Breakdown Of Translational Symmetry Research Articles

Localized Surface Magnon Modes in Cubic Ferromagnetic Lattices

A theoretical formalism for calculating the bulk and surface spin modes in Heisenberg semi-infinite lattices is presented on a ferromagnetic cubic network of spins, coupled via nearest and next-nearest neighbors exchange interactions. The magnetic surface can be considered as semi-infinite slabs at the end of the bulk structures. The breakdown of translation symmetry, in the normal direction of the surface, gives rise to localized spin wave modes in its neighborhood. The localized magnon spectrum is derived as elements of a Landauer-type scattering matrix, in the three cubic lattices sc, bcc and fcc. The magnon properties are simulated and determined numerically for each cubic lattice by using the matching technique. The observed fluctuations in the numerical results demonstrate the interference magnon effects between scattered spinwaves and the localized magnon states, generated by the surface region with characteristic Fano resonances. In cubic leads, the localized surface spin states are sensitive to the local magnetic coupling and the incident direction in the surface boundary. In this contribution, the normalized energy of spinwaves arising from the absence of translation symmetry is analyzed for each cubic system as a function of the exchange integral parameters. This addresses the dependence of the surface magnon on the different possibilities of the of the exchange parameters variation from softening to hardening in the neighborhood of the surface region.

Partial breakdown of translation symmetry at a structural quantum critical point associated with a ferroelectric soft mode

We report that complete suppression of a phonon-driven structural phase transition causes partial breakdown of a three-dimensional translation symmetry in a well-defined sublattice. This state is revealed for a dielectric compound, ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{Al}}_{2}{\mathrm{O}}_{4}$, that comprises an ${\mathrm{AlO}}_{4}$ network incorporated into a hexagonal Ba(Sr) sublattice. Pair distribution function analyses and inelastic neutron scattering experiments provide clear-cut evidence of the ${\mathrm{AlO}}_{4}$ network forming a continuum of Al-O short-range correlations similar to glasses, whereas the Ba(Sr) sublattice preserves the original translational symmetry. This glassy network significantly dampens the phonon spectrum and transforms it into the broad one resembling those typically observed in glass materials.

Measuring phonon dispersion at an interface.

The breakdown of translational symmetry at heterointerfaces leads to the emergence of new phonon modes localized at the interface1. These modes have an essential role in thermal and electrical transport properties in devices, especially in miniature ones wherein the interface may dominate the entire response of the device2. Although related theoretical work began decades ago1,3-5, experimental research is totally absent owing to challenges in achieving the combined spatial, momentum and spectral resolutions required to probe localized modes. Here, using the four-dimensional electron energy-loss spectroscopy technique, we directly measure both the local vibrational spectra and the interface phonon dispersion relation for an epitaxial cubic boron nitride/diamond heterointerface. In addition to bulk phonon modes, we observe modes localized at the interface and modes isolated from the interface. These features appear only within approximately one nanometre around the interface. The localized modes observed here are predicted to substantially affect the interface thermal conductance and electron mobility. Our findings provide insights into lattice dynamics at heterointerfaces, and the demonstrated experimental technique should be useful in thermal management, electrical engineering and topological phononics.

Ground-State Phases of Alternating-Bond S = 1 Diamond Chains

The ground-state phases of alternating-bond spin-1 diamond chains are investigated. Each ground state consists of an array of spin clusters separated by singlet dimers owing to an infinite number of local conservation laws. If no singlet dimers are present, the ground state is equivalent to that of a spin chain with infinite length.For strong frustration, we find a series of quantum phase transitions as in the case of alternating-bond mixed diamond chains with spins 1 and 1/2. For intermediate frustration, we find the nonmagnetic Haldane or dimer phases according to whether the bond alternation is weak or strong. For weak frustration and weak bond alternation, we find the ferrimagnetic states with spontaneous magnetizations $m=1/6$ and 1/3 per site. The ferrimagnetic state with $m=1/6$ is accompanied by a spontaneous translational symmetry breakdown. This phase vanishes for strong bond alternation.

Ground State Phases of Distorted S = 1 Diamond Chains

The ground states of distorted $S=1$ diamond chains are investigated for two types of distortion called type A and B [J. Phys. Soc. Jpn. 79 (2010) 114703]. For the type A distortion, Haldane phases with and without spontaneous translational symmetry breakdown are present for large values of parameter $\lambda$ that parametrize the strength of frustration. For small $\lambda$, the Haldane phase and two quantized ferrimagnetic phases in the undistorted chain remain stable even for strong distortion. In contrast, for the type B distortion, the quantized ferrimagnetic phases with and without spontaneous translational symmetry breakdown are present for large $\lambda$. The partial ferrimagnetic phases emerge between them. For small $\lambda$, two quantized ferrimagnetic phases remain and the partial ferrimagnetic phases also emerge between them. The Haldane phase between the two kinds of ferrimagnetic phases turns into a topologically trivial double Haldane phase for strong distortion.

