize by conventional optical methods; however, they can offer important indicators for improving crystallization conditions. Additionally, the advent and availability of ultrafast X-ray free-electron lasers now allows single-pulse diffraction from individual protein nanocrystals for structure determination. However, these and other applications of nanocrystals currently suffer major bottlenecks in sample characterization, limiting their broader utility. Second harmonic generation correlation spectroscopy (SHG-CS) is being developed to address this key characterization need. Under tight focus and high laser intensity, highly-ordered (crystalline) material lacking inversion symmetry; including the vast majority of protein crystals but not simple salt crystals, amorphous protein, solvents, etc.; allow for second harmonic generation, the frequency doubling of light. This provides a way to selectively track crystalline protein particles in solution. The size of the particles can be determined by taking advantage of the fact that the particles diffuse through solution. The amount of time spent in a particular pixel along with the size of the pixel allows diffusion constants to be extracted by autocorrelation. Although in principle, SHG-CS is similar to fluorescence correlation spectroscopy, practical challenges arising from the use of high average power (>100 mW) require constant motion of the beam. The coherence of SHG can also present unique challenges and potential opportunities compared to fluorescence-based correlation methods.