arXiv subjects

S. M. Shapiro

Publications and source records attributed to S. M. Shapiro.

17 recordsLinked to original sources

arXiv2010

Einstein Modes in the Phonon Density of States of the Single-Filled Skutterudite Yb$_{0.2}$Co$_{4}$Sb$_{12}$

Measurements of the phonon density of states by inelastic neutron \emph{time-of-flight} scattering and specific heat measurements along with first principles calculations, provide compelling evidence for the existence of an Einstein oscillator ("\emph{rattler}") at $ω_{E1} \approx$ 5.0 meV in the filled skutterudite Yb$_{0.2}$Co$_{4}$Sb$_{12}$. Multiple dispersionless modes in the measured density of states of Yb$_{0.2}$Co$_{4}$Sb$_{12}$ at intermediate transfer energies (14 meV $\leq$ \emph{$ω$} $\leq$ 20 meV) are exhibited in both the experimental and theoretical \emph{density-of-states} of the Yb-filled specimen. A peak at 12.4 meV is shown to coincide with a second Einstein mode at \emph{$ω_{E2} \approx$} 12.8 meV obtained from heat capacity data. The local modes at intermediate transfer energies are attributed to altered properties of the host CoSb$_{3}$ cage as a result of Yb-filling. It is suggested that these modes are owed to a complementary mechanism for the scattering of heat-carrying phonons in addition to the mode observed at $ω_{E1} \, \approx$ 5.0 meV. Our observations offer a plausible explanation for the significantly-higher \textit{dimensionless figures of merit} of filled skutterudites, compared to their parent compounds.

cond-mat.mtrl-sci↗

arXiv2010

Robust charge and magnetic order under electric field and current in the multiferroic LuFe(2)O(4)

We performed elastic neutron scattering measurements on the charge- and magnetically-ordered multiferroic material LuFe(2)O(4). An external electric field along the [001] direction with strength up to 20 kV/cm applied at low temperature (~100 K) does not affect either the charge or magnetic structure. At higher temperatures (~360 K), before the transition to three-dimensional charge-ordered state, the resistivity of the sample is low, and an electric current was applied instead. A reduction of the charge and magnetic peak intensities occurs when the sample is cooled under a constant electric current. However, after calibrating the real sample temperature using its own resistance-temperature curve, we show that the actual sample temperature is higher than the thermometer readings, and the "intensity reduction" is entirely due to internal sample heating by the applied current. Our results suggest that the charge and magnetic orders in LuFe(2)O(4) are unaffected by the application of external electric field/current, and previously observed electric field/current effects can be naturally explained by internal sample heating.

cond-mat.mtrl-sci↗

arXiv2008

Observation of a continuous phase transition in a shape-memory alloy

Elastic neutron-scattering, inelastic x-ray scattering, specific-heat, and pressure-dependent electrical transport measurements have been made on single crystals of AuZn and Au_{0.52}Zn_{0.48} above and below their martensitic transition temperatures (T_M=64K and 45K, respectively). In each composition, elastic neutron scattering detects new commensurate Bragg peaks (modulation) appearing at Q = (1.33,0.67,0) at temperatures corresponding to each sample's T_M. Although the new Bragg peaks appear in a discontinuous manner in the Au_{0.52}Zn_{0.48} sample, they appear in a continuous manner in AuZn. Surprising us, the temperature dependence of the AuZn Bragg peak intensity and the specific-heat jump near the transition temperature are in favorable accord with a mean-field approximation. A Landau-theory-based fit to the pressure dependence of the transition temperature suggests the presence of a critical endpoint in the AuZn phase diagram located at T_M*=2.7K and p*=3.1GPa, with a quantum saturation temperature θ_s=48.3 +/- 3.7K.

cond-mat.mtrl-sci↗

arXiv2007

Anomalous phonon behavior in the high temperature shape memory alloy: TiPd:Cr

Ti50 Pd50-xCrx is a high temperature shape memory alloy with a martensitic transformation temperature strongly dependent on the Cr composition. Prior to the transformation a premartensitic phase is present with an incommensurate modulated cubic lattice with wave vector of q0=(0.22, 0.22, 0). The temperature dependence of the diffuse scattering in the cubic phase is measured as a function temperature for x=6.5, 8.5, and 10 at. %. The lattice dynamics has been studied and reveals anomalous temperature and q-dependence of the [110]-TA2 transverse phonon branch. The phonon linewidth is broad over the entire Brillouin zone and increases with decreasing temperature, contrary to the behavior expected for anharmonicity. No anomaly is observed at q0. The results are compared with first principles calculation of the phonon structure.

cond-mat.mtrl-sci↗

arXiv2006

Observation of Phasons in Magnetic Shape Memory Alloy Ni2MnGa

An inelastic neutron scattering study of the lattice dynamics of the martensite phase of the ferromagnetic shape memory alloy, Ni2MnGa, reveals the presence of well-defined phasons associated with the charge density wave (CDW) resulting from Fermi surface (FS) nesting. The velocity and the temperature dependence of the phason are measured as well as the anomalous [110]-TA2 phonon.

