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An STM to Measure Phase Differences in Superconductors

Grad student Anita Roychowdhury examines a dilution refrigerator for cooling the STM device.

Grad student Anita Roychowdhury examines a dilution refrigerator for cooling the STM device.

Twenty years after the discovery of high temperature superconductors, the mechanisms that cause those materials to lose electrical resistance remain unknown. To understand the phenomenon, researchers will have to determine precisely which aspects of a material’s atomic configuration contribute to superconductivity, and to measure telltale differences at the atomic scale between very slightly different arrangements in a material’s crystal lattice.

Now a JQI/UMD team is building a device to do just that: a new kind of scanning tunneling microscope (STM) that can detect the subtle difference in quantum “phase” between tiny adjacent regions on the surface of a sample.

In quantum mechanics, properties of objects are described in terms of wave functions. Just as two waves can be in or out of phase, quantum states of matter -- individual electrons or, in the case of superconductors, entire arrays of electrons that share the same phase -- vary from point to point depending on their atomic surroundings.

This can lead to distinctive effects. For example, if two pieces of the same superconducting material are separated by a very thin insulating layer, a current will flow across the barrier if there is a phase difference between the two sides. This arrangement, called a Josephson junction, is thus a sensitive detector of phase difference.

However, such a measurement only reveals an averaged phase difference across a stationary junction that is typically much larger than atomic scale.

Determining how the phase difference is related to specific atomic structure will require comparing the phase at a fixed reference point to the phase at various points on the surface. In addition, the junction itself must small enough to measure at the atomic scale.”

At present, there is no device capable of making those measurements. So Bob Anderson, Chris Lobb and Fred Wellstood of JQI, and colleagues from UMD and LPS, are building a unique variation on the conventional STM.

That design involves a needle-like metal probe that moves above an atomic surface. Thanks to their wave-like quantum nature, electrons can “tunnel” across the gap between the surface and the probe; and the closer the probe, the more electrons cross the gap. Tracking the current that reaches the probe provides a detailed picture of the surface topology.

JQI’s version will use a superconducting probe that moves across a superconducting surface -- in effect, a mobile Josephson junction.

As the probe reaches points at which the phase is different -- because of a defect or grain boundary in the material, a “doping” atom in the atomic lattice, the presence of a “microdomain” or pocket of different materials in a heterogenous sample, or some other cause -- the device will record a change in current.

Investigators can then look for correlations between phase and atomic structure to better understand the nature of hightemperature (high Tc) superconductors that require less expensive refrigeration to work.

For example, even within a defect-free high-Tc material, the phase changes depending on which way the lattice is oriented with respect to the detector. Take the same crystal, rotate it by 90 degrees, and the phase will be different. But no one knows exactly what aspect of the structure causes the difference.

In addition, because phase difference is a distinctive property of superconductors, the new STM will also be able to identify exactly which regions are superconducting and which are not.

The team plans to begin on well-characterized high-Tc substances such as yttrium barium copper oxide (YBCO) to calibrate and test the device. Then they will move on to target materials such as bismuth strontium calcium copper oxide (BSCCO).

First, however, they have to determine whether they can construct a superconducting STM capable of detecting the very faint signals of phase difference between two microscopic regions. To eliminate thermal noise, the system will be cooled to about 20 thousandths of a degree above absolute zero using a device called a dilution refrigerator that employs two isotopes of helium in a kind of evaporative cooling.

But low temperature alone will not be enough to do the job. Small quantum effects have an irreducible degree of uncertainty, and the signal strength is expected to be in the range of 1 billionth of an ampere. To minimize the uncertainty effects, the STM junction will be coupled to a second, larger junction that will serve to stabilize the system.

If the STM team -- which includes LPS’ Dan Sullivan, Michael Dreyer and graduate student Anita Roychowdhury -- succeeds, it will help answer one of the most urgent questions in condensed-matter physics. And their findings could prove crucial in the effort to create the next generation of high-Tc superconductors that can operate at even higher temperatures.