Raman silent modes in vertically aligned undoped ZnO nanorods

We report the observation of Raman silent modes of B1low (276cm−1) and B1high (582cm−1) in vertically aligned ZnO nanorods due to the breakdown of translational symmetry. The structural studies reveal the high crystalline nature of the ZnO nanorods on the lattice mismatched silicon substrates. The dominant donor bound exciton emission and the phonon replicas signify the good quality of the nanorods which substantiate that the anomalous Raman modes could not be attributed to the intrinsic point defects. Further, our results show that the observed silent modes of wurtzite-ZnO become Raman active due to the breakdown of the wave-vector selection rule by loss of translational symmetry induced by nanorods geometry.

Studies on the complex behavior of optical phonon modes in wurtzite (ZnO)1−x (Cr2O3) x

This paper reports on the use of phonon spectra obtained with laser Raman spectroscopy for the uncertainty concerned to the optical phonon modes in pure and composite ZnO1−x (Cr2O3) x . Particularly, in previous literature, the two modes at 514 and 640 cm−1 have been assigned to ZnO are not found for pure ZnO in our present study. The systems investigated for the typical behavior of phonon modes with 442 nm as excitation wavelength are the representative semiconductor (ZnO)1−x (Cr2O3) x (x = 0, 5, 10 and 15 %). Room temperature Raman spectroscopy has been demonstrated polycrystalline wurtzite structure of ZnO with no structural transition from wurtzite to cubic with Cr2O3. The incorporation of Cr3+ at most likely on the Zn sub-lattice sites is confirmed. The uncertainty of complex phonon bands is explained by disorder-activated Raman scattering due to the relaxation of Raman selection rules produced by the breakdown of translational symmetry of the crystal lattice and dopant material. The energy of the E 2 (high) peak located at energy 53.90 meV (435 cm−1) due to phonon–phonon anharmonic interaction increases to 54.55 meV (441 cm−1). A clear picture of the dopant-induced phonon modes along with the B 1 silent mode of ZnO is presented and has been explained explicitly. Moreover, anharmonic line width and effect of dislocation density on these phonon modes have also been illustrated for the system. The study will have a significant impact on the application where thermal conductivity and electrical properties of the materials are more pronounced.

SCATTERING BY AN ADSORBED CHAIN ON METALLIC HEXAGONAL SURFACE

We develop a theoretical approach to calculate the scattering and localization properties in the inhomogeneous hexagonal nanostructure system with adsorbed chain. Our numerical results are presented for three different positions of the defect, Top, Hollow and Bridge. The breakdown of translation symmetry induced by the inhomogeneity, gives rise to localized elastic wave modes at its neighborhood. The Landauer and Buttiker method is used to analyze the elastic wave scattering phenomena for atomic sites that constitute a minimum representation set in the neighborhood of the defect. The transmission and reflection probabilities as well as the phonons average transmittance across and the vibration density of states are determined for a large band of incident scattering energies. The coherent coupling between the localized elastic wave and the propagating modes of the perfect hexagonal structure leads to Fano resonances in the phononic conductance spectra of the studied structure. This is directly related to the measurable physical thermal conductivity of the perturbed domain.

Morphology of InN nanorods using spectroscopic Raman imaging

We report the vibrational properties of vertical and oblique InN nanorods (NRs) grown by molecular beam epitaxy (MBE). Surface optical (SO) Raman mode at 561 cm−1, belonging to E1 symmetry [SO(E1)], is identified along with symmetry allowed Raman modes of E2(low), E2(high), and E1(LO) at 87, 489, and 589 cm−1, respectively, corresponding to wurtzite InN phase. Usually, SO phonon modes arise due to breakdown of translational symmetry of surface potential at surface defects, which are attributed by the surface roughness. Intensity distribution of E1(LO) and SO(E1) phonon modes over a specified area have been analysed using Raman area mapping with an optical resolution of 400 nm. Imaging with E1(LO) phonon mode, originating from the bulk of the sample, distinguishes the vertical NRs alone. We are able to resolve NR morphologies in both vertical and oblique cases with additional Raman mapping analysis of SO(E1) phonon mode, emerging from the surface irregularities, which are confined to the tip of MBE grown NRs. Copyright © 2013 John Wiley & Sons, Ltd.

Optical Response Spectra of Surfaces and Interfaces

Recent status of optical response spectra calculations is reviewed, illustrating various bulk, surface, and interface systems. One can obtain fruitful information on atomic and electronic structures using the freedom of light such as energy (wave length) and polarization. Moreover, by explaining the spectral shape origin in reflectance difference spectroscopy, we show that the optical response of surface and interface has quite different features and needs different calculation methods from those of bulk systems, reflecting the breakdown of translational symmetry.

