Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session B22: Optical Studies of Strongly Correlated Systems |
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Sponsoring Units: DCMP Room: 202A |
Monday, March 2, 2015 11:15AM - 11:27AM |
B22.00001: Spatially ordered transit through a canonical Mott transition revealed by cryogenic nano-imaging A.S. McLeod, E. van Heumen, J.G. Ramirez, S. Wang, T. Saerbeck, S. Guenon, M. Goldflam, L. Anderegg, P. Kelly, A. Mueller, M.K. Liu, I.K. Schuller, D.N. Basov We report on temperature-dependent (24K-300K) near-field infrared (IR) imaging of the canonical Mott insulator V$_{2}$O$_{3}$ across its temperature-driven metal-insulator transition. This was accomplished using a home-built s-SNOM (scattering-type scanning near-field optical microscope) affording unprecedented spatial resolution ($\sim$ 20 nm) to surface optical properties with simultaneously acquired AFM topography at \textit{cryogenic temperatures}. Our V$_{2}$O$_{3}$ thin film is found to exhibit extreme nano-scale electronic heterogeneity near the Mott transition (170K) from paramagnetic metal to antiferromagnetic insulator. A sequence of nano-IR images acquired across the transition provides a direct probe of the metal/insulator fill fraction in accord with an observed percolation-driven resistive transition. We resolve dynamic evolution of electronic phases and a crossover from long- to short-range spatial correlations. Identification of the attendant V$_{2}$O$_{3}$ structural transition by X-ray diffraction reveals an unexpected decoupling of Mott and structural transitions. Supported by nano-IR imaging of remnant metallic puddles below the Mott transition, these observation point towards a novel low-temperature metallic phase. [Preview Abstract] |
Monday, March 2, 2015 11:27AM - 11:39AM |
B22.00002: New Insights into the Diverse Electronic Phases of a Novel Vanadium Dioxide Polymorph: A Terahertz Spectroscopy Study James Lourembam, Amar Srivastava, Chan La-o-vorakiat, Helene Rotella, Thirumalai Venkatesan, Elbert Chia A remarkable feature of vanadium dioxide is that it can be synthesized in a number of polymorphs. The conductivity mechanism in the metastable layered polymorph VO$_{2}(B)$ thin films has been investigated by terahertz time-domain spectroscopy (THz-TDS). In VO$_{2}(B)$, a critical temperature of 240 K marks the appearance of a non-zero Drude term in the observed complex conductivity, indicating the evolution from a pure insulating state towards a metallic state. In contrast, the THz conductivity of the well-known VO$_{2}(M$1) is well fitted only by a modification of the Drude model to include backscattering. We also identified two different THz conductivity regimes separated by temperature in these two polymorphs. The electronic phase diagram is constructed, revealing that the width and onset of the metal-insulator transition in the B phase develop differently from the $M$1 phase. [Preview Abstract] |
Monday, March 2, 2015 11:39AM - 11:51AM |
B22.00003: On the Repeatability of Domain Formation and Growth During the Metal-Insulator Transition in Vanadium Dioxide Films T.J. Huffman, Peng Xu, M.M. Qazilbash, Bong-Jun Kim, Hyun-Tak Kim Nanoscale phase coexistence between insulating and metallic domains has been observed in films of vanadium dioxide (VO2) using scattering-type scanning near-field infrared microscopy (s-SNIM). When insulating VO2 transitions to the metallic phase, small regions of the metallic phase first nucleate, and then grow as the metal-insulator transition (MIT) progresses. It is an open question if the patterns of insulating and metallic VO2 in a given scan area are reproducible upon repeated thermal cycling across the MIT. To investigate this matter, we image the same area of a VO2 film with s-SNIM over multiple thermal cycles through the MIT. In this way, we uncover the relative contributions of deterministic and random events occurring at the nanoscale during the progress of the MIT. Our experiments reveal the nature of phase coexistence in VO2 films and the real-space dynamics of the MIT. [Preview Abstract] |
Monday, March 2, 2015 11:51AM - 12:03PM |