cond-mat.mtrl-sci↗

arXiv2006

Structural Phase Transitions

Gen Shirane began studying ferroelectrics while he was still based in Japan in the early 1950s. It was therefore natural that when he arrived at Brookhaven and began specialising in neutron scattering that he would devote much of his energy and expertise studying structural phase transitions. We review the ground breaking experiments that showed the behaviour of antiferroelectrics and ferroelectrics were reasonably described in terms of the soft mode concept of structural phase transitions. This work lead directly to Gen being awarded the Buckley prize and, jointly with John Axe, awarded the Warren prize. We then describe his work on incommensurate phase transitions and in particular how the giant Kohn anomalies are responsible for the structural instabilities in one-dimensional metals. Finally Gen carefully investigated the central peak and the two-length scale phenomena that occur at most if not all transitions. Due to Gens elegant experimental work we know a great deal about both of these effects but in neither case is theory able to explain all of his results

cond-mat.mtrl-sci↗

arXiv2006

Lattice dynamics of the high temperature shape memory alloy Nb-Ru

Nb-Ru is a high temperature shape memory alloy that undergoes a Martensitic transformation from a parent cubic b-phase into a tetragonal b' phase at TM 900 C. Measurements of the phonon dispersion curves show that the [110]-TA2 phonon branch, corresponding in the q=0 limit to the elastic constant C'=1/2(C11-C12) has an anomalous temperature dependence. Nearly the entire branch softens with decreasing temperature as TM is approached. The temperature dependence of the low-q phonon energies suggests that the elastic constants would approach 0 as T approaches TM, indicating a second order transition. No additional lattice modulation is observed in the cubic phase.

cond-mat.mtrl-sci↗

arXiv2005

Strain phase diagram and domain orientation in SrTiO$_3$ thin films

SrTiO$_3$ thin films were used as a model system to study the effects of strain and epitaxial constraint on structural phase transitions of perovskite films. The basic phenomena revealed will apply to a variety of important structural transitions including the ferroelectric transition. Highly strained SrTiO$_3$ films were grown on different substrates, providing both compressive and tensile strain. The measured strain-temperature phase diagram is qualitatively consistent with theory, however the increase in the phase transition temperature is much larger than predicted. Due to the epitaxial strain and substrate clamping, the SrTiO$_3$ lattice is tetragonal at all temperatures. The phase transitions involve only changes in internal symmetry. The low temperature phase under tensile strain has a unique structure with orthorhombic $Cmcm$ space group but a tetragonal lattice, an interesting consequence of epitaxial constraint.

cond-mat.other↗

arXiv2004

Tunable Thermal Expansion Behavior in the Intermetallic YbGaGe

We investigate the effects of carbon and boron doping on the thermal expansion in the hexagonal (P63/mmc) intermetallic YbGaGe. X-ray powder diffraction was used to measure the lattice constants on pure and doped (C or B at nominal levels of 0.5 %) samples from T~10 K to T~300 K. Also measured were resistivity, specific-heat, and magnetic susceptibility. While the pure YbGaGe samples exhibit positive thermal volume expansion, (V300K-V10K)/V300K = 0.94%, the volume expansion in the lightly C and B-doped samples, contract and tend towards zero volume expansion. Such a strong response with such light doping suggests that the underlying mechanism for the reported zero volume expansion is substitutional disorder, and not the previously proposed valence fluctuations.

cond-mat.mtrl-sci↗

arXiv2004

Polarized beam operation of the Hybrid Spectrometer at the pulsed Spallation Neutron Source

The concept of a neutron Hybrid Spectrometer (HYSPEC) combines the time-of-flight spectroscopy with the focusing Bragg optics and incorporates a polarized beam option. Here we describe the polarization analysis scheme proposed for HYSPEC and quantify its performance via the Monte-Carlo simulations. We find that the broadband supermirror-bender transmission polarizers provide reasonably good polarization analysis capability within about 8-10 meV energy window for scattered neutron energies in the thermal range up to about 25 meV.

cond-mat.other↗

arXiv2004

Structural phase transitions in epitaxial perovskite films

Three different film systems have been systematically investigated to understand the effects of strain and substrate constraint on the phase transitions of perovskite films. In SrTiO$_3$ films, the phase transition temperature T$_C$ was determined by monitoring the superlattice peaks associated with rotations of TiO$_6$ octahedra. It is found that T$_C$ depends on both SrTiO$_3$ film thickness and SrRuO$_3$ buffer layer thickness. However, lattice parameter measurements showed no sign of the phase transitions, indicating that the tetragonality of the SrTiO$_3$ unit cells was no longer a good order parameter. This signals a change in the nature of this phase transition, the internal degree of freedom is decoupled from the external degree of freedom. The phase transitions occur even without lattice relaxation through domain formation. In NdNiO$_3$ thin films, it is found that the in-plane lattice parameters were clamped by the substrate, while out-of-plane lattice constant varied to accommodate the volume change across the phase transition. This shows that substrate constraint is an important parameter for epitaxial film systems, and is responsible for the suppression of external structural change in SrTiO$_3$ and NdNiO$_3$ films. However, in SrRuO$_3$ films we observed domain formation at elevated temperature through x-ray reciprocal space mapping. This indicated that internal strain energy within films also played an important role, and may dominate in some film systems. The final strain states within epitaxial films were the result of competition between multiple mechanisms and may not be described by a single parameter.

