V. Thampy | Brookhaven National Laboratory (original) (raw)

Papers by V. Thampy

Spin fluctuations may play a key role in metals where superconductivity appears as a magnetic pha... more Spin fluctuations may play a key role in metals where superconductivity appears as a magnetic phase is suppressed under pressure or with chemical substitution. The suppressed magnetism is manifested as a gap in the spin fluctuation spectrum and a spin resonance to which the spectral weight is shifted. We have studied the effect of high magnetic fields on this resonance.

ABSTRACT Spin fluctuations may play a key role in metals where superconductivity appears as a mag... more ABSTRACT Spin fluctuations may play a key role in metals where superconductivity appears as a magnetic phase is suppressed under pressure or with chemical substitution. The suppressed magnetism is manifested as a gap in the spin fluctuation spectrum and a spin resonance to which the spectral weight is shifted. We have studied the effect of high magnetic fields on this resonance. While fine structure is observed, these features do not shift with field and persist in zero field and in the normal state. Temperature difference spectra are however, significantly broadened in high fields.

Nature materials, 2016

Measuring how the magnetic correlations evolve in doped Mott insulators has greatly improved our ... more Measuring how the magnetic correlations evolve in doped Mott insulators has greatly improved our understanding of the pseudogap, non-Fermi liquids and high-temperature superconductivity. Recently, photo-excitation has been used to induce similarly exotic states transiently. However, the lack of available probes of magnetic correlations in the time domain hinders our understanding of these photo-induced states and how they could be controlled. Here, we implement magnetic resonant inelastic X-ray scattering at a free-electron laser to directly determine the magnetic dynamics after photo-doping the Mott insulator Sr2IrO4. We find that the non-equilibrium state, 2 ps after the excitation, exhibits strongly suppressed long-range magnetic order, but hosts photo-carriers that induce strong, non-thermal magnetic correlations. These two-dimensional (2D) in-plane Néel correlations recover within a few picoseconds, whereas the three-dimensional (3D) long-range magnetic order restores on a flue...

$Research paper thumbnail of Determination of a Magnetic Component to the Superconducting Condensation Energy for Fe $ _ {\ mathrm {1+\ delta}} $ Se $ _ {\ mathrm {x}} $ Te $ _ {\ mathrm {1-x}} $$

Physical Review B, 2014

ABSTRACT Neutron scattering is used to probe magnetic interactions as superconductivity develops ... more ABSTRACT Neutron scattering is used to probe magnetic interactions as superconductivity develops in optimally doped Fe 1+δ Se x Te 1−x. Applying the first moment sum rule to comprehensive neutron scattering data, we extract the change in magnetic exchange energy [J R−R S R · S R ] in the superconducting state referenced to the normal state. Oscillatory changes are observed for Fe-Fe displacements |R| < ξ, where ξ = 1.3(1) nm is the superconducting coherence length. Dominated by a large reduction in the second nearest neighbor exchange energy [−1.2(2) meV/Fe], the overall reduction in magnetic interaction energy is H mag = −0.31(9) meV/Fe. Comparison to the superconducting condensation energy E SC = −0.013(1) meV/Fe, which we extract from specific heat data, suggests the modified magnetism we probe drives superconductivity in Fe 1+δ Se x Te 1−x .

Physics Procedia, 2013

We report substantial improvements in a compact wide angle neutron spin filter system that was re... more We report substantial improvements in a compact wide angle neutron spin filter system that was recently employed on the Multi-Axis Crystal Spectrometer at the Center for Neutron Research at the U.S. National Institute of Standards and Technology. The apparatus consists of a cylindrical 3 He polarizer cell and wide-angle 3 He analyzer cells, a vertical solenoid to provide a uniform magnetic field, and a shielded radio-frequency solenoid for the polarizer cell. Nuclear magnetic resonance is employed to reverse the polarization in the polarizer cell and monitor the 3 He polarization in all cells. The first experiment using this apparatus was carried out with cylindrical analyzer cells with limited angular coverage due to low polarizations in fused quartz cells. We present results for aluminosilicate glass analyzer cells that cover 110 • and have long relaxation times (100 h to 400 h). Using two 100 W diode bars spectrally narrowed with chirped volume Bragg gratings, we have obtained 65 % -80 % 3 He polarization in these cells. The 3 He polarization has been measured by neutron transmission and electron paramagnetic resonance. Additional progress includes an improved holding field solenoid and decreased spin-flip losses.

$Research paper thumbnail of Control of Tetrahedral Coordination and Superconductivity in FeSe_{0.5}Te_{0.5} Thin Films$

Physical Review Letters, 2010

We demonstrate a close relationship between superconductivity and the dimensions of the Fe-Se(Te)... more We demonstrate a close relationship between superconductivity and the dimensions of the Fe-Se(Te) tetrahedron in FeSe0.5Te0.5. This is done by exploiting thin film epitaxy, which provides controlled biaxial stress, both compressive and tensile, to distort the tetrahedron. The Se/Te height within the tetrahedron is found to be of crucial importance to superconductivity, in agreement with the scenario that (π, π) spin fluctuations promote superconductivity in Fe superconductors.

