Publications (179)312.19 Total impact

[Show abstract] [Hide abstract]
ABSTRACT: We measured the spectroscopic response of stripeand vortexcontaining ErMnO 3 in order to uncover the electronic properties of the domain walls. We quantify Born effective charge and polarization differences using the lattice behavior, analyze the local rare earth environment from the f manifold excitations, and reveal how shifts in the charge transfer excitations impact the band gap. The increased Born charge, polarization, and band gap in the vortexcontaining material are brought together with a discussion of hybridization and wall density effects. The domain wall optical constants are strongly frequency dependent.Physical Review B 09/2014; 90(12):121303(R). DOI:10.1103/PhysRevB.90.121303 · 3.66 Impact Factor 
Article: A broadband silicon quarterwave retarder for farinfrared spectroscopic circular dichroism
[Show abstract] [Hide abstract]
ABSTRACT: The high brightness, broad spectral coverage and pulsed characteristics of infrared synchrotron radiation enable timeresolved spectroscopy under throughputlimited optical systems, as can occur with the highfield magnet cryostat systems used to study electron dynamics and cyclotron resonance by farinfrared techniques. A natural extension for magnetospectroscopy is to sense circular dichroism, i.e. the difference in a material's optical response for left and right circularly polarized light. A key component for spectroscopic circular dichroism is an achromatic 1/4 wave retarder functioning over the spectral range of interest. We report here the development of an inline retarder using total internal reflection in highresistivity silicon. We demonstrate its performance by distinguishing electronic excitations of different handednesses for GaAs in a magnetic field. This 1/4 wave retarder is expected to be useful for farinfrared spectroscopy of circular dichroism in many materials.Infrared Physics & Technology 08/2014; 67. DOI:10.1016/j.infrared.2014.09.004 · 1.46 Impact Factor 
[Show abstract] [Hide abstract]
ABSTRACT: Optical properties of hexagonal multiferroic oxides RMnO3, where R = Ho, Er, Tm, Yb, and Lu, have been studied in the farinfrared spectral range between 100 and 2000 cm(1) and temperatures between 1.5 and 300 K by means of several experimental techniques: Mueller matrix spectroscopic ellipsometry, rotating analyzer ellipsometry, and optical transmission spectroscopy. Spectra of the optical phonons are described in terms of the temperature dependencies of their frequency, damping, and oscillator strength. For all studies, oxide materials' clear signatures of the spinphonon interaction have been found below the temperature of the antiferromagnetic phase transition TN due to magnetic ordering of Mn3+ spins. A decrease of the ionic radius for R3+ ions between Ho3+ and Lu3+ in the corresponding RMnO3 compounds resulted in systematic variation of the frequency for several optical phonons. A magnetic excitation at similar to 190 cm(1) was observed at low temperatures below TN and interpreted as resulting from twomagnon absorption.Physical Review B 07/2014; 90(2):024307. DOI:10.1103/PhysRevB.90.024307 · 3.66 Impact Factor 
[Show abstract] [Hide abstract]
ABSTRACT: The characteristics of topological insulators are manifested in both their surface and bulk properties, but the latter remain to be explored. Here we report bulk signatures of pressureinduced band inversion and topological phase transitions in Pb$_{1x}$Sn$_x$Se ($x=$0.00, 0.15, and 0.23). The results of infrared measurements as a function of pressure indicate the closing and the reopening of the band gap as well as a maximum in the free carrier spectral weight. The enhanced density of states near the band gap in the topological phase give rise to a steep interband absorption edge. The change of density of states also yields a maximum in the pressure dependence of the Fermi level. Thus our conclusive results provide a consistent picture of pressureinduced topological phase transitions and highlight the bulk origin of the novel properties in topological insulators.Physical Review Letters 06/2014; 113(9). DOI:10.1103/PhysRevLett.113.096401 · 7.73 Impact Factor 
Conference Paper: Probing the pressureinduced topological phase transition in BiTeI.
[Show abstract] [Hide abstract]
ABSTRACT: X. Xi; C. Ma; Z. Liu; Z. Chen; W. Ku; H. Berger; C. Martin; D. B. Tanner; and G. L. Carr:APS March Meeting March 3  7, 2014 Denver, Colorado USA; 03/2014 
