Advancing the experimental frontier
THz spectroscopy in underdoped cuprate superconductors
The under-doped part of the cuprate phase diagram presents a confusing phenomenology that is inconsistent with the conventional theory of metals. There is now a large body of evidence for strong superconducting correlations even in those regions of the phase diagram that do not exhibit bulk superconductivity. Recently for example, Ong and coworkers have demonstrated a Nernst effect that persists up to temperatures far above the resistive Tc in underdoped samples . This Nernst signal has been attributed to vortex motion.
To advance our understanding of this important phase of matter, and as part of the IQM quest to develop new experimental tools for condensed matter physics, we are using time-domain THz spectroscopy to characterize the regime with putative strong superconducting fluctuations in underdoped La2−xSrxCuO4 films.
This project is proceeding in collaboration with Ivan Bozovic of Brookhaven National Laboratory, who is providing the high quality epitaxial thin films. THz spectroscopy allows us to characterize the charge fluctuation spectrum through measurements of the complex conductivity at energies below the superconducting gap, but well above the static DC limit. The imaginary part of the conductivity allows us access to the superfluid stiffness, which is a fundamental quantity that sets the energy scale for phase twists of the superconducting order parameter. The finite frequency of the probe means that we can sample fluctuations on short time and length scales.
In Fig. 4, the THz superfluid stiffness is presented over a broad frequency range from low temperatures up to and through the superconducting transition. There are a number of interesting aspects. First there is a specific temperature where frequency dependence is acquired in the superfluid stiffness. Such frequency dependence is indicative of an increasing role of fluctuations. Moreover, the higher the frequency scale we use, the shorter time scale we probe and the larger the superconducting signal is. Interestingly, the temperature where the stiffness acquires its frequency dependence is very close to the universal prediction for occurrence of a Kosterlitz-Thouless-Bereninskii phase transition (red dotted line), which is a vortex unbinding transition that occurs in 2D superconductors and superfluids. That this temperature (14 K) is substantially below the temperature of the bulk Tc (20 K given by the solid red line) shows that there is a regime even in the superconducting state where the superconducting correlations have 2D character. These fluctuations persist smoothly through the superconducting transition at 20 K and continue as high as 9 K above the transition. It is also remarkable that superconducting correlations are only detectable up to about 29 K in a Tc=20 K sample. This is far below the maximum temperature for the Nernst effect  and may indicate that the Nernst effect measurements have been misattributed. Further work is now underway to characterize the fluctuation regime over a larger doping range and to analyze the explicit temperature dependence of the characteristic time scale of fluctuations. In this way we can put precise bounds on the strength of the fluctuations to determine whether they can account for the phenomenology of underdoped cuprates. Our newly developed time-domain THz spectrometer is thus providing unique new information on a key problem in contemporary condensed matter physics.
Very high efficiency inelastic neutron scattering on the MACS spectrometer
After several years of work by the PI and engineers and technicians at JHU and at the NIST Center for Neutron Research, the Multi Axis Crystal Spectrometer at the NCNR is now taking data [14-15]. Fig. 5 (a) shows an image of MACS. The instrument features, to our knowledge, the most intense monochromatic cold neutron beam in the world (up to 5×108n/cm2/s) and an innovative twenty channel detection system. Fig. 5 (b-c) show the kind of data that MACS is particularly effective at collecting: A slice through momentum space at fixed energy transfer, , here 1.5 meV and 2.75 meV. These data were obtained in approximately 6 hours from 1 cm3 of the spin-1/2 quantum antiferromagnet TeVO4. While the material was thought to be a quasi-one-dimensional alternating spin chain, it is quite clear from the data that it is in fact at least quasi-two-dimensional. Fig. 6 shows the most recent test experiment, which is a measurement of quantum critical spinon scattering in copper pyrazine nitrate as a function of temperature.
The IQM will have access to up to 20% of the beam time on MACS. This will enable early experiments with high discovery potential probing inelastic magnetic scattering in magnetic and superconducting materials developed at Princeton and JHU under IQM auspices.
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