$Research paper thumbnail of Friedel-Like Oscillations from Interstitial Iron in Superconducting Fe_{1+y}Te_{0.62}Se_{0.38}$

Physical Review Letters, 2012

Using polarized and unpolarized neutron scattering, we show that interstitial Fe in superconducti... more Using polarized and unpolarized neutron scattering, we show that interstitial Fe in superconducting Fe(1+y)Te(1-x)Se(x) induces a magnetic Friedel-like oscillation that diffracts at Q⊥=(1/2 0) and involves &amp;gt;50 neighboring Fe sites. The interstitial &amp;gt;2μ(B) moment is surrounded by compensating ferromagnetic four-spin clusters that may seed double stripe ordering in Fe(1+y)Te. A semimetallic five-band model with (1/2 1/2) Fermi surface nesting and fourfold symmetric superexchange between interstitial Fe and two in-plane nearest neighbors largely accounts for the observed diffraction.

$Research paper thumbnail of Magnetic excitations in underdoped Ba(Fe_{1−x}Co_{x})_{2}As_{2} with x=0.047$

Physical Review B, 2012

ABSTRACT The magnetic excitations in the paramagnetic-tetragonal phase of underdoped Ba(Fe${}_{0.... more ABSTRACT The magnetic excitations in the paramagnetic-tetragonal phase of underdoped Ba(Fe${}_{0.953}$Co${}_{0.047}$)${}_{2}$As${}_{2}$, as measured by inelastic neutron scattering, can be well described by a phenomenological model with purely diffusive spin dynamics. At low energies, the spectrum around the magnetic ordering vector mathbfQmathrmAFM{\mathbf{Q}}_{\mathrm{AFM}}mathbfQmathrmAFM consists of a single peak with elliptical shape in momentum space. At high energies, this inelastic peak is split into two peaks across the direction perpendicular to mathbfQmathrmAFM{\mathbf{Q}}_{\mathrm{AFM}}mathbfQmathrmAFM. We use our fittings to argue that such a splitting is not due to incommensurability or propagating spin-wave excitations, but is rather a consequence of the anisotropies in the Landau damping and in the magnetic correlation length, both of which are allowed by the tetragonal symmetry of the system. We also measure the magnetic spectrum deep inside the magnetically ordered phase, and find that it is remarkably similar to the spectrum of the paramagnetic phase, revealing the strongly overdamped character of the magnetic excitations.

$Research paper thumbnail of Comparison of charge modulations in La_{1.875}Ba_{0.125}CuO_{4} and YBa_{2}Cu_{3}O_{6.6}$

Physical Review B, 2013

ABSTRACT A charge modulation has recently been reported in (Y,Nd)Ba$_2$Cu$_3$O$_{6+x}$ [Ghiringhe... more ABSTRACT A charge modulation has recently been reported in (Y,Nd)Ba$_2$Cu$_3$O$_{6+x}$ [Ghiringhelli {\em et al.} Science 337, 821 (2013)]. Here we report Cu L_3L_3L_3 edge soft x-ray scattering studies comparing the lattice modulation associated with the charge modulation in YBa$_2$Cu$_3$O$_{6.6}$ with that associated with the well known charge and spin stripe order in La$_{1.875}$Ba$_{0.125}$CuO$_4$. We find that the correlation length in the CuO$_2$ plane is isotropic in both cases, and is 259pm9259 \pm 9259pm9 \AA for La$_{1.875}$Ba$_{0.125}$CuO$_4$ and 55pm1555 \pm 1555pm15 \AA for YBa$_2$Cu$_3$O$_{6.6}$. Assuming weak inter-planar correlations of the charge ordering in both compounds, we conclude that the order parameters of the lattice modulations in La$_{1.875}$Ba$_{0.125}$CuO$_4$ and YBa$_2$Cu$_3$O$_{6.6}$ are of the same order of magnitude.

$Research paper thumbnail of Magnetic fluctuations and quasi-static magnetism in optimally doped FeSe $ _ {0.4} $ Te $ _ {0.6} $$

ABSTRACT Magnetic Fluctuations in the optimally doped 11-type iron superconductor FeSe0.4Te0.6 we... more ABSTRACT Magnetic Fluctuations in the optimally doped 11-type iron superconductor FeSe0.4Te0.6 were examined using inelastic neutron scattering on the MACS instrument at NIST. In the normal state at T=25K we find strong low energy fluctuations through an extended area of the (hk0) zone that includes and connects the high symmetry (1/2,0,0) and (1/2,1/2,0) points. In the superconducting state intensity at the (1/2,1/2,0) location is depleted for phi = 1.5 meV as spectral weight is transferred to the 6.5 meV resonance. Low energy and quasi-elastic scattering however remains at (1/2,0,0). In the (HHL) zone we observed striped features indicating shorter range correlations along c. While glassy magnetism and superconductivity coexist in our samples, they are associated with distinct parts of momentum space. Work at JHU was supported by DoE through DE-FG02-08ER46544.

Nature Materials, 2010

The iron chalcogenide Fe 1+y (Te 1-x Se x ) is structurally the simplest of the Fe-based supercon... more The iron chalcogenide Fe 1+y (Te 1-x Se x ) is structurally the simplest of the Fe-based superconductors 1-3 . Although the Fermi surface is similar to iron pnictides 4-5 , the parent compound Fe 1+y Te exhibits antiferromagnetic order with in-plane magnetic wave-vector (0) 6 . This contrasts the pnictide parent compounds where the magnetic order has an 2 in-plane magnetic wave-vector () that connects hole and electron parts of the Fermi surface 7-8 . Despite these differences, both the pnictide and chalcogenide Fesuperconductors exhibit superconducting spin resonances around (π, π) 9-11 , suggesting a common symmetry for their superconducting order parameter. A central question in this burgeoning field is therefore how (π, π) superconductivity can emerge from a (π, 0) magnetic instability 12 . Here, we report that the magnetic soft mode evolving from the (π, 0)-type magnetic long-range order is associated with weak charge carrier localization.