arXiv subjects

T. Timusk

Publications and source records attributed to T. Timusk.

49 records · Page 3Linked to original sources

arXiv2002

The oxygen isotope effect in the ab-plane reflectance of underdoped YBa_2Cu_3O_{7-delta}

We have measured the effect of oxygen isotope substitution on the ab-plane reflectance of underdoped YBCO. The frequency shift of the transverse optic phonons due to the substitution of O-16 by O-18 yields an isotope effect of the expected magnitude for copper-oxygen stretching modes with alpha=0.5 +- 0.1. The reflectance shoulder at 400 - 500 cm^-1 shows a much smaller exponent of alpha=0.1 +- 0.1 in the normal state and alpha=0.23+- 0.1 in the superconducting state. These observations suggest that the shoulder is of electronic origin and not due to a phonon mode as has been suggested recently.

cond-mat.supr-con↗

arXiv2001

Infrared and optical properties of pure and cobalt-doped LuNi_2B_2C

We present optical conductivity data for Lu(Ni$_{1-x}$Co$_x$)$_2$B$_2$C over a wide range of frequencies and temperatures for x=0 and x=0.09. Both materials show evidence of being good Drude metals with the infrared data in reasonable agreement with dc resistivity measurements at low frequencies. An absorption threshold is seen at approximately 700 cm-1. In the cobalt-doped material we see a superconducting gap in the conductivity spectrum with an absorption onset at 24 +/- 2 cm-1 = 3.9$ +/- 0.4 k_BT_c suggestive of weak to moderately strong coupling. The pure material is in the clean limit and no gap can be seen. We discuss the data in terms of the electron-phonon interaction and find that it can be fit below 600 cm-1 with a plasma frequency of 3.3 eV and an electron-phonon coupling constant lambda_{tr}=0.33 using an alpha^{2}F(omega) spectrum fit to the resistivity.

cond-mat.supr-con↗

arXiv2001

Incoherent Interplane Conductivity of kappa-(BEDT-TTF)2Cu[N(CN)2]Br

The interplane optical spectrum of the organic superconductor kappa-(BEDT-TTF)2Cu[N(CN)2]Br was investigated in the frequency range from 40 to 40,000 cm-1. The optical conductivity was obtained by Kramers-Kronig analysis of the reflectance. The absence of a Drude peak at low frequency is consistent with incoherent conductivity but in apparent contradiction to the metallic temperature dependence of the DC resistivity. We set an upper limit to the interplane transfer integral of tb = 0.1 meV. A model of defect-assisted interplane transport can account for this discrepancy. We also assign the phonon lines in the conductivity to the asymmetric modes of the ET molecule.

cond-mat.supr-con↗

arXiv1999

Scattering Rate Gap in the IR Response of HgBa_2Ca_2Cu_3O_{8+delta}

The ab-plane optical spectra of one underdoped and one nearly optimally doped single crystal of HgBa_2Ca_2Cu_3O_{8+delta} were investigated in the frequency range from 40 to 40,000 cm^-1. The frequency dependent scattering rate was obtained by Kramers Kronig analysis of the reflectance. Both crystals have a scattering rate gap of about 1000 cm^-1 which is much larger than the 700 cm^-1 gap seen in optical studies of several cuprates with maximum Tc around 93 K. There appears to be a universal scaling between scattering rate gap and maximum Tc for the cuprate superconductors.

cond-mat.supr-con↗

arXiv1999

The pseudogap in high-temperature superconductors: an experimental survey

We present an experimental review of the nature of the pseudogap in the cuprate superconductors. Evidence from various experimental techniques points to a common phenomenology. The pseudogap is seen in all high temperature superconductors and there is general agreement on the temperature and doping range where it exists. It is also becoming clear that the superconducting gap emerges from the normal state pseudogap. The d-wave nature of the order parameter holds for both the superconducting gap and the pseudogap. Although an extensive body of evidence is reviewed, a consensus on the origin of the pseudogap is as lacking as it is for the mechanism underlying high temperature superconductivity.

cond-mat.supr-con↗

arXiv1999

Infrared properties of exotic superconductors

The infrared spectra of the non-traditional superconductors share certain common features. The lack of a gap signature at $2Δ$ and the residual conductivity are the consequence of a d-wave order parameter. The high $T_c$ materials, the organic conductors and the heavy Fermion materials have a strong mid-infrared absorption band which can be interpreted as strong coupling of the carriers to electronic degrees of freedom which leads to a breakdown of the Fermi liquid picture. The cuprates and the organic charge transfer salts are unique in possessing an intrinsic low dimensionality. The charge transport normal to the highly conducting direction is incoherent down to the lowest temperatures and frequencies.

cond-mat.supr-con↗

arXiv1998

Temperature Evolution of the Pseudogap State in the Infra-Red Response of Underdoped La_(2-x)Sr_xCuO_4

The ab-plane optical spectra of two single crystals of underdoped La_(2-x)Sr_xCuO_4 were investigated. The reflectivity of La_(1.87)Sr_(0.13)CuO_4 has been measured in the frequency range 30- 9,000 cm-1 (0.004 -- 1 eV) both parallel and perpendicular to the CuO_2 planes, whereas La_(1.86)Sr_(0.14)CuO_4 was studied only in the ab-plane. The extended Drude model shows that the frequency-dependent effective scattering rate 1/tau(omega)is strongly suppressed below the high-frequency straight-line extrapolation, a signature of the pseudogap state. This suppression can be seen from temperatures below the superconducting transition up to 400 K. In the case of underdoped LSCO the straight-line extrapolation is temperature independent below 200 K, whereas above 200 K there is a strong temperature dependence of the high-frequency 1/tau(omega,T). The out-of-plane direction is also examined for evidence of the pseudogap state.