B22.00004: Optical Spectroscopy of Fermi Liquids Thomas Timusk, Jesse Hall, Nathan Armstrong, Kim Mortimer, Mahsa Movassagh, Urmas Nagel, Toomas R\~o\~om, Yoshiteru Maeno, Andrew Mackenzie, Byeong Min, Yong Kwon Metallic materials are normally characterized as Fermi liquids if their low temperature dc resistivity has a $T^2$ temperature dependence. It was shown by Gurzhi that there is is also a frequency dependence and the overall resistivity is given by $\rho(\omega,T)=C(\omega^2+b\pi^2T^2)$, where the scaling constant $b=4$ for a Fermi liquid with umklapp scattering [1]. A survey of literature shows that where spectroscopic experiments exist, $b=4$ is hardly ever observed [2]. We will present spectroscopic data on LiFeAs, Sr$_2$RuO$_4$ and Sr$_3$Ru$_2$O$_7$, three materials that show $T^2$ resistivity at low temperatures and discuss their excitation spectra.\\[4pt] [1] R. N. Gurzhi, Sov. Phys. JETP {\bf 14}, 886 (1962).\\[0pt] [2] U. Nagel {\it et al.} PNAS {\bf 109}, 19161 (2012). [Preview Abstract] |
Monday, March 2, 2015 12:03PM - 12:15PM |
B22.00005: Closing of the pseudogap in Fe$_{\mathbf{1.03}}$Te C.C. Homes, Y.M. Dai, J. Schneeloch, R.D. Zhong, Q. Li, G.D. Gu, A. Akrap The optical properties of strongly-correlated Fe$_{1.03}$Te have been measured over a wide frequency range for light polarized in the \emph{a-b} planes at temperatures above and below the structural and magnetic transition, $T_N\simeq 68$~K. For $T>T_N$, in the paramagnetic state, the resistivity is increasing with decreasing temperature, and the optical conductivity is flat over much of the infrared region, except for a weak Drude-like response at low frequency. Below $T_N$, in the antiferromagnetic state, there is dramatic increase in the low-frequency conductivity with a commensurate transfer of spectral weight (area under the conductivity curve) from high to low energy. The roughly constant value of the scattering rate indicates that it is the plasma frequency ($\omega_p$) that is increasing. This increase in $\omega_p^2\propto n/m^\ast$ is associated with the closing of the pseudogap on the electron pocket resulting in an increase in the number of carriers ($n$). In addition, below $T_N$ the effective mass ($m^\ast$) is also thought to decrease. Both effects lead to an increase in $\omega_p$ on the electron pocket.\footnote{Y. M. Dai {\em et al.}, Phys. Rev. B {\bf 90}, 121114(R) (2014).} [Preview Abstract] |
Monday, March 2, 2015 12:15PM - 12:27PM |
B22.00006: Optical spectroscopy of the metallic Nd$_{\mathrm{1-x}}$TiO$_3$ system Nathan Armstrong, Athena Sefat, Jing Yang, John Greedan, Thomas Timusk The neodymium titanate system, Nd$_{\mathrm{1-x}}$TiO$_{3}$, exhibits two metal-to-insulator transitions at $x=$ 0.08 and $x=$ 0.24. The $n$-type metallic regime between the two transitions exhibits the Fermi liquid $T^{2}$ DC resistivity over a large range of temperatures. We have measured the reflectivity of the $x=$ 0.15 and $x=$ 0.2 samples from 4 meV to 5.5 eV at temperature from 15 K to 300 K. Previous optical work by Yang \textit{et al.} measured an insulating, a semiconducting, and a metallic sample. The metallic sample was found to have the Fermi liquid $\omega^{2} $dependence in the scattering rate. We reevaluate the Yang \textit{et al.} data for the $x=$ 0.095 metallic sample, just inside the MIT, and compare it to our samples deeper in the metallic regime that have $T^{2}$ coefficients an order of magnitude smaller. [Preview Abstract] |
(Author Not Attending)
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B22.00007: Resilient quasiparticles in Ruthenates: transport properties within LDA+DMFT method Xiaoyu Deng, Kristjan Haule, Gabriel Kotliar Many Rutheniums are strongly correlated metals with Fermi Liquid behavior found only a small temperature scale. Non-Fermi signatures appear in both their resistivity and optical conductivity. We study the transport properties of a set of Ruthenates within first principle methods in combination with dynamical mean field theory and find reasonable agreement with experimental findings. The non-Fermi-liquid features are attributed to the temperature dependence of resilient quasiparticles, which survives above the Fermi liquid temperature scale and exhibits a strong temperature dependence in their effective mass enhancement and scattering rate. [Preview Abstract] |
Monday, March 2, 2015 12:39PM - 12:51PM |