[Show abstract] [Hide abstract]
ABSTRACT: We report the observation of two signatures of a pressureinduced topological quantum phase transition in the polar semiconductor BiTeI using xray powder diffraction and infrared spectroscopy. The xray data confirm that BiTeI remains in its ambientpressure structure up to 8 GPa. The lattice parameter ratio c/a shows a minimum between 2.02.9 GPa, indicating an enhanced caxis bonding through p_{z} band crossing as expected during the transition. Over the same pressure range, the infrared spectra reveal a maximum in the optical spectral weight of the charge carriers, reflecting the closing and reopening of the semiconducting band gap. Both of these features are characteristics of a topological quantum phase transition and are consistent with a recent theoretical proposal.Physical Review Letters 10/2013; 111(15):155701. DOI:10.1103/PhysRevLett.111.155701 · 7.73 Impact Factor 
[Show abstract] [Hide abstract]
ABSTRACT: Vanadium dioxide (VO2) undergoes a phase transition at a temperature of 340 K between an insulating monoclinic M1 phase and a conducting rutile phase. Accurate measurements of possible anisotropy of the electronic properties and phonon features of VO2 in the insulating monoclinic M1 and metallic rutile phases are a prerequisite for understanding the phase transition in this correlated system. Recently, it has become possible to grow single domain untwinned VO2 microcrystals which makes it possible to investigate the true anisotropy of VO2. We performed polarized transmission infrared microspectroscopy on these untwinned microcrystals in the spectral range between 200 cm1 and 6000 cm1 and have obtained the anisotropic phonon parameters and low frequency electronic properties in the insulating monoclinic M1 and metallic rutile phases. We have also performed ab initio GGA+U total energy calculations of phonon frequencies for both phases. We find our measurements and calculations to be in good agreement.Physical review. B, Condensed matter 07/2013; 87(11). DOI:10.1103/PhysRevB.87.115121 · 3.66 Impact Factor 
[Show abstract] [Hide abstract]
ABSTRACT: Finitedifference timedomain methods are increasingly being used to develop, model and analyze the response of materials, including engineered metamaterials that may contain superconductors. Though simple and useful expressions for the timedomain susceptibility exist for basic metals and dielectrics, the timedomain response for a superconductor has not been developed, mainly because the frequencydependent expressions themselves are rather complex. In this paper we present a simple approximate expression for the timedomain susceptibility of a superconductor for the hbar/2Delta time scale (where Delta is the BCS energy gap) that fulfills causality requirements, and demonstrate its ability to model the transmission and reflection of a fullygapped superconductor in the THz region. By allowing Delta to be a function of current, we also show how this model function can be used to describe nonlinear microwave response in superconductors.Superconductor Science and Technology 07/2013; 26(11). DOI:10.1088/09532048/26/11/114001 · 2.80 Impact Factor 
[Show abstract] [Hide abstract]
ABSTRACT: Reflection and transmission as a function of temperature (7–300 K and 5–300 K respectively) have been measured on single crystals of the multiferroic compound FeTe2O5Br utilizing light spanning from the far infrared to the visible. The complex dielectric function and other optical properties were obtained via KramersKronig analysis and by fits to a DrudeLortentz model. Analysis of the anisotropic excitation spectra via DrudeLorentz fitting and lattice dynamical calculations have led to the observation of 43 of the 53 modes predicted along the b axis of the monoclinic cell. The phonon response parallel to the a and c axes are also presented. Assignments to groups (clusters) of phonons have been made and trends within them are discussed in light of our calculated displacement patterns.Physical review. B, Condensed matter 06/2013; 87(22). DOI:10.1103/PhysRevB.87.224108 · 3.66 Impact Factor 
[Show abstract] [Hide abstract]