cond-mat.supr-con↗

arXiv1997

Temperature Dependence of the FIR Reflectance of LaSrGaO4

The reflectance of single crystal LaSrGaO4 has been measured from approx 50 to 40000 cm^-1 along the "a" and "c" axis. The optical properties have been calculated from a Kramers-Kronig analysis of the reflectance for both polarizations. The reflectance curves have been fit using a product of Lorentzian oscillators.

cond-mat.mtrl-sci↗

arXiv1997

New Far Infrared Vibrational Mode in Zn Doped CuGeO3

We report on far infrared measurements on Zn and Si doped crystals of the spin-Peierls compound CuGeO3. Zn doping has the effect of introducing several new absorption lines, polarized in the ab-plane, between 5 and 55 cm-1. The intensity of the absorption grows with Zn concentration but saturates above 2% Zn. One line at 10 cm-1 loses intensity above 4K, and a second line at 20 cm-1 is absent at low temperatures but grows to peak at about 40K in agreement with a three level model with two excited states 10 and 30 cm-1 above the ground state. As the doping is increased these lines broaden, and a temperature independent absorption developes over the entire range from 5 to 55 cm-1. These features are magnetic field independent up to 16T and are absent in Si doped samples. We suggest the new absorption is due to localized lattice modes of the zinc ion and the surrounding GeO4 tetrahedra.

cond-mat↗

arXiv1997

Doping Dependence of the Pseudogap State in the ab plane IR responce of La(2-x)Sr(x)CuO(4)

The ab plane optical spectra of two single crystals of La(2-x)Sr(x)Cu(4), one overdoped and one underdoped, were investigated. We observe a gap-like depression of the effective scattering rate below 700 cm(-1) in both systems. This feature persists up to 300 K in the underdoped sample with the concentration of Sr x=0.14 but loses prominance at temperatures above 300 K in the overdoped regime (x=0.22). Below 700 cm(-1) the scattering rate is temperature dependent and superlinear in frequency for both samples. Above this frequency the effective scattering rate becomes linear in frequency and is temperature independent in the case of the underdoped La(1.86)Sr(0.14)CuO(4) up to 300 K. On the other hand, the overdoped La(1.76)Sr(0.22)CuO(4) shows a scattering rate temperature dependence above 700 cm(-1) at all temperatures. This behaviour of the frequency and temperature dependent scattering rates is a signature of a pseudogap state in other HTSC materials and suggests that both the under and overdoped single-layer HTSC systems Ls(2-x)Sr(x)CuO(4) have a pseudogap at temperatures exceeding 300 K.

cond-mat.supr-con↗

arXiv1996

Pseudogap State in High-$T_c$ Superconductors: an Infrared Study

We report on a study of the electromagnetic response of three different families of high-T_c superconductors that in combination allowed us to cover the whole doping range from under- to overdoped. The discussion is focused on the ab-plane charge dynamics in the {\it pseudogap state} which is realized in underdoped materials below a characteristic temperature T^*; a temperature that can significantly exceed the superconducting transition temperature T_c. We explore the evolution of the pseudogap response by changing the doping level, by varying the temperature from the above to below T^*, or by introducing impurities in the underdoped compounds. We employ a memory-function analysis of the ab-plane optical data that allows us to observe the effect of the pseudogap most clearly. We compare the infrared data with other experimental results, including c-axis optical response, dc transport, and angular resolved photoemission.

cond-mat.supr-con↗

arXiv1996

The Imaginary Part of the Optical Conductivity of $Ba_{1-x}K_xBiO_3$

The frequency dependence of the imaginary part of the infrared conductivity is calculated for a superconductor. Sharp structure, characteristic of superconductivity with an order parameter with s-wave symmetry, appears in the BCS limit as a minimum at a frequency equal to twice the gap value. This structure scales with temperature but gets progressively smeared and shifted as impurity scattering is increased. The relationship between low frequency results and the zero frequency limit is investigated. Experimental results on $\BKBO$ are also presented. The low frequency imaginary part of the conductivity displays a minimum at $2Δ\approx 12$ meV, and provides unequivocal evidence of an s-wave superconducting order parameter. Strong coupling (Eliashberg) results show similar trends. Using this formulation we find that the electron-phonon coupling in $\BKBO$ must necessarily be small, with coupling constant $λ\approx 0.2$, in agreement with conclusions drawn from measurements of the real part of the conductivity. Thus $\BKBO$ is an s-wave superconductor that is not driven by the electron-phonon interaction.

cond-mat↗

arXiv1995

Optical properties along the c axis of YBa_2Cu_3O_{6+x}, for x=0.50 -> 0.95: evolution of the pseudogap

The reflectance of high-quality, unpolished single crystals of YBa_2Cu_3O_{6+x}, for the doping range x=0.50 -> 0.95, has been measured with radiation polarized along the c axis from approx 50 cm^{-1} to 5000 cm^{-1} between 10 K and 300 K. In highly doped (x=0.95) material, the normal-state conductivity shows a metallic response. For intermediate dopings (x=0.85 -> 0.90) the conductivity is no longer metallic, increasing with temperature, and for low dopings (x=0.50 -> 0.80) this behavior is clearly seen to be caused by a pseudogap at approx 280 cm^{-1}, which develops well above T_c. In the superconducting state, in the optimally-doped material, a gap-like depression develops below T_c but there is residual conductivity to very low frequency indicating either an anisotropic gap, or a gap with nodes. In the underdoped materials the superconductivity-induced changes to the conductivity are harder to see, but it appears that the weight of the condensate originates at frequencies much higher than the conventional weak-coupling value of 3.5k_BT_c.

cond-mat↗