Modulation Wave Vector Research Articles

Highly compact magneto-optical switches for metal-dielectric-metal plasmonic waveguides.

We introduce highly compact resonant-cavity-enhanced magneto-optical switches for metal-dielectric-metal (MDM) plasmonic waveguides. The field profile of the fundamental mode of a MDM waveguide in which the metal is subject to an externally applied static magnetic field is asymmetric. The static magnetic field induced asymmetry, which enhances or reduces the coupling between the waveguide and a side-coupled resonator, and the relatively large induced wave vector modulation are used to design a Fabry-Perot cavity magneto-optical switch, consisting of a MDM waveguide side-coupled to two MDM stub resonators. The on and off states correspond to either the presence or the absence of the externally applied static magnetic field.

Quantization of Electromagnetic Modes in a Hyperbolic Negative-Index Layered Superconductor Slab

We theoretically study the infrared reflectivity and transmissivity of a high-temperature layered superconductor slab. Both infrared spectra exhibit very narrow Fabry–Perot resonances associated with the quantization of the wave vector of the TM electromagnetic modes. The resonances are observed in a pass band where the refractive index of the layered superconductor is negative. The pass band of negative dispersion is above the Josephson plasma frequency which appears in the expression for the effective permittivity component, corresponding to the direction perpendicular to the layers. It was found that the Fabry–Perot resonances undergo a blue shift as the slab thickness or the angle of incidence are increased. Moreover, the quantized electromagnetic modes turn out to be quasi-longitudinal because of the strong anisotropy of the infrared dielectric response of the layered superconductor.

Superspace description of trimethyltin hydroxide at T = 100 K

Abstract At low temperatures the metalorganic compound trimethyltin hydroxide, (CH3)3SnOH, possesses a commensurately modulated crystal structure, the modulation wave vector can be described as q = 1 2 c ∗ . ${\bf{q}}\; = \;{1 \over 2}{{\bf{c}}^ * }.$ The crystal structure is studied by analysing single-crystal X-ray diffraction data within the (3+1)-dimensional superspace approach and superspace group P21212(00γ)00s. The corresponding twofold superstructure has space group symmetry P212121. The structure is characterised by polymeric chains running along c-axis, generated by Sn–O–Sn bridges between neighbouring Sn atoms and packed in a distorted hexagonal pattern and linked via C–H···O interstrand hydrogen bonds along the (orthorhombic) directions [110] and [11̅0], but not along [100].

Electromagnetic fluctuation spectra of collective oscillations in magnetized Maxwellian equal mass plasmas for low-frequency waves

Recently, the general electromagnetic fluctuation theory for magnetized plasmas has been used to study the steady-state fluctuation spectra and the total intensity of low-frequency collective weakly damped modes for parallel wave vectors in Maxwellian plasmas. Now, we address the same question with respect to an arbitrary direction of the wave-vector. Here, we analyze this problem for equal mass plasmas. These plasmas are a very good tool to study various plasma phenomena, as they considerably facilitate the theoretical consideration and at the same time provide with their clear physical picture. Finally, we compare our results in the limiting case of parallel wave vectors with the previous study.

Theory of the jitter radiation in a magnetized plasma accompanying a temperature gradient

Abstract The linear stability of a magnetized plasma accompanying a temperature gradient is reexamined by using plasma kinetic theory. We propose that the anisotropic velocity distribution function should be decomposed into two components. One is proportional to the temperature gradient parallel to the background magnetic field. The other is proportional to the temperature gradient perpendicular to the background magnetic field. Since the amplitude of the anisotropic velocity distribution function is proportional to the heat conductivity, and the heat conductivity perpendicular to the magnetic field is strongly reduced, the first component of the anisotropic velocity distribution function is predominant. The anisotropic velocity distribution function induced by the temperature gradient along the background magnetic field drives plasma kinetic instability and circular polarized magnetic plasma waves are excited. We show that the instability is almost identical to the Weibel instability in the weakly magnetized plasma. However, in the case of the instability caused by the temperature gradient, whether wave vectors of modes are parallel to or antiparallel to the background magnetic field, the growth rate of one mode is suppressed and the growth rate of the other mode is enhanced due to the background magnetic field. In the strongly magnetized plasma, one mode is stabilized and only one of the modes remains unstable. The formulae for the jitter radiation spectrum emitted by relativistic electrons when they travel through the magnetized plasma with the plasma waves driven by the instability are deduced at the first time. We show that the synchrotron emission and the jitter radiation are simultaneously emitted from the same relativistic electron. The jitter radiation is expected to be circularly polarized but with a very small polarization degree since almost the same amounts of left-handed and right-handed circular polarized magnetic waves are excited by the instability.