ABSTRACT: We report conventional and timeresolved infrared spectroscopy on LaFeAsO$_{1x}$F$_x$ superconducting thin films. The farinfrared transmission can be quantitatively explained by a twocomponent model including a conventional swave superconducting term and a Drude term, suggesting at least one carrier system has a full superconducting gap. Photoinduced studies of excess quasiparticle dynamics reveal a nanosecond effective recombination time and temperature dependence that agree with a recombination bottleneck in the presence of a full gap. The two experiments provide consistent evidence of a full, nodeless though not necessarily isotropic, gap for at least one carrier system in LaFeAsO$_{1x}$F$_x$.Physical review. B, Condensed matter 04/2013; 87(18). DOI:10.1103/PhysRevB.87.180509 · 3.66 Impact Factor 
[Show abstract] [Hide abstract]
ABSTRACT: The vortexstate electrodynamics of swave superconductors has been studied by infrared spectroscopy. Farinfrared transmission and reflection spectra of superconducting NbTiN and NbN thin films were measured in a magnetic field perpendicular to the film surface, and the optical conductivity was extracted. The data show clear reduction of superconducting signature. We consider the vortex state as a twocomponent effective medium of normal cores embedded in a BCS superconductor. The spectral features are well explained by the MaxwellGarnett theory. Our analysis supports the presence of magneticfieldinduced pairbreaking effects in the superconducting component outside of the vortex cores.Physical review. B, Condensed matter 04/2013; 87(18). DOI:10.1103/PhysRevB.87.184503 · 3.66 Impact Factor 
[Show abstract] [Hide abstract]
ABSTRACT: We have developed a simple timedomain electric susceptibility model for a BCS type superconductor, valid for the spectral range spanning the optical energy gap frequency φ˜2δ and TTC. The expression can be used in Finite Difference Time Domain (FDTD) calculations for propagating electromagnetic waves through systems containing superconductor materials, including metamaterials. Since the energy gap appears explicitly, it can be varied as a function of time to describe nonlinear and nonequilibrium effects as observed in microwave experiments. We use the expression in a FDTD calculation for the transmission through and reflection from a thin film of NbN on a substrate, and compare with both conventional frequency domain calculations as well as actual experimental results. 
[Show abstract] [Hide abstract]
ABSTRACT: We have performed both conventional as well as timeresolved farinfrared spectroscopy on LaFeAsO1xFx pnictide thin films. The conventional spectroscopy results can be fit using a simple gapped superconductor + normal conductor twocomponent model. Absorption by quasiparticles in a gap system with nodes is a plausible explanation for the normal component [Lobo et al. Phys. Rev. B 82, 100506(R) (2010)]. The timeresolved study is performed by laserpump, farIR probe spectroscopy using synchrotron radiation at NSLS beamline U4IR. A laser pulse breaks superconducting pairs and the synchrotron probe is used to sense the recombination process. In contrast to the picosecond response observed for cuprate superconductors, we observe a nanosecond response typical of a fully gapped superconductor where phononbottleneck effects slow the effective recombination rate. This result suggests the presence of a full isotropic gap, as might exist at lower energies due to electronic scattering [Carbotte et al. Phys. Rev. B 81, 104510 (2010)]. 
[Show abstract] [Hide abstract]
ABSTRACT: FarIR spectra of magnetoelectric (ME) and multiferroic materials are in the focus of modern experimental and theoretical studies. Bianisotropic optical properties of these materials require consideration of not only dielectric susceptibility tensor ɛ(φ) but also magnetic permeability μ(φ) and ME α(φ) tensors that cannot be distinguished from a single transmission or reflection spectrum. We report on the application of Mueller matrix spectroscopic ellipsometry (MMSE) for studies of elementary excitations in multiferroic materials such as TbMnO3, TbMn2O5, and TbFe3(BO3)4 single crystals. We show that magnetic, electric, and ME dipole excitations, such as magnons, phonons, and electromagnons can be distinguished from each other using a single MM measurement without introducing any modeling arguments. The fit of MM spectra based on the Berreman's 4x4 propagation matrix formalism allowed us to determine parameters of electromagnon excitations separating the electric ɛ(φ) and ME α(φ) tensors components. 
[Show abstract] [Hide abstract]
