Cassandra R. Hunt

Cassandra R. Hunt
University of California, Berkeley | UCB · Department of Physics

Doctor of Philosophy

About

25
Publications
2,677
Reads
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964
Citations
Additional affiliations
November 2010 - present
Max Planck Department for Structural Dynamics
Position
  • Non-equilibrium, light-induced superconductivity in cuprates
Description
  • MIR, THz spectroscopy, pump-probe, time domain spectroscopy, phase transitions, non-equilibrium, high temperature superconductivity, cuprates, YBCO
November 2010 - May 2015
Center for Free-Electron Laser Science
Position
  • PhD Student
February 2009 - present
University of Illinois, Urbana-Champaign
Position
  • Point contact spectroscopy investigations of iron-based superconductors
Description
  • multi-gap superconductors, unconventional superconductivity, pnictides, chalcogenides, point contact spectroscopy
Education
August 2008 - May 2015
August 2004 - June 2008

Publications

Publications (25)
Article
Optical excitation of apical oxygen vibrations in YBa$_{2}$Cu$_{3}$O$_{6+\delta}$ has been shown to enhance its c-axis superconducting-phase rigidity, as evidenced by a transient blue shift of the equilibrium inter-bilayer Josephson plasma resonance. Surprisingly, a transient c-axis plasma mode could also be induced above T$_{c}$ by the same apical...
Article
Full-text available
We use femtosecond resonant soft x-ray diffraction to measure the optically stimulated ultrafast changes of charge density wave correlations in underdoped YBa2Cu3O6.6. We find that when coherent interlayer transport is enhanced by optical excitation of the apical oxygen distortions, at least 50% of the in-plane charge density wave order is melted....
Article
Full-text available
We address the kinetic competition between charge striped order and superconductivity in La$_{1.675}$Eu$_{0.2}$Sr$_{0.125}$CuO$_{4}$. Ultrafast optical excitation is tuned to a mid-infrared vibrational resonance that destroys charge order and promptly establishes transient coherent interlayer coupling in this material. This effect is evidenced by t...
Article
Nonlinear optical excitation of infrared active lattice vibrations has been shown to melt magnetic or orbital orders and to transform insulators into metals. In cuprates, this technique has been used to remove charge stripes and promote superconductivity, acting in a way opposite to static magnetic fields. Here, we show that excitation of large-amp...
Article
Full-text available
Superconducting interlayer coupling is induced above Tc in the stripe phase of La1.885Ba0.115CuO4 by excitation with near-infrared laser pulses. A Josephson Plasma Resonance is detected by terahertz-frequency reflectivity measurements. Key to this measurement is the ability to probe the optical properties at frequencies as low as 150 GHz, detecting...
Conference Paper
By using femtosecond pulses in the mid-infrared, we resonantly excite an infrared-active phonon mode in the high-temperature superconductor YBa2Cu3Ox . The electronic properties of the driven state, probed with ultra-broadband time-domain terahertz spectroscopy, are highly unconventional.
Article
Full-text available
In conventional superconductors, the finite superfluid density is quantified by a zero-frequency peak in the optical conductivity. In high Tc cuprates, the layered structure gives rise to additional c-axis excitations of the superfluid. For those cuprates with two superconducting layers per unit cell, the superfluid density is shared between two co...
Article
Full-text available
We use point contact spectroscopy (PCS) to probe the superconducting properties of electron doped $\rm{Ba(Fe_{1-x}Co_x)_2As_2}$ ($\rm{x = 0.05, 0.055, 0.07, 0.08}$) and hole doped $\rm{Ba_{0.8}K_{0.2}Fe_2As_2}$. PCS directly probes the low energy density of states via Andreev reflection, revealing two distinct superconducting gaps in both compound...
Article
Point contact spectroscopy reveals a gap-like feature above the magnetic and structural transition temperatures for underdoped Ba(Fe1-xCox)2As2, SrFe2As2 and Fe1+yTe. The conductance spectrum starts showing an enhancement at temperatures as high as 177 K for BaFe2As2 (TN ~ 132 K) and 250 K for SrFe2As2 (TN ~ 192 K). Possible origins for this enhanc...
Article
Full-text available
Strong optical fields at THz frequencies can be used to modulate the crystallographic structure of a solid along a chosen normal-mode coordinate, creating dynamical states with new properties. Here, 20-THz femtosecond pulses are used to resonantly drive large-amplitude oscillations of the out-of-plane apical oxygen positions in YBa2Cu3O6+x. A light...
Article
Full-text available
High intensity optical pulses at THz frequencies can be tuned to specific vibrational resonances and dynamically modulate the crystallographic structure of solids along a chosen normal-mode coordinate. This technique can be used to tip the balance between competing orders, or, as shown in the present paper, to create dynamically modulated states wi...
Article