Brillouin light scattering study of transverse mode coupling in confined yttrium iron garnet/barium strontium titanate multiferroic

Using the space-resolved Brillouin light scattering spectroscopy we study the transformation of dynamic magnetization patterns in a bilayer multiferroic structure. We show that in the comparison with a single yttrium iron garnet (YIG) film magnetization distribution is transformed in the bilayer structure due to the coupling of waves propagating both in an YIG film (magnetic layer) and in a barium strontium titanate slab (ferroelectric layer). We present a simple electrodynamic model using the numerical finite element method to show the transformation of eigenmode spectrum of confined multiferroic. In particular, we demonstrate that the control over the dynamic magnetization and the transformation of spatial profiles of transverse modes in magnetic film of the bilayer structure can be performed by the tuning of the wavevectors of transverse modes. The studied confined multiferroic stripe can be utilized for fabrication of integrated dual tunable functional devices for magnonic applications.

Superconductivity in the orthorhombic phase of thermoelectric CsPbxBi4−xTe6 with 0.3≤x≤1.0

Experimental measurements clearly reveal the presence of bulk superconductivity in the CsPbxBi4−xTe6 (0.3≤x≤1.0) materials, i.e. the first member of the thermoelectric series of Cs[PbmBi3Te5+m], these materials have the layered orthorhombic structure containing infinite anionic [PbBi3Te6]− slabs separated with Cs+ cations. Temperature dependences of electrical resistivity, magnetic susceptibility, and specific heat have consistently demonstrated that the superconducting transition in Cs0.96Pb0.25Bi3.75Te6.04 occurs at Tc=3.1K, with a superconducting volume fraction close to 100% at 1.8K. Structural study using aberration-corrected STEM/TEM reveals a rich variety of microstructural phenomena in correlation with the Pb-ordering and chemical inhomogeneity. The superconducting material Cs0.96Pb0.25Bi3.75Te6.04 with the highest Tc shows a clear ordered structure with a modulation wave vector of q≈a*/2+c*/1.35 on the a–c plane. Our study evidently demonstrates that superconductivity deriving upon doping of narrow-gap semiconductor is a viable approach for exploration of novel superconductors.

Premartensite to martensite transition and its implications for the origin of modulation inNi2MnGaferromagnetic shape-memory alloy

We present here results of temperature dependent high resolution synchrotron x-ray powder diffraction study of sequence of phase transitions in Ni2MnGa. Our results show that the incommensurate martensite phase results from the incommensurate premartensite phase, and not from the austenite phase assumed in the adaptive phase model. The premartensite phase transforms to the martensite phase through a first order phase transition with coexistence of the two phases in a broad temperature interval (~40K), discontinuous change in the unit cell volume as also in the modulation wave vector across the transition temperature and considerable thermal hysteresis in the characteristic transition temperatures. The temperature variation of the modulation wave vector q shows smooth analytic behaviour with no evidence for any devilish plateau corresponding to an intermediate or ground state commensurate lock-in phases. The existence of the incommensurate 7M like modulated structure down to 5K suggests that the incommensurate 7M like modulation is the ground state of Ni2MnGa and not the Bain distorted tetragonal L10 phase or any other lock-in phase with a commensurate modulation. These findings can be explained within the framework of the soft phonon model.