ABSTRACT: We developed farIR spectroscopic ellipsometer at the U4IR beamline of the National Synchrotron Light Source in Brookhaven National Laboratory. This ellipsometer is able to measure both, rotating analyzer and fullMueller matrix spectra using rotating retarders, and wiregrid linear polarizers. We utilize exceptional brightness of synchrotron radiation in the broad spectral range between about 20 and 4000 cm. Fouriertransform infrared (FTIR) spectrometer is used for multiwavelength data acquisition. The sample stage has temperature variation between 4.2 and 450 K, wide range of θ2θ angular rotation, χ tilt angle adjustment, and XYZ translation. A LabVIEWbased software controls the motors, sample temperature, and FTIR spectrometer and also allows to run fully automated experiments with preprogrammed measurement schedules. Data analysis is based on Berreman's 4 × 4 propagation matrix formalism to calculate the Mueller matrix parameters of anisotropic samples with magnetic permeability μ ≠ 1. A nonlinear regression of the rotating analyzer ellipsometry and∕or Mueller matrix (MM) spectra, which are usually acquired at variable angles of incidence and sample crystallographic orientations, allows extraction of dielectric constant and magnetic permeability tensors for bulk and thinfilm samples. Applications of this ellipsometer setup for multiferroic and ferrimagnetic materials with μ ≠ 1 are illustrated with experimental results and simulations for TbMnO and DyFeO single crystals. We demonstrate how magnetic and electric dipoles, such as magnons and phonons, can be distinguished from a single MM measurement without adducing any modeling arguments. The parameters of magnetoelectric components of electromagnon excitations are determined using MM spectra of TbMnO.The Review of scientific instruments 02/2013; 84(2):023901. DOI:10.1063/1.4789495 · 1.58 Impact Factor 
[Show abstract] [Hide abstract]
ABSTRACT: Reflection and transmission as a function of temperature (5300 K) have been measured on single crystals of the multiferroic compound FeTe$_{2}$O$_{5}$Br utilizing light spanning the far infrared to the visible portions of the electromagnetic spectrum. The complex dielectric function and optical properties were obtained via KramersKronig analysis and by fits to a DrudeLortentz model. Analysis of the anisotropic excitation spectra via DrudeLorentz fitting and lattice dynamical calculations have lead to the observation of all 52 IRactive modes predicted in the $ac$ plane and 43 or the 53 modes predicted along the b axis of the monoclinic cell. Assignments to groups (clusters) of phonons have been made and trends within them are discussed in light of our calculated displacement patterns. 
[Show abstract] [Hide abstract]
ABSTRACT: We developed farIR spectroscopic ellipsometer at the U4IR beamline of the National Synchrotron Light Source in Brookhaven National Laboratory. This ellipsometer is able to measure both, rotating analyzer and fullMueller matrix spectra using rotating retarders, and wiregrid linear polarizers. We utilize exceptional brightness of synchrotron radiation in the broad spectral range between about 20 and 4000 cm −1 . Fouriertransform infrared (FTIR) spectrometer is used for multiwavelength data acquisition. The sample stage has temperature variation between 4.2 and 450 K, wide range of θ –2θ angular rotation, χ tilt angle adjustment, and XYZ translation. A LabVIEWbased software controls the motors, sample temperature, and FTIR spectrometer and also allows to run fully automated experiments with preprogrammed measurement schedules. Data analysis is based on Berreman's 4 × 4 propagation matrix formalism to calculate the Mueller matrix parameters of anisotropic samples with magnetic permeability μ = 1. A nonlinear regression of the rotating analyzer ellipsometry and/or Mueller matrix (MM) spectra, which are usually acquired at variable angles of incidence and sample crystallographic orientations, allows extraction of dielectric constant and magnetic permeability tensors for bulk and thinfilm samples. Applications of this ellipsometer setup for multiferroic and ferrimagnetic materials with μ = 1 are illustrated with experimental results and simulations for TbMnO 3 and Dy 3 Fe 5 O 12 single crystals. We demonstrate how magnetic and electric dipoles, such as magnons and phonons, can be distinguished from a single MM measurement without adducing any modeling arguments. The parameters of magnetoelectric components of electromagnon excitations are determined using MM spectra of TbMnO 3 . © 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4789495]The Review of scientific instruments 01/2013; 84:023901. DOI:10.1063/1.4789495] · 1.58 Impact Factor 