cond-mat.other↗

arXiv2003

Dynamics of an anisotropic Haldane antiferromagnet in strong magnetic field

We report the results of elastic and inelastic neutron scattering experiments on the Haldane gap quantum antiferromagnet Ni(C5D14N2)2N3(PF6) performed at mK temperatures in a wide range of magnetic field applied parallel to the S = 1 spin chains. Even though this geometry is closest to an ideal axially symmetric configuration, the Haldane gap closes at the critical field Hc~4T, but reopens again at higher fields. The field dependence of the two lowest magnon modes is experimentally studied and the results are compared with the predictions of several theoretical models. We conclude that of several existing theories, only the recently proposed model [Zheludev et al., cond-mat/0301424 ] is able to reproduce all the features observed experimentally for different field orientations.

cond-mat↗

arXiv2003

Anomalous Phase Transition in Strained SrTiO$_3$ Thin Films

We have studied the cubic to tetragonal phase transition in epitaxial SrTiO$_3$ films under various biaxial strain conditions using synchrotron X-ray diffraction. Measuring the superlattice peak associated with TiO$_6$ octahedra rotation in the low temperature tetragonal phase indicates the presence of a phase transition whose critical temperature is a strong function of strain, with T$_C$ as much as 50K above the corresponding bulk temperature. Surprisingly, the lattice constants evolve smoothly through the transition with no indication of a phase change. This signals an important change in the nature of the phase transition due to the epitaxy strain and substrate clamping effect. The internal degrees of freedom (TiO$_6$ rotations) have become uncoupled from the overall lattice shape.

cond-mat↗

arXiv2002

Charge transfer in the high dielectric constant materials CaCu3Ti4O12 and CdCu3Ti4O12

The cubic perovskite-related ceramic CaCu3Ti4O12 has a very high static dielectric constant ~10000 at room temperature (RT), which drops to about 100 below about 100 K. Substituting Cd for Ca reduces the RT value of the dielectric constant by over an order of magnitude. The large the dielectric constant may be due to an internal barrier layer capacitance (IBLC) effect. Infrared optical properties show a low-frequency mode that increases dramatically in strength at low temperature, suggesting a change in the effective charges and a breakdown of the IBLC model due to a semiconductor-to-insulator transition.

cond-mat.str-el↗

arXiv2000

Field-induced 3- and 2-dimensional freezing in a quantum spin liquid

Field-induced commensurate transverse magnetic ordering is observed in the Haldane-gap compound \nd by means of neutron diffraction. Depending on the direction of applied field, the high-field phase is shown to be either a 3-dimensional ordered Néel state or a short-range ordered state with dominant 2-dimensional spin correlations. The structure of the high-field phase is determined, and properties of the observed quantum phase transition are discussed.

cond-mat.str-el↗

arXiv1999

Unconventional ferromagnetic and spin-glass states of the reentrant spin glass Fe0.7Al0.3

Spin excitations of single crystal Fe0.7Al0.3 were investigated over a wide range in energy and reciprocal space with inelastic neutron scattering. In the ferromagnetic phase, propagating spin wave modes become paramagnon-like diffusive modes beyond a critical wave vector q0, indicating substantial disorder in the long-range ordered state. In the spin glass phase, spin dynamics is strongly q-dependent, suggesting remnant short-range spin correlations. Quantitative model for S(energy,q) in the ``ferromagnetic'' phase is determined.

cond-mat.dis-nn↗

arXiv1997

Magnetic excitations and structural change in the S=1/2 quasi-one-dimensional magnet Sr_{14-x}Y_{x}Cu_{24}O_{41} (0<x<1)

Neutron scattering measurements have been performed on the S=1/2 quasi-one-dimensional system Sr_{14-x}Y_{x}Cu_{24}O_{41}, which has both simple chains and two-leg ladders of copper ions. We observed that when a small amount of yttrium is substituted for strontium, which is expected to reduce the number of holes, the dimerized state and the structure in the chain are changed drastically. The inelastic peaks originating from the dimerized state of the chain becomes broader in energy but not in momentum space. This implies that the dimerized state becomes unstable but the spin correlations are unchanged with yttrium substitution. Furthermore, it was observed that nuclear Bragg peak intensities originating from the chain show strong temperature and x dependence, which suggests that the chains slide along the c axis as temperature and x are varied.

cond-mat.str-el↗