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Title: ULTRAFAST OPTICAL PUMP TERAHERTZ-PROBE SPECTROSCOPY OF STRONGLY CORRELATED ELECTRON MATERIALS

Abstract

No abstract prepared.

Authors:
; ;
Publication Date:
Research Org.:
Los Alamos National Lab., NM (US)
Sponsoring Org.:
US Department of Energy (US)
OSTI Identifier:
785020
Report Number(s):
LA-UR-01-4821
TRN: AH200137%%133
DOE Contract Number:
W-7405-ENG-36
Resource Type:
Conference
Resource Relation:
Conference: Conference title not supplied, Conference location not supplied, Conference dates not supplied; Other Information: PBD: 1 Aug 2001
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; ELECTRONS; SPECTROSCOPY; OPTICAL PUMPING

Citation Formats

R. D. AVERITT, A. J. TAYLOR, and ET AL. ULTRAFAST OPTICAL PUMP TERAHERTZ-PROBE SPECTROSCOPY OF STRONGLY CORRELATED ELECTRON MATERIALS. United States: N. p., 2001. Web.
R. D. AVERITT, A. J. TAYLOR, & ET AL. ULTRAFAST OPTICAL PUMP TERAHERTZ-PROBE SPECTROSCOPY OF STRONGLY CORRELATED ELECTRON MATERIALS. United States.
R. D. AVERITT, A. J. TAYLOR, and ET AL. Wed . "ULTRAFAST OPTICAL PUMP TERAHERTZ-PROBE SPECTROSCOPY OF STRONGLY CORRELATED ELECTRON MATERIALS". United States. doi:. https://www.osti.gov/servlets/purl/785020.
@article{osti_785020,
title = {ULTRAFAST OPTICAL PUMP TERAHERTZ-PROBE SPECTROSCOPY OF STRONGLY CORRELATED ELECTRON MATERIALS},
author = {R. D. AVERITT and A. J. TAYLOR and ET AL},
abstractNote = {No abstract prepared.},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Wed Aug 01 00:00:00 EDT 2001},
month = {Wed Aug 01 00:00:00 EDT 2001}
}