QPS reveals a conductance enhancement at a temperature, Tonset, for RFe2As2 (R=Ca, Sr, Ba) and Fe1.13Te. For Ba/Sr Fe2As2 and Fe1.13Te the enhancement survives well above the magnetic and structural transition temperatures (Ba: TN˜132 K, Tonset˜175 K; Sr: TN˜192 K, Tonset˜240 K; Fe1.13Te: TN˜60 K, Tonset˜75 K) while for most CaFe2As2 junctions it d...
Article
Full-text available
We use point-contact spectroscopy (PCS) to probe AEFe2As2 (AE=Ca, Sr, Ba) and Fe1+yTe. For AE=Sr,Ba we detect orbital fluctuations above TS while for AE=Ca these fluctuations start below TS. Co doping preserves the orbital fluctuations while K doping suppresses it. The fluctuations are only seen at those dopings and temperatures where an in-plane r...
Article
We use point contact spectroscopy to probe AEFe2As2 (AE = Ca, Sr, Ba) and Fe1+yTe. For AE = Sr, Ba we detect orbital fluctuations above TS while for AE = Ca these fluctuations start below TS. Co doping preserves the orbital fluctuations while K doping suppresses it. The fluctuations are only seen at those dopings and temps. where an in-plane resist...
Article
Full-text available
At this centenary of the discovery of superconductivity, the design of new and more useful superconductors remains as enigmatic as ever. These materials play crucial roles both for fundamental science and applications, and they hold great promise in addressing our global energy challenge. The recent discovery of a new class of high-temperature supe...
Article
Full-text available
Point contact spectroscopy reveals a gap-like feature above the magnetic and structural transition temperatures for underdoped $Ba(Fe_{1-x}Co_x)_2As_2$, $SrFe_2As_2$ and $Fe_{1+y}Te$. The conductance spectrum starts showing an enhancement at temperatures as high as 177 K for $BaFe_2As_2$ ($T_N$ $\sim$ 132 K) and 250 K for $SrFe_2As_2$ ($T_N$ $\sim$...
Article
We present PCS results on Ba(Fe1-xCox)2As2 and Fe1+yTe. The superconducting (S) crystals (x=0.08) show multigap like Andreev peaks. The non-S crystals (x=0.015, y=0.03) also show a conductance enhancement with split peaks at low temperatures (T). This conductance enhancement does not match with the bulk antiferromagnetic (AFM) transition T and surv...
Article
We present point contact spectroscopy results on single crystal Co-doped SrFe2As2. Two sets of Andreev-like enhancements in conductance are seen with nominally c-axis contacts. For temperatures up to Tc = 14.5 K, the conductance is fit to a Blonder-Tinkham-Klapwijk (BTK) model extended to two independent bands with lifetime broadening [1]. We also...
Article
Full-text available
The superconducting order parameter in the iron-chalcogenide superconductor FeTe$_{0.55}$Se$_{0.45}$ ($T_\textrm{c}$ = 14.2 K) is investigated by point-contact Andreev reflection spectroscopy. The energy gap magnitude (3.8 meV at 1.70 K) and temperature dependence as extracted from the Andreev conductance spectra reveal strong-coupling superconduct...
Article
We report point-contact conductance measurements on the iron chalcogenide superconductors, Fe1+yTe1-xSex. The excess Fe atoms are known to occupy the interstitial sites in the Te-Se plane, affecting the superconductivity as well as the magnetism in this family. For a compound having nominal values of y=0 and x=0.45, a single superconducting transit...
Article
We present point-contact spectroscopy (PCS) measurements on single crystal Fe1+yTe1-xSex using a nanoscale Au tip contact. Analysis of PCS measurements using BTK theory[1] is well-established, and recent work[2] that extends BTK to two-band superconductors with a relative band phase may shed new light on the nature of the SC order parameter (OP) of...
Article
We present point contact spectroscopy measurements on single crystal Fe1+yTe1-xSex using a nanometer scale gold tip, as a function of applied magnetic field, temperature, doping and contact resistance. The superconducting samples exhibit a zero bias peak that persists up to 5K above Tc while the x=0 compound shows a dip at zero bias, unaffected by...
Article
Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2008. Includes bibliographical references (p. 43-45). The transition to Advanced LIGO introduces new sensitivity requirements for the LIGO interferometers. When light scatters away from the main laser beam, then scatters off the beam tube and returns to the main beam, noise is...
Article
Most extensive optical studies of lightning have been conducted from orbit, and the statistics of events collected from earth are relatively poorly documented. The time signatures of optical power measured in the presence of clouds are inevitably affected by scattering,which can distort the signatures by extending and delaying the amplitude profile...
Article
A vibrating plate of granular material can exhibit unexpected emergent be- havior, including the formation of oscillons. Oscillons are stable, localized, particle-like clumps of vibrating material that are capable of drifting across the plate as well as attracting and repelling other oscillons. This paper details the characteristics of these strang...