Dielectric Properties of HDOBAMBC Close to the Smectic A–Chiral Smectic C (AC*) Transition

The temperature dependence of the dielectric susceptibility χg is recalculated by making the elastic modulus K temperature dependent for the chiral smectic C phase in HDOBAMBC. For this calculation, the experimental data for the spontaneous polarization PS, tilt angle θ and the helical pitch ρ are used from literature close to the AC* transition in HDOBAMBC. The temperature dependences of the wave vector of the helical modulation q, the elastic free energy F and the critical electric field EC are also calculated by using the temperature dependent elastic modulus K for the chiral smectic C phase close to the AC* transition in this ferroelectric liquid crystal.It is found that the critical behaviors of the dielectric susceptibility χg, the helical modulation q, elastic free energy F and the electric field EC, are modified as the temperature dependence of the elastic modulus K is taken into account. Predictions given here can be compared with the experimental measurement in the chiral smectic phase close to the AC* transition in HDOBAMBC.

X-ray induced lock-in transition of cycloidal magnetic order in a multiferroic perovskite manganite

Gd${}_{1-x}$Tb${}_{x}$MnO${}_{3}$ oxides are characterized by two main competing states that exhibit both ferroelectric and multiferroic ordering. Both states involve cycloidal magnetic order that drives ferroelectricity. In their new article, a collaboration of researchers from Japan discovered that exposure of Gd${}_{0.5}$Tb${}_{0.5}$MnO${}_{3}$ to x-ray irradiation induces a reversible transition into a new hidden state, which is also characterized by cycloidal order, but with a different modulation wave vector. The ability to manipulate the state with x-rays suggests a novel route for photocontrol of multiferroic materials.

Effect of pointlike impurities ondx2−y2charge-density waves in cuprate superconductors

Many cuprate superconductors possess an unusual charge-ordered phase that is characterized by an approximate ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ intraunit cell form factor and a finite modulation wave vector ${\mathbf{q}}^{*}$. We study the effects of impurities on this charge-ordered phase via a single-band model in which bond order is the analog of charge order in the cuprates. Impurities are assumed to be pointlike and are treated within the self-consistent $t$-matrix approximation. We show that suppression of bond order by impurities occurs through the local disruption of the ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ form factor near individual impurities. Unlike $d$-wave superconductors, where the sensitivity of ${T}_{c}$ to impurities can be traced to a vanishing average of the ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ order parameter over the Fermi surface, the response of bond order to impurities is dictated by a few Fermi surface ``hotspots.'' The bond order transition temperature ${T}_{\mathrm{bo}}$ thus follows a different universal dependence on impurity concentration ${n}_{i}$ than does the superconducting ${T}_{c}$. In particular, ${T}_{\mathrm{bo}}$ decreases more rapidly than ${T}_{c}$ with increasing ${n}_{i}$ when there is a nonzero Fermi surface curvature at the hotspots. Based on experimental evidence that the pseudogap is insensitive to Zn doping, we conclude that a direct connection between charge order and the pseudogap is unlikely. Furthermore, the enhancement of stripe correlations in the La-based cuprates by Zn doping is evidence that this charge order is also distinct from stripes.

Crystal structure determination of incommensurate modulated martensite in Ni–Mn–In Heusler alloys

The crystal structure of modulated martensite in Mn-rich off-stoichiometric Ni2Mn1.44In0.56 alloy was determined with high-resolution powder neutron diffraction and synchrotron X-ray diffraction in the frame of (3+1)-dimensional superspace theory. The average crystal structure and the modulation wave vector were firstly derived by analyzing the reflection separations induced by the martensitic transformation on the basis of the transformation orientation inheritance. This treatment could be applied to predetermine the modulated structures of materials with displacive structural transformation. The crystal structure of modulated martensite was finally refined by the Rietveld method. Results show that the martensite possesses an incommensurate 6M modulated structure of superspace group I2/m(α0γ)00, with lattice parameters a=4.3919(4)Å, b=5.6202(1)Å, c=4.3315(7)Å, and β=93.044(1)°, and the modulation wave vector q=0.343(7) c*. The detailed site occupations for extra-Mn atoms with respect to the stoichiometric case were investigated by ab initio calculations. The extra-Mn atoms have a preference to be uniformly dispersed. A threefold layered superstructure in the 3-dimensional space was proposed to approximately describe the incommensurate modulated structure. This 6M superstructure model is considered to be representative for off-stoichiometric Ni–(Co)–Mn–In modulated martensite with martensitic transformation around room temperature. The present study is expected to offer an important basis for reliable crystallographic and microstructural characterizations on Ni–Mn–In alloys, so as to understand the underlying mechanisms of their multifunctional magneto-responsive properties.