B22.00008: Abrupt changes in electronic relaxation and lattice dynamics across the structural phase transition in lightly doped Ca$_{2}$RuO$_{4}$ observed via time-resolved optical reflectivity Hao Chu, Darius Torchinsky, Liuyan Zhao, Patrick Rall, Jasminka Terrace, Gang Cao, David Hsieh Ca$_{2}$RuO$_{4}$ is a multiband strongly correlated electron system that undergoes a structural phase transition at Ts ~ 360K that is concomitant with an insulator-to-metal transition and a rearrangement of orbital occupancy. Understanding its structural and electronic response to ultrafast optical excitation can provide insight about the microscopic mechanism of this phase transition.We report temperature and fluence dependent time resolved optical reflectivity measurements from lightly doped Ca$_{2}$RuO$_{4}$ single crystals. Abrupt changes in both the electronic relaxation dynamics and multiple lattice vibrational modes are observed, including the softening of two optical phonon modes as Ts is approached. We will discuss the relevance of our results to existing theories of the mechanism underlying the structural phase transition in Ca$_{2}$RuO$_{4}$ as well as the possibility of photo-inducing this phase transition on ultrafast time scales. [Preview Abstract] |
Monday, March 2, 2015 12:51PM - 1:03PM |
B22.00009: THz investigation of non-Drude transport in the ferromagnetic metal SrRuO$_3$ Grace Bosse, Y. Lubashevsky, D. E. Shai, C. Adamo, D. G. Schlom, K. M. Shen, N. P. Armitage While the highly correlated complex oxide perovskite ferromagnet SrRuO$_3$ has been studied for decades, interest remains in its unusual transport properties. In view of this, we present time-domain terahertz conductivity measurements taken from room temperature to 5K on a low disorder film of SrRuO$_3$. Previous optical measurements have shown a deviation from Fermi liquid theory transport predictions in this material. We investigate these deviations in the context of both an extended Drude model analysis and the previously used fractional power law form. The high quality of our film, reflected in its large residual resistivity ratio, allows us to better isolate the inelastic scattering channels, which were likely masked by disorder in earlier studies. We also comment on the possible ferromagnetic resonance present in our optical data. [Preview Abstract] |
Monday, March 2, 2015 1:03PM - 1:15PM |
B22.00010: Observation of a third, previously unknown charge-density-wave order in RTe$_3$ by optical spectroscopy N.L. Wang, B.F. Hu, R.Y. Chen, Y. Huang, T. Dong The occurrences of collective quantum states, such as superconductivity and charge- or spin-density-waves (CDWs or SDWs), are among the most fascinating phenomena in solids. Rare-earth tri-telluride RTe$_3$ (R being an element of the lanthanide family) represents an interesting and well-known CDW family in which the CDW phase transition temperature could be tuned by changing the size of the rare earth element. Previously it was known that the light rare-earth element R based compounds have only one CDW order or phase transition, while for the four heavy rare-earth RTe$_3$ (R=Tm, Er, Ho, Dy) compounds, a second CDW order with the wave vector $\textbf{q}_2$$\sim$1/3 $\textbf{a}^{\ast}$ perpendicular to the first one developed at lower temperature. Here we report the observation of a third CDW order in the series by optical spectroscopy probe. This third CDW order also evolves systematically with the size of R element. With increased chemical pressure, the first and third CDW orders are both substantially suppressed and compete with the second one by depleting the low energy spectral weight. A complete phase diagram for the multiple CDW orders in this series is established. We acknowledge B. Cheng and R. H. Yuan for their help in the experiments. [Preview Abstract] |
Monday, March 2, 2015 1:15PM - 1:27PM |
