# Presentation: 702735

## Detecting the latent heat of Bragg glass melting in a classic type-II superconductor

**X. S. Ling, Brown University,**** ****DMR-0406626**** **

- Type-II superconductors are being used in MRI machines, superconducting magnets in accelerators, etc.  Yet, one of the most fundamental questions concerning these materials as to whether they are truly superconducting has not been fully resolved.  For example, the PI’s recent neutron scattering studies showed a Bragg glass transition in a classic type-II superconductor Nb, which would signify a true superconducting phase in magnetic fields.  However, a true thermodyanmic phase transition should also be accompanied by a latent heat signature in specific heat measurements.  Such measurements are performed on the Nb sample studied by neutron scattering.  Finally, the long sought-after signature of Bragg glass melting has bee observed.  The left panel shows the variation of sample temperature (while the base temperature held fixed) as a function of magnetic field.  The two negative peaks indicate latent heat of phase transitions near Hc1 and Hc2.  The right panel is a phase diagram of the same sample.

**Notes:**
To appreciate the importance of establishing the existence or absence of a phase transition in vortex state, one needs only to know that ice and water are made of the same stuff, yet their properties are drastically different.  A true superconducting state is like ice, has “rigidity” against the force generated by an electric current, and as a result, does not dissipate energy, hence called true superconductivity.

Simple metals such as Al and Pb are called type-I superconductors because their magnetic properties are simple: it expels magnetic flux up to a critical magnetic field above which the superconductivity is quenched.  This system is well understood in the context of BCS theory: electrons pair up to avoid bad collisions with the ions, thereby avoiding resistance.  Strong magnetic fields break up the “Cooper pairs” and quench the superconductivity.

For technologically important materials such as Nb, NbTi, and HTSC, the BCS theory for electron pairing is not enough to describe their behavior in a magnetic field.  For these materials, their magnetic properties are determined by how the Abrikosov vortices behave.  For decades, scientists thought that the vortices are in a slushy fluid state and will never form a vortex solid phase.  In that scenario, the superconductors are not really superconducting. 

In the past decade, this old picture was critically re-examined, both theoretically and experimentally.  Our neutron scattering experiment provided the most critical information regarding the existence of a vortex solid-liquid transition in Nb.  This work was described in two Physical Review Letters in 2001 and 2003.

The present experiment is designed to work the following way: the sample, a Nb crystal, is held in a vacuum can with copper wires serve as heat link between the sample and the copper can.  The massive copper can is held at a constant temperature by controlling the cold helium gas flow.  When the magnetic field is ramped up, the sample temperature should change reflecting the change in its heat capacity, and is detected using a lock-in amplifier (low excitation current).  When the Bragg glass (vortex lattice) melts, it absorbs heat from the Nb atomic lattice, resulting a lower sample temperature, and a negative peak. 

The new observations of latent heat at the Bragg glass transitions (one near Hc1 and the other near Hc2 and at the peak effect) provide the last major evidence establishing a Bragg glass transition in Nb.  A manuscript reporting this major result is being prepared for PRL.

## Detecting the latent heat of Bragg glass melting in a classic type-II superconductor

- 1. X.S. Ling, S.R. Park, B.A. McClain, S.M. Choi, D.C. Dender, and J.W. Lynn, Physical Review Letters _86_, 712 (2001).
- 2. S.R. Park, S.M. Choi, D.C. Dender, J.W. Lynn, and X.S. Ling, Physical Review Letters _91_, 167003 (2003).
- 3. N. Daniilidis, I. Dimitrov, C. Elbaum, V. Mitrovic, and X.S. Ling, manuscript in preparation.

**Education:**
- Two Ph.D. students Nikos Daniilidis and Ivo Dimitrov are currently carrying their Ph.D. thesis studies under the support of the DMR program.
- Students are being trained in superconductivity, neutron scattering, low-temperature physics, ultrasensitive electrical measurements.

**Detecting the latent heat of Bragg glass melting in a classic type-II superconductor**

**X. S. Ling, Brown University,**** ****DMR-0406626**** **

**Society Impact:**
- With the eventual depletion of world’s fossil energy sources, sooner or later humanity will have to confront the fundamental issues of energy carrying and storage.  High-temperature superconductors will one day become a major carrier of electricity.  The study of basic physics of vortex matter in these materials will lay a solid foundation for the emerging superconducting technology.