Charge order in the pseudogap phase of cuprate superconductors

Charge ordering instabilities are studied in a multiorbital model of cuprate superconductors. A known, key feature of this model is that the large local Coulomb interaction in the Cu orbitals generates local moments with short range antiferromagnetic (AF) correlations. The strong simplifying ansatz that these moments are static and ordered allows us to explore a regime not generally accessible to weak-coupling approaches. The AF correlations lead to a pseudogap-like reconstruction of the Fermi surface. We find that the leading charge instability within this pseudogap-like state is to a phase with a spatially modulated transfer of charge between neighbouring oxygen px and py orbitals accompanied by weak modulations of the charge density on the Cu orbitals. As a prime result of the AF Fermi-surface reconstruction, the wavevectors of the charge modulations are oriented along the crystalline axes with a periodicity that agrees quantitatively with experiments. This suggests a resolution to a discrepancy between experiments, which find axial order, and previous theoretical calculations, which find modulation wavevectors along the Brillouin zone diagonal. The axial order is stabilized by hopping processes via the Cu4s orbital, which is commonly not included in model analyses of cuprate superconductors. The main implication of our results is that charge order emerges from the pseudogap state, and is not the primary source of the pseudogap.

Phase diagram of (Li(1-x)Fe(x))OHFeSe: a bridge between iron selenide and arsenide superconductors.

Previous experimental results have shown important differences between iron selenide and arsenide superconductors which seem to suggest that the high-temperature superconductivity in these two subgroups of iron-based families may arise from different electronic ground states. Here we report the complete phase diagram of a newly synthesized superconducting (SC) system, (Li1-xFex)OHFeSe, with a structure similar to that of FeAs-based superconductors. In the non-SC samples, an antiferromagnetic (AFM) spin-density-wave (SDW) transition occurs at ∼127 K. This is the first example to demonstrate such an SDW phase in an FeSe-based superconductor system. Transmission electron microscopy shows that a well-known √5×√5 iron vacancy ordered state, resulting in an AFM order at ∼500 K in AyFe2-xSe2 (A = metal ions) superconductor systems, is absent in both non-SC and SC samples, but a unique superstructure with a modulation wave vector q = (1)/2(1,1,0), identical to that seen in the SC phase of KyFe2-xSe2, is dominant in the optimal SC sample (with an SC transition temperature Tc = 40 K). Hence, we conclude that the high-Tc superconductivity in (Li1-xFex)OHFeSe stems from the similarly weak AFM fluctuations as FeAs-based superconductors, suggesting a universal physical picture for both iron selenide and arsenide superconductors.

FFLO-wave-vector Lock-in Effect in Quasi-1D Superconductors

We study the phase transition into the Fulde-Ferrell-Larkin-Ovchinnikov state in high magnetic field in quasi-one dimensional superconductors within the quasi-classical formalism, taking into account the interchain Josephson coupling and the paramagnetic spin splitting. We show that anomalies in the field-direction dependence of the upper critical field when the magnetic field length equals to the FFLO period, previously described in [29], are characterized by the lock-in effect of the FFLO modulation wave vector, which is governed by the magnetic length.

Charge order and its connection with Fermi-liquid charge transport in a pristine high-T(c) cuprate.

Electronic inhomogeneity appears to be an inherent characteristic of the enigmatic cuprate superconductors. Here we report the observation of charge-density-wave correlations in the model cuprate superconductor HgBa2CuO(4+δ) (T(c)=72 K) via bulk Cu L3-edge-resonant X-ray scattering. At the measured hole-doping level, both the short-range charge modulations and Fermi-liquid transport appear below the same temperature of about 200 K. Our result points to a unifying picture in which these two phenomena are preceded at the higher pseudogap temperature by q=0 magnetic order and the build-up of significant dynamic antiferromagnetic correlations. The magnitude of the charge modulation wave vector is consistent with the size of the electron pocket implied by quantum oscillation and Hall effect measurements for HgBa2CuO(4+δ) and with corresponding results for YBa2Cu3O(6+δ), which indicates that charge-density-wave correlations are universally responsible for the low-temperature quantum oscillation phenomenon.