Conference:
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  • We have used optical-pump far-infrared probe spectroscopy to probe the low energy electron dynamics of high temperature superconductors and colossal magnetoresistance manganites. For the superconductor YBa2Cu3O7, picosecond conductivity measurements probe the interplay between Cooper-pairs and quasiparticles. In optimally doped films, the recovery time for long-range phase-coherent pairing increases from {approx}1.5 ps at 4K to {approx}3.5 ps near Tc, consistent with the closing of the superconducting gap. For underdoped films, the measured recovery time is temperature independent (3.5 ps) in accordance with the presence of a pseudogap. Ultrafast picosecond measurements of optically induced changes in the absolute conductivity of La0:7M0:3MnO3 thinmore » films (M = Ca, Sr) from 10K to {approx}0.9Tc reveal a two-component relaxation. A fast, {approx}2 ps, conductivity decrease arises from optically induced modification of the effective phonon temperature. The slower component, related to spin-lattice relaxation, has a lifetime that increases upon approaching Tc from below in accordance with an increasing spin specific heat. Our results indicate that for T<<Tc, the conductivity is determined by incoherent phonons while spin fluctuations dominate near Tc.« less
  • Multiferroic materials have attracted much interest in the past decade, due not only to their novel device applications, but also their manifestations of coupling and interactions between different order parameters (particularly electric polarization and magnetic order). Recently, much attention has been focused on perovskite manganites, RMnO{sub 3} (R = rare earth ions), due to the discovery of a large magnetoelectric effect in these materials. The first member of this family to be discovered was TbMnO{sub 3} (TMO), which is now well established as a typical magnetoelectric multiferroic. Extensive experimental and theoretical studies have already been done on single crystal TMOmore » (SC-TMO). In brief, SC-TMO, with a distorted orthorhombic perovskite structure, has an antiferromagnetic (AFM) phase transition at T{sub N} {approx}40 K with sinusoidally ordered Mn moments. Below T{sub FE} {approx} 28 K, ferroelectric (FE) order develops owing to the appearance of cycloidal spiral spin structure. In contrast, there are relatively few reports describing the properties of TMO thin films (typically grown on SrTiO{sub 3} (STO) substrates). In general, thin films can enable new functionality in materials, as their physical parameters can be changed by modifying their structure via strain imposed by the substrate. Strain in particular has the potential to directly couple FE and FM orders, which is very rare. This could benefit electronic device applications by providing low power consumption, high speed operation, and greater electric/magnetic field controllability. Previous investigations of magnetic properties in TMO films revealed an unexpected ferromagnetic (FM) order, in contrast to SC-TMO. However, several important questions regarding these films are still unanswered for instance: (1) What mechanism induces FM order? (2) Can FM, sinusoidal AFM and spiral AFM (or FE) orders coexist? (3) Can FM order be coupled to FE order? To fully understand these unique materials, experimental techniques capable of dynamically unraveling the interplay between these degrees of freedom on an ultrafast timescale are needed. Here, we use ultrafast optical pump-probe spectroscopy to reveal coexisting coupled magnetic orders in epitaxial TMO thin films grown on (001)-STO, which were not observed in previous work. Our temperature (T)-dependent transient differential reflectivity ({Delta}R/R) measurements show clear signatures of sinusoidal AFM, spiral AFM (FE) and FM phases developing as the film thickness changes. We carry out first-principle density functional theory (DFT) calculations to explain the coupling between AFM/FE and FM orders. These results reveal that the coupling between different magnetic orders observed in our multiferroic TMO thin films may offer greater control of functionality as compared to bulk single crystal multiferroics.« less
  • 5f electrons in the Mott insulator UO{sub 2} produce intriguing electronic states and dynamics, such as strong correlation and f-f excitations. We have performed femtosecond pump-probe reflectivity measurements on a single crystal UO{sub 2} at temperatures 5-300 K to study the ultrafast dynamics of photoexcited 5f electrons. The laser pulses at 400 nm pump 5 f electrons across the Mott gap, while those at 800 nm probe the pump-induced change of reflectivity. We find temperature-dependent excitation and relaxation processes and long-lived acoustic phonons, and extract picosecond risetimes and microsecond relaxation times at low temperatures. The observed slow relaxation is ascribedmore » to the decay of Hubbard excitons formed by U{sup 3+}-U{sup 5+} pairs.« less
  • Cooper pair-breaking dynamics (PBD) have been resolved for the first time in MgB{sub 2}. We present an analysis of the PBD using the Rothwarf-Taylor model, enabling the determination of the bare quasiparticle recombination and phonon pair-breaking rates. Nonequilibrium effects have been studied in superconductors since the 1960's. One of the primary goals of such investigations has been to determine the bare quasiparticle recombination rate. However, directly measuring this rate is difficult since, as first noted by Rothwarf and Taylor, other phenomena contribute to the measurements. In the case of superconductors, the bare quasiparticle recombination rate is masked due to pair-breakingmore » by above gap phonons. To date, it has not been possible to extract the bare quasiparticle recombination rate. In the case of conventional superconductors, one problem is that the superconducting transition temperatures are quite low, making detailed fluence dependent measurements all but impossible. However, with the discovery of MgB{sub 2} (T{sub c} = 39K) it is now possible to perform detailed fluence dependent ultrafast measurements on a conventional (i.e. phonon mediated pairing) superconductor. Our recent experiments of the picosecond photoinduced far-infrared conductivity dynamics on MgB{sub 2} have, for the first time, revealed the dynamics of Cooper pair breaking following photoexcitation. Here we present a detailed analysis of the pair breaking dynamics which suggests that in MgB{sub 2} photoexcitation is initially followed by energy relaxation to high frequency phonons instead of, as commonly assumed, e-e thermalization. Furthermore, the bare quasiparticle recombination rate and the probability for pair-breaking by phonons have been determined for the first time.« less
  • We have demonstrated accurate observations of terahertz (THz) dielectric response due to photoexcited carriers in a Si plate via single-shot optical-pump and THz-probe spectroscopy. In contrast to conventional THz time-domain spectroscopy, this spectroscopic technique allows single-shot detection of the THz response of materials at a given delay time between the pump and THz pulses, thereby sufficiently extending the time interval between the pump pulses. As a result, we can accurately measure the dielectric properties of materials, while avoiding artifacts in the response caused by the accumulation of long-lived photoexcited carriers. Using our single-shot scheme, the transmittance of a Si platemore » was measured in the range of 0.5–2.5 THz with different pump fluences. Based on a Drude model analysis, the optically induced complex dielectric constant, plasma frequency, and damping rate in the THz region were quantitatively evaluated.« less