Investigating quantum phase transitions using designer-anvil pressure cells

Lead Research Organisation: Royal Holloway University of London
Department Name: Physics

Abstract

Discovery in correlated electron systems research occurs on the boundary of known low temperature states. Next to applied magnetic field, pressure, which affords precise control over the lattice density, is the vehicle of choice for traversing quantum matter phase diagrams and seeking out novel states of matter. This project represents a radical departure in high pressure anvil cell technology: We will structure leads and detection circuits into ceramic anvils by microlithographic methods. The resulting designer anvils provide the reliability, ease of use and versatility necessary for a novel investigation into the high-pressure superconducting state of the archetypal correlated electron superconductor CeCu2Si2. In this intriguing material it appears that two distinct mechanisms can induce superconductivity in different regions of the pressure-temperature phase diagram. Both interactions originate in the Coulomb repulsion between electrons, but one - an effective magnetic interaction - dominates when the material is tuned close to the onset of magnetic order, while the other - an effective charge density interaction - prevails close to a putative transition of the lattice, in which the unit cell volume and hence the lattice density undergoes a weak first order change. In order to investigate the density change quantum phase transition in CeCu2Si2 and in related materials, a detailed study of the pressure dependence of resistivity, heat capacity and thermal expansion will be carried out into the 100 kbar regime. While the investigated pairing mechanism may have ramifications throughout the strongly correlated electron field, including the high-Tc superconductors, the envisaged high pressure technology will be beneficial for a large community of researchers interested in transport or thermodynamic experiments under anvil cell conditions.

Publications

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Duncan W (2010) Quantum phase transitions in NbFe 2 and Ca 3 Ru 2 O 7 Quantum phase transitions in NbFe 2 and Ca 3 Ru 2 O 7 in physica status solidi (b)

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Cuk T (2010) Signatures of pressure-induced superconductivity in insulating Bi 1.98 Sr 2.06 Y 0.68 CaCu 2 O 8 + d in Physical Review B

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Duncan WJ (2010) High pressure study of BaFe2As2--the role of hydrostaticity and uniaxial stress. in Journal of physics. Condensed matter : an Institute of Physics journal

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Welzel OP (2011) Patterned anvils for high pressure measurements at low temperature. in The Review of scientific instruments

 
Description One of the biggest scientific surprises of the twentieth century, at least in the field of condensed matter research, was the discovery of superconductivity: the electrons in some metals can collectively develop a form of quantum order which is rigid to perturbation and resists an applied magnetic field with a persistent current. Superconductivity and magnetism are the two most widely known forms of electronic quantum order found in dense electronic matter at low temperature, but there are many others.

We tend to find such new forms of electronic quantum order on the boundary of a known electronic state at low temperature. This observation motivates work on high pressure instrumentation: next to applied magnetic field, pressure, which affords precise control over the lattice density, is the vehicle of choice for traversing quantum matter phase diagrams and thereby for seeking out novel states of electronic quantum matter. Varying the lattice spacing through applied pressure effectively produces an infinity of crystals from a single parent specimen, continuously, reversibly, and without introducing disorder.

Most experimental probes of the low temperature state require that electrical leads are present in direct proximity to the sample, or are even directly connected to the sample. Having to feed wires into the tiny sample volume available in pressure cells is a tremendous technical challenge and has slowed down progress in the field for a long time. In this project, we have made a radical departure from conventional high pressure anvil cell technology: we pattern very fine metallic tracks and detection circuits onto the surface of high pressure anvils made of moissanite (single crystal silicon carbide) or alumina (polycrystalline Al2O3). While these anvil materials are not as strong as diamond they have the advantage of being comparatively inexpensive. This will help the technique spread to other groups.

The resulting anvils provide the reliability, ease of use and versatility necessary for investigating the high pressure states of correlated electron systems by electrical resistivity or Hall effect measurements. We have also succeeded in patterning a tiny thirty-turn coil onto the tip of a high pressure anvil. This will enable measurements of magnetic properties under pressure or indirect, contactless measurements of the electrical resistivity by detecting the skin depth at high frequency.

Using this technique, we have investigated the high pressure phase diagrams of Ca3Ru2O7 and BaFe2As2. At low pressure, the ruthenate material Ca3Ru2O7 undergoes two phase transitions on cooling. With increasing pressure, we find that both transitions are suppressed, but a new ordered phase arises above about 60,000 bar. This phase eventually disappears at about 100,000 bar. By measuring the Hall effect under pressure, we could furthermore show that the carrier density increases by two order of magnitude near 30,000 bar, where the lower of the two ordered phases is suppressed. This is evidence for a reconstruction of the Fermi surface without underlying lattice symmetry breaking, a phenomenon which is under discussion in the context of the underdoped copper-oxide superconductors, and which we are now investigating more closely by means of quantum oscillation measurements under pressure using the same patterned anvil technique.
In the iron pnictide BaFe2As2 high pressure has been known to induce superconductivity at high temperature, but the precise form of the superconducting phase diagram has been controversial. Our study helped to resolve this controversy. We could show that the quality of the pressure environment, in particular the presence of uniaxial stresses on the sample, strongly influences the pressure at which superconductivity is observed.

These outcomes motivate fresh lines of investigation, which rely on the newly available high pressure techniques.
Exploitation Route The new high pressure methods developed as part of this project enable new lines of research which can speed up materials discovery and investigation. They have already led to a licencing agreement with a British instrument maker.
Sectors Electronics,Energy