B22.00011: Antiferromagnetic fluctuations in the metallic state of organic superconductor $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_{2}$]Br observed by Raman scattering Natalia Drichko, Rudi Hackl, John Schlueter Charge properties of the $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_{2}$]$X$ ($X$=Cl, Br) materials are well studied and are well explained within the approach of the Hubbard model, but magnetic properties leave some questions. In particular, the presence of anti-ferromagnetic (AF) fluctuations in the metallic state close to Mott Insulator $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_{2}$]Br observed by NMR and other methods, but do not agree with the observed Fermi-liquid behavior. We study magnetic excitations in these materials by Raman scattering. In B$_{1g}$ polarization at temperatures below 100 K we observe a band at 500 cm$^{-1}$, assigned to two-magnon excitations basing on results for AF-ordered Mott insulator $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_{2}$]Cl. The band shows very similar intensity and position for the metallic $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_{2}$]Br. We discuss this result in terms of two possible interpretations, AF fluctuations vs metal-AF insulator mixed phase close to the line of the first order phase transition between Mott Insulator and metallic states. [Preview Abstract] |
Monday, March 2, 2015 1:27PM - 1:39PM |
B22.00012: Optical Conductivity in the Cuprates and Gauge/Gravity duality Brandon Langley, Philip Phillips The recent claim that inhomogeneous charge densities in a bulk gravitational setting are capable of yielding power law optical conductivities observed in the cuprates is examined critically. We first show that charge density in the Q-lattice proposal by Donos et al. [1] is actually homogeneous and hence is not a satisfactory candidate for addressing this issue. We then construct a two-scalar field system that can interpolate between the holographic lattice of Horowitz et al. [2] and the Q-lattice of Donos et al. and explore how the power law in the optical conductivity turns on as a function of the inhomogeneity and the mixing of higher harmonics. \\[4pt] [1] A. Donos and J. P. Gauntlett, ``Holographic Q-lattices,'' JHEP 1404 (2014) 040, arXiv:1311.3292 [hep-th].\\[0pt] [2] G. T. Horowitz, J. E. Santos, and D. Tong, ``Optical Conductivity with Holographic Lattices,'' JHEP 1207 (2012) 168, arXiv:1204.0519 [hep-th]. [Preview Abstract] |
Monday, March 2, 2015 1:39PM - 1:51PM |
B22.00013: Optical Conductivity in the Cuprates from Unparticle Kridsanaphong Limtragool, Jimmy Hutasoit, Philip Phillips The optical conductivity of optimally doped cuprates above the superconducting dome exhibits a universal power law of the form, $\omega^{-\frac{2}{3}}$. Unparticles, scale-invariant matter with an algebraic propagator, is a candidate to explain this phenomenon. We explore the possibility of using unparticle to produce such power law behavior. We apply unparticle-gauge couplings and linear response theory at finite temperature to calculate the optical conductivity. We find that simply expanding a four-point correlation function using Wick's theorem is not sufficient to obtain the power law. We investigate the role played by non-Wick processes in determining the power law [Preview Abstract] |
Monday, March 2, 2015 1:51PM - 2:03PM |
B22.00014: Why a quantum wire can act as an optical amplifier Manvir Kushwaha We discuss the fundamental issues associated with the magnetoplasmon excitations in a semiconducting quantum wire characterized by a harmonic confining potential and subjected to an applied (perpendicular) magnetic field. Essentially, we focus on the device aspects of the intersubband collective (magnetoroton) excitation, which observes a negative group velocity (NGV) between maxon and roton. Consequently, it leads to tachyon-like (superluminal) behavior without one's having to introduce the negative energies. Existence of the NGV is a clear manifestation of a medium with population inversion brought about due to a metastable state caused by the magnetic field that satisfies the condition $B> B_{th}$; $B_{th}$ being the threshold value below which the magnetoroton does not exist. The interest in NGV is based on anomalous dispersion in a medium with inverted population, so that gain instead of absorption occurs at the frequencies of interest. A medium with an inverted population has the remarkable ability of amplifying a small optical signal of definite wavelength, i.e., it can serve as an optical amplifier. Examining the life-time of magnetorotons leads us to infer that relatively smaller magnetic fields are optimal. [Preview Abstract] |
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