|Fluctuations and Phase Transitions in Superconductors and Ferromagnets|
A remarkable consequence of research on high-temperature
superconductors is a new picture of superconductivity in a magnetic field.
Research on the conventional superconductors led to the belief that, while
the resistance might be immeasurably small, it would never be zero: While
there are strong superconducting fluctuations, there is not a true superconducting
phase transition in a magnetic field. New experiments and theories since
1989 led to the opposite conclusion, very often based on scaling to analyze
current-voltage (I-V) characteristics.
While data should undoubtedly obey scaling if a phase
transition exists, work at Maryland argues that the converse is more subtle
[D. R. Strachan et al., Phys. Rev. Lett. 87, 067007 (2001)].
The work showed that standard I-V scaling is much too flexible to allow
conclusions to be drawn about the presence or absence of a phase transition.
Based on a new experimental criterion, it appeared that data are actually
more consistent with the absence of a phase transition.
The objective of this work is to experimentally study
the temperature vs. field phase diagram of type-II superconductors, to
determine if there is a transition in non-zero field, and to determine
the critical exponents of that transition if there is, as well as the
exponents of the zero-field transition. (The exponents reported in the
literature vary widely for both transitions.) The methods used include
DC current-voltage (I-V) and high-frequency (45 MHz to 50 GHz) conductivity
measurements on the same samples, providing independent experiments. Extreme
sensitivity of DC measurements to noise, and of both measurements to sample
inhomogeneity, is dealt with by improved filtering and sample characterization.
In addition, the new experimental criterion proposed above is used to
determine when a scaling can be properly used. It was discovered that finite-size and finite length-scale effects, due to the finite-frequency currents, are rife in these experiments (see the figure below). We have found that the static critical behavior is consistent with the 3D-XY universality class. However we find that the dynamical critical behavior is consistent with model-E dynamics, with a dynamical scaling exponent z ~ 1.5 (arXiv:0803.0969v2).
Related work has examined the critical behavior of magnetization at the ferromagnetic/paramagnetic second order phase transition in single crystal manganites (paper 73).
The intellectual merit of this work is an improved understanding
of phase transitions in superconductors and in other systems. More broadly,
the project will reaffirm that questioning and testing widely-accepted
results is an essential part of the scientific process. Other broad impacts
include continued support for training of graduate and undergraduate students.
Some relevant papers: (All papers can be downloaded from the full publication list)
128. Hua Xu, Su Li, Steven M. Anlage, C. J. Lobb , M. C. Sullivan , Kouji Segawa and Yoichi Ando , “Universal critical behavior in single crystals and films of YBa2Cu3O7-d,” Phys. Rev. B 80 , 104518 (2009). arXiv:0803.0969v2 pdf
73. Andrew Schwartz, Mark Scheffler, and Steven M. Anlage, "Determination of the Magnetization Scaling Exponent for Single Crystal La 0.8Sr0.2MnO3 by Broadband Microwave Surface Impedance Measurements," Phys. Rev. B (Rapid Communications) 61, R870-R873 (2000). pdf
49. J. C. Booth, Dong-Ho Wu, S. Qadri, E. Skelton, M. S. Osofsky, Alberto
Pique, and Steven M. Anlage, "Large Dynamical Fluctuations in the
Microwave Conductivity of YBa2Cu3O7-d ," Phys. Rev. Lett. 77,
4438 (1996). pdf
45. Steven M. Anlage, J. Mao, J. C. Booth, D.-H. Wu, and
J. L. Peng, "Fluctuations in the Microwave Conductivity of YBa2Cu3O7-d Single Crystals in zero dc magnetic field," Phys. Rev. B 53,
2792-2796 (1996). pdf
40. Dong-Ho Wu, J. C. Booth, and Steven M. Anlage, "Frequency and Field Variation of Vortex Dynamics in YBa2Cu3O7-d ," Phys. Rev. Lett. 75, 525 (1995). pdf
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