@article{reagor1986inertial,

title = {Inertial dynamics of pinned charge-density-wave condensates. I. NbSe 3},

author = {D Reagor and S Sridhar and G Gruner},

year  = {1986},

date = {1986-01-01},

journal = {Physical Review B},

volume = {34},

number = {4},

pages = {2212},

publisher = {APS},

abstract = {Conductivity measurements are reported for the linear-chain compound 

NbSe

3

 in the frequency range 9–100 GHz. In both charge-density-wave (CDW) states, which develop at 

T

1

=144 K and at 

T

2

=59 K, a strongly frequency-dependent response is observed with Reσ(ω) decreasing with increasing frequency. The behavior is described in terms of a harmonic oscillator response, and in the measured spectral range the expression Re

σ

CDW

(ω)=(

ne

2

τ/

m

*

)[1 /(1+

ω

2

τ

2

)] provides an excellent account of our findings. We evaluate the effective mass 

m

*

 and damping constant τ from a fit to the above equation. The measured effective mass is only weakly temperature dependent, and in both CDW states is in good qualitative agreement with those determined using the theory of Lee, Rice, and Anderson. We also discuss the temperature-dependent damping constant, and a comparison with data taken in the radio-frequency range is given.



Received 28 October 1985

DOI:https://doi.org/10.1103/PhysRevB.34.2212



©1986 American Physical Society},

keywords = {Collective Excitations},

pubstate = {published},

tppubtype = {article}

}

@article{sridhar1986inertial,

title = {Inertial dynamics of pinned charge-density-wave condensates. II. Orthorhombic TaS 3},

author = {S Sridhar and D Reagor and G Gruner},

year  = {1986},

date = {1986-01-01},

journal = {Physical Review B},

volume = {34},

number = {4},

pages = {2223},

publisher = {APS},

abstract = {Complex conductivity 

σ

CDW

(

ω

,T) measurements in the ranges 10–110 GHz and 20–300 K are reported on the charge-density-wave (CDW) material orthorhombic 

TaS

3

. In the CDW state between approximately 130 and 210 K, the complex conductivity obeys a Drude behavior 

σ

CDW

(ω)=(

ne

2

τ/

m

*

)(1+i ωτ), which we associate with damped inertial response of the CDW. The measurements, together with earlier results in the radio-frequency spectral range, enable us to extract all the phenomenological parameters: damping constant τ, effective mass 

m

*

, pinning frequency 

ω

0

, and maximum CDW conductivity 

σ

max

. The experimental results for these parameters are compared with available microscopic theories. The full dynamic response of the CDW between 

10

7

 and 

10

11

 Hz is also presented and shows the conductivity resonance well below the single-particle gap, representing both the relaxational and inertial dynamic response of the collective CDW mode. A comparison is made to similar results on the related CDW compound 

NbSe

3

.



Received 28 October 1985

DOI:https://doi.org/10.1103/PhysRevB.34.2223



©1986 American Physical Society},

keywords = {Collective Excitations},

pubstate = {published},

tppubtype = {article}

}

@article{sridhar1986nonequilibrium,

title = {Nonequilibrium dynamics of quasiparticles in superconductors},

author = {S Sridhar and JE Mercereau},

year  = {1986},

date = {1986-01-01},

journal = {Physical Review B},

volume = {34},

number = {1},

pages = {203},

publisher = {APS},

abstract = {Small changes δ

R

s

 in the 10-GHz surface resistance 

R

s

 of thin superconducting films of Sn and In, caused by an induced static current 

J

0

 have been measured. δ

R

s

 was quadratic in 

J

0

 and both increases (δ

R

s

>0) and decreases (δ

R

s

<0) of absorption were observed. The increase is associated mainly with thermodynamic depression of the gap parameter due to the static current. On the other hand, the decrease of absorption, which implies an enhancement of superconductivity, is incompatible with thermodynamic analysis. A nonequilibrium analysis is presented, according to which the combined static and high-frequency currents cause a nonthermal, time-dependent oscillation of the quasiparticle distribution, and which is out of phase with the driving high-frequency field. The counter oscillation leads to an enhancement of high-frequency supercurrent and decrease of dissipative normal current, and hence results in a decrease of absorption. The analysis describes the experimental results for very thin films of Sn both qualitatively and quantitatively, and with no adjustable parameters. These results lead to the conclusion that at high frequencies, the superconductor responds to time-dependent perturbations in an inherently nonadiabatic, nonequilibrium manner even for vanishingly small high-frequency fields. The implications for several unexplained experiments on the magnetic field dependence of 

R

s

 are discussed.