Haldane Phases and Ferrimagnetic Phases with Spontaneous Translational Symmetry Breakdown in Distorted Mixed Diamond Chains with Spins 1 and 1/2

The ground states of two types of distorted mixed diamond chains with spins 1 and 1/2 are investigated using exact diagonalization, DMRG, and mapping onto low-energy effective models. In the undistorted case, the ground state consists of an array of independent spin-1 clusters separated by singlet dimers. The lattice distortion induces an effective interaction between cluster spins. When this effective interaction is antiferromagnetic, several Haldane phases appear with or without spontaneous translational symmetry breakdown (STSB). The transition between the Haldane phase without STSB and that with ( n +1)-fold STSB ( n =1, 2, and 3) belongs to the same universality class as the ( n +1)-clock model. In contrast, when the effective interaction is ferromagnetic, the quantized and partial ferrimagnetic phases appear with or without STSB. An effective low-energy theory for the partial ferrimagnetic phase is presented.

Nanosphericals and nanobundles of ZnO: Synthesis and characterization

ZnO nanosphericals and nanobundles well dispersion have been synthesized using [( N, N′-bis(salicylaldehydo)ethylenediamine)zinc(II)]; [Zn(salen)] as precursor via two methods. Nanosphericals of ZnO has been prepared via thermal decomposition of [Zn(salen)] in the presence of oleylamine at 290 °C for 90 min. Also nanobundle of ZnO has been synthesized via thermolysis of [Zn(salen)] in the air at 500 °C for 5 h. The as-synthesized products were characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and scanning electronic microscopy (SEM). The room temperature photoluminescence (PL) spectra of both nanostructures are dominated by the green emission attributed to the oxygen vacancy ( V O) related donor–acceptor transition. Presence of several infrared (IR) inactive vibrational modes in the Fourier transform infrared (FT-IR) absorption spectra of the samples indicates a breakdown of translational symmetry in the nanostructures induced by native defects.

Thermolytic Growth of ZnO Nanocrystals: Morphology Control and Optical Properties

ZnO nanostructures with triangular and rod-like morphologies could be grown by thermolysis of zinc acetate in oleylamine solvent using oleic acid as surfactant. Concentration of the zinc precursor in the reaction mixture had a strong effect on the final morphology of the ZnO nanostructures. While a low precursor concentration results in triangular morphology, high precursor concentrations favor rod-like morphologies of the nanostructures. The room temperature photoluminescence (PL) spectra of both nanostructures are dominated by the green emission attributed to the oxygen vacancy (VO) related donor−acceptor transition. Presence of several infrared (IR) inactive vibrational modes in the Fourier transform infrared (FT-IR) absorption spectra of the samples indicates a breakdown of translational symmetry in the nanostructures induced by native defects. Possible mechanisms of the morphology evolution and the origin of observed visible emissions in the nanostructures are discussed.

Hidden vortex lattices in a thermally paired superfluid

We study the evolution of rotational response of a statistical mechanical model of two-component superfluid with a nondissipative drag interaction as the system undergoes a transition into a paired superfluid phase at finite temperature. The transition manifests itself in a change of (i) vortex-lattice symmetry and (ii) nature of the vortex state. Instead of a vortex lattice, the system forms a highly disordered tangle which constantly undergoes merger and reconnecting processes involving different types of vortices with a ``hidden'' breakdown of translation symmetry.

Raman mapping of a single-layer to double-layer graphene transition

We report on confocal Raman spectroscopy on a few-layer graphene flake. Adjacent single- and double-layer graphene sections allow mapping the transition in vibrational and electronic properties to a second stacked graphene sheet and with it a weak interlayer coupling. Most prominently the width of the D' peak doubles upon going from a single to a double layer, which can be explained within the double-resonant Raman model. The intensities of the G and G' lines decrease at the crossover to a single layer. Contrary to the G' line the G peak position shifts to higher wave numbers, however, not uniformly over the entire section: its frequency fluctuates spatially. The Raman map of the D line intensity shows a non-zero contribution at the boundaries of the flake and the individual sections, which can be attributed either to defects and disorder or to the breakdown of translational symmetry, whereas within the flake no D line signal is detected.

Spatially Resolved Raman Spectroscopy of Single- and Few-Layer Graphene

We present Raman spectroscopy measurements on single- and few-layer graphene flakes. By using a scanning confocal approach, we collect spectral data with spatial resolution, which allows us to directly compare Raman images with scanning force micrographs. Single-layer graphene can be distinguished from double- and few-layer by the width of the D' line: the single peak for single-layer graphene splits into different peaks for the double-layer. These findings are explained using the double-resonant Raman model based on ab initio calculations of the electronic structure and of the phonon dispersion. We investigate the D line intensity and find no defects within the flake. A finite D line response originating from the edges can be attributed either to defects or to the breakdown of translational symmetry.