[Show abstract] [Hide abstract]
ABSTRACT: Quasiparticle recombination in a superconductor with an swave gap is typically dominated by a phonon bottleneck effect. We have studied how a magnetic field changes this recombination process in metallic thinfilm superconductors, finding that the quasiparticle recombination process is significantly slowed as the field increases. While we observe this for all field orientations, we focus here on the results for a field applied parallel to the thin film surface, minimizing the influence of vortices. The magnetic field disrupts the timereversal symmetry of the pairs, giving them a finite lifetime and decreasing the energy gap. The field could also polarize the quasiparticle spins, producing different populations of spinup and spindown quasiparticles. Both processes favor slower recombination; in our materials we conclude that strong spinorbit scattering reduces the spin polarization, leaving the fieldinduced gap reduction as the dominant effect and accounting quantitatively for the observed recombination rate reduction.Physical review. B, Condensed matter 11/2012; 87(14). DOI:10.1103/PhysRevB.87.140502 · 3.66 Impact Factor 
[Show abstract] [Hide abstract]
ABSTRACT: Infrared reflection and transmission as a function of temperature have been measured on single crystals of Cu3Bi(SeO3)2O2Cl. The complex dielectric function and optical properties along all three principal axes of the orthorhombic cell were obtained via KramersKronig analysis and by fits to a DrudeLorentz model. Below 115 K, 16 additional modes [8(E∥â)+6(E∥b̂)+2(E∥ĉ)] appear in the phonon spectra; however, powder xray diffraction measurements do not detect a new structure at 85 K. Potential explanations for the new phonon modes are discussed. Transmission in the far infrared as a function of temperature has revealed magnetic excitations originating below the magnetic ordering temperature (Tc∼24 K). The origin of the excitations in the magnetically ordered state will be discussed in terms of their response to different polarizations of incident light, behavior in externally applied magnetic fields, and the anisotropic magnetic properties of Cu3Bi(SeO3)2O2Cl as determined by dc susceptibility measurements.Physical review. B, Condensed matter 11/2012; 86(17). DOI:10.1103/PhysRevB.86.174104 · 3.66 Impact Factor 
[Show abstract] [Hide abstract]
ABSTRACT: Infrared reflection and transmission as a function of temperature have been measured on single crystals of Cu$_{3}$Bi(SeO$_{3}$)$_{2}$O$_{2}$Cl. The complex dielectric function and optical properties along all three principal axes of the orthorhombic cell were obtained via KramersKronig analysis and by fits to a DrudeLorentz model. Below 115 K, 16 additional modes (8(E$\parallel\hat{a}$)+6(E$\parallel\hat{b}$)+2(E$\parallel\hat{c}$)) appear in the phonon spectra; however, powder xray diffraction measurements do not detect a new structure at 85 K. Potential explanations for the new phonon modes are discussed. Transmission in the far infrared as a function of temperature has revealed magnetic excitations originating below the magnetic ordering temperature ($T_{c}\sim$24 K). The origin of the excitations in the magnetically ordered state will be discussed in terms of their response to different polarizations of incident light, behavior in externallyapplied magnetic fields, and the anisotropic magnetic properties of Cu$_{3}$Bi(SeO$_{3}$)$_{2}$O$_{2}$Cl as determined by d.c. susceptibility measurements.
Publication Stats
2k  Citations  
312.19  Total Impact Points  
Top Journals
Institutions

1998–2014

Brookhaven National Laboratory
 • National Synchrotron Light Source
 • Physics Department
New York City, New York, United States


2007

Los Alamos National Laboratory
ЛосАламос, California, United States


2003

University of Maryland, College Park
 Institute for Research in Electronics and Applied Physics (IREAP)
CGS, Maryland, United States


2002

Pohang University of Science and Technology
 Department of Physics
Antō, North Gyeongsang, South Korea


1990–1996

University of Florida
 Department of Physics
Gainesville, FL, United States


1983–1990

Emory University
 Department of Physics
Atlanta, Georgia, United States


1981

The Ohio State University
 Department of Physics
Columbus, OH, United States