Received 24 January 1986

DOI:https://doi.org/10.1103/PhysRevB.34.203



©1986 American Physical Society},

keywords = {Collective Excitations},

pubstate = {published},

tppubtype = {article}

}

@article{sridhar1985inertial,

title = {Inertial Dynamics of Charge-Density Waves in Ta S 3 and Nb Se 3},

author = {S Sridhar and D Reagor and G Gruner},

year  = {1985},

date = {1985-01-01},

journal = {Physical review letters},

volume = {55},

number = {11},

pages = {1196},

publisher = {APS},

abstract = {Frequency-dependent complex-conductivity measurements are reported in the charge-density-wave states of Ta S 3 and Nb Se 3, in the 10—100-GHz spectral region. A Drude-type behavior associated with damped inertial response of the condensate is observed for the first time. The fundamental parameters that characterize the dynamics of the collective mode (damping constant, pinning energy, and effective mass) are obtained. The results for the effective mass and damping are compared with microscopic theories.},

keywords = {Collective Excitations},

pubstate = {published},

tppubtype = {article}

}

@article{reagor1985inertial,

title = {Inertial charge-density-wave dynamics in (TaSe 4) 2 I},

author = {D Reagor and S Sridhar and M Maki and G Gruner},

year  = {1985},

date = {1985-01-01},

journal = {Physical Review B},

volume = {32},

number = {12},

pages = {8445},

publisher = {APS},

abstract = {Frequency-dependent conductivity measurements are reported in the charge-density-wave (CDW) state of the linear-chain compound (TaSe 4) 2 I in the spectral range 4.5–94 GHz. In contrast to other CDW systems which are overdamped, the response is underdamped, with a resonant frequency of approximately 35 GHz, a resonance width of approximately 20 GHz, and an effective mass of 10 4. The large effective mass, large resonant frequency, and small damping are a consequence of the strong electron-phonon coupling in this system.},

keywords = {Collective Excitations},

pubstate = {published},

tppubtype = {article}

}

@article{javadi1985giant,

title = {Giant conductivity resonance in the spin-density-wave state of an organic conductor},

author = {HHS Javadi and S Sridhar and G Grüner and Long Chiang and F Wudl},

year  = {1985},

date = {1985-01-01},

journal = {Physical review letters},

volume = {55},

number = {11},

pages = {1216},

publisher = {APS},

abstract = {Frequency-dependent conductivity measurements between 4.5 and 35 GHz are reported in the spin-density-wave (SDW) state of the linear-chain organic conductor (TMTSF) 2 P F 6, where TMTSF denotes tetramethyltetraselenafulvalenium. A giant conductivity resonance, implying long relaxation times, is observed at frequencies well below the single-particle gap. The frequency dependence is strongly depressed by small amounts of impurities. Either collective spin-density-wave conduction or excitations of the spin-density-wave mode are responsible for the observed frequency-dependent response.},

keywords = {Collective Excitations},

pubstate = {published},

tppubtype = {article}

}

@article{sridhar1985complex,

title = {Complex conductivity measurements between 26 and 110 GHz using complex impedance bridges},

author = {S Sridhar and D Reagor and G Gruner},

year  = {1985},

date = {1985-01-01},

journal = {Review of scientific instruments},

volume = {56},

number = {10},

pages = {1946--1952},

publisher = {American Institute of Physics},

abstract = {A method is described of measuring complex conductivity based upon the measurement of the impedance of a sample placed in a waveguide using a complex impedance bridge. From the observed changes in the bridge parameters, viz., attenuation and phase shift, the impedance and, hence, the complex conductivity are inferred. The principles and details of operation of three bridges covering the frequency range 26 to 110 GHz are discussed. The measurement technique was verified for lossless samples and for lossy resistive wires. An example of temperature and frequency dependent complex conductivity measurements on a linear chain material is presented.},

keywords = {Collective Excitations},

pubstate = {published},

tppubtype = {article}

}