Structural phase transition associated with van Hove singularity in 5d transition metal compound IrTe2

We have investigated the electronic states of IrTe2 by using angle-resolved photoemission spectroscopy to elucidate the origin of the structural phase transition. Both the Ir 4f and Te 4d core level spectra exhibit dramatic splitting below the phase transition temperature Ts, suggesting that there exist two inequivalent Ir and Te sites with distinctly different electronic states in the distorted phase. The band related to the saddle points at the Fermi level (EF) is strongly reconstructed, which removes the van Hove singularity from EF below Ts. The wavevector connecting the adjacent saddle points is consistent with the in-plane superstructure modulation wavevector. These results indicate that the phase transition in IrTe2 is intimately associated with the saddle points. As the van Hove singularity mainly originates from the Te px+py orbitals, the Te 5p electronic states play a dominant role in the structural phase transition.

Exploring intertwined orders in cuprate superconductors

The concept of intertwined orders has been introduced to describe the cooperative relationship between antiferromagnetic spin correlations and electron (or hole) pair correlations that develop in copper-oxide superconductors. This contrasts with systems in which, for example, charge-density-wave (CDW) order competes for Fermi surface area with superconductivity. La2−xBaxCuO4 with x=0.125 provides an example in which the ordering of spin stripes coincides with the onset of two-dimensional superconducting correlations. The apparent frustration of the interlayer Josephson coupling has motivated the concept of the pair-density-wave superconductor, a state that theoretical calculations show to be energetically competitive with the uniform d-wave superconductor. Even at x=0.095, where there is robust superconductivity below 32K in zero field, the coexistence of strong, low-energy, incommensurate spin excitations implies a spatially modulated and intertwined pair wave function. Recent observations of CDW order in YBa2Cu3O6+x and other cuprate families have raised interesting questions regarding the general role of charge modulations and the relation to superconductivity. While there are differences in the doping dependence of the modulation wave vectors in YBa2Cu3O6+x and La2−xBaxCuO4, the maximum ordering strength is peaked at the hole concentration of 1/8 in both cases. There are also possible connections with the quantum oscillations that have been detected about the same hole concentration but at high magnetic fields. Resolving these relationships remains a research challenge.

Synthesis and characterization of the layered iron-selenideNa0.8Fe1.6Se2

An iron-selenide Na0.8Fe1.6Se2 single crystal has been successfully synthesized using a self-flux method. The electrical resistivity measurement shows that this material exhibits semiconducting behavior in the whole temperature range, with an anomalous increment of resistivity at T-s similar to 595 K. By varying the concentrations of Na and Fe, a small volume of superconducting phase could be achieved with a critical temperature of T-c similar to 34 K. Structural characterization shows that, similarly to K0.8Fe1.6Se2, the Na0.8Fe1.6Se2 phase exhibits clear superstructure with a modulation wave vector of q = (3/5,1/5,0) caused by the Fe-vacancy order within the a-b plane.

Cu3Sn – understanding the systematic absences

The intermetallic compound Cu3Sn has previously been described as a long-period antiphase boundary superstructure of the Cu3Ti structure type. While the compound itself has been reported as a tenfold and an eightfold superstructure, ternary doped alloys show shorter repetitions. Interestingly, the diffraction patterns of these compounds show non-crystallographic absences that cannot be explained using the superstructure models. Since the compound exhibits phase broadening, these models are not satisfactory because the paucity of observed data does not allow for a refinement of the composition. Here, an alternative, superspace model in the orthorhombic space group Xmcm(0β0)000 is proposed, with the centering vectors (0,0,0,0) and (½,0,0,½). The presence of the non-crystallographic absences is explained as a result of a dominating occupational modulation that is accompanied by a weaker displacive modulation. In consistency with the EDXS results, the composition has been refined to Cu3 + xSn from single-crystal X-ray diffraction data. It is further demonstrated that by varying the length and the direction of the modulation wavevector in the superspace model, the ternary Cu3Sn compounds and other colored hexagonal close packing (h.c.p.) structures can be produced.