Raman study of lattice dynamic behaviors in phosphorus-doped ZnO films

Phosphorus-induced lattice dynamic behaviors in ZnO:P epilayers grown by the metalorganic chemical vapor deposition technique have been studied using the Raman scattering method. Additional modes around 504, 520, 655, and 866cm−1 are attributed to the disorder-activated modes due to the breakdown of translational symmetry by P doping, well supported by the reported ab initio calculations of lattice dynamics in w-ZnO. Two modes around 364 and 478cm−1 are assigned to the local vibrational modes of Zn–P and P–O pairs, respectively. The correlation of transport and vibrational properties demonstrates the complex doping mechanism and the amphoteric nature of P dopant in ZnO. In addition, the redshift of 2 longitudinal optical multiphonon around 1154cm−1 is possibly originated from the variation of short-range forces in ZnO uniaxial lattice caused by P incorporation.

Quantum vs Classical Magnetization Plateaus of S=1/2 Frustrated Heisenberg Chains

The competition between quantum and classical magnetization plateaus of S=1/2 frustrated Heisenberg chains with modified exchange couplings is investigated. The conventional S=1/2 frustrated Heisenberg chains is known to exhibit a 3-fold degenerate udu-type classical plateau at 1/3 of the saturation magnetization accompanied by the spontaneous Z_3 translational symmetry breakdown. The stability of this plateau phase against period 3 exchange modulation which favors the 00u-type quantum plateau state (00= singlet dimer) is studied by bosonization, renormalization group and numerical diagonalization methods. The ground state phase diagram and the spin configuration in each phase are numerically determined. The translationally invariant Valence Bond Solid-type model with 4-spin and third neighbor interactions, which has the exact 00u-type quantum plateau state, is also presented. The phase transition to the classical udu-type ground state is also observed by varying the strength of 4-spin and third neighbor interactions. The relation between these two types of models with quantum plateau states is discussed.

Vibration spectra of soliton boundaries separating nanostructured 2D hexagonal lattice domains

We investigate the vibration spectra at domain boundaries called heavy and superheavy solitons that separate phase domains in hexagonal two dimensional lattices. The breakdown of translation symmetry in the 2D plane, normal to a soliton domain boundary, gives rise to localized vibration effects. The matching method is used to calculate the characteristic vibration Green functions, spectral densities, and densities of states, for the vibration components of the individual atomic sites that constitute a complete representation of the heavy and superheavy soliton domain boundaries. The variations of these vibration spectra due to changes in the homogeneous elastic constraints in the soliton domain boundary are calculated. It is shown that the calculated vibration spectra are sensitive and explicit signatures for the softening and hardening effects. The theoretical results yield criteria under certain experimental conditions, as regards the interpretation of the measurements of the elastic constraints in the domain boundaries.

Is the Roton in Superfluid4He the Ghost of a Bragg Spot?

It has always been assumed that the roton in 4He had to do with local vorticity—hence the name! We present here an alternate view: the roton is viewed as a “soft mode”, precursor of a crystallization instability. In such a picture the liquid is “nearly solid”, and the long observed similarities of heat propagation in liquid and solid phases are naturally explained. In this qualitative paper we consider three models successively. A lattice gas with one atom per site displays a Mott localization transition, as shown by the Bangalore group. The important result is the vanishing of the superfluid order parameter (condensate fraction N o) at the transition. There is no breakdown of translational symmetry and consequently no soft mode. Another lattice model with half filling, with an added nearest neighbour repulsion, was studied by Matsubara and Matsuda in the early 1950s it displays a first order transition between a superfluid and a localized charge density wave state. The excitation spectrum has a soft mode at zone edge near the transition, signalling the proximity of the CDW instability. Finally, we consider the realistic situation of a continuous system with no preexisting lattice. We approach the problem from the limit N o = 0 instead of the ideal gas N o = N. When N o = 0 the quasiparticle spectrum and the charge density spectrum are decoupled. The latter should have a soft mode ω=ωm if crystallization is close. That soft mode is a normal state property that has nothing to do with superfluidity. A small N o acts to hybridize quasiparticles and density fluctuations: the resulting anticrossing lowers ωm 2 as well as the ground state energy. We show that N o is bounded for two reasons: (i) if ω m 2 turns negative, the liquid is unstable towards freezing (ii) depletion due to quantum fluctuation exceeds N o if the latter is too large. The resulting upper bound noindent for N/N o is ≪ 1, a consequence of the deep roton minimum. The whole paper is qualitative, based on outrageous simplifications in order to make algebra tractable.