Research Projects

☞  Quantum Solid He-4: Is it a "superslid" showing both superfluidity and crystallinity at temperatures below 0.5 K?  There are  many unsual  effects observed  in quantum solid He-4 at low temperatures.   At Rutgers, compound TO techniques were developed to study frequency dependent dynamics, hysteretic effects were discovered, He-3 impurity effects were measured in detail, aging-glass phenomena different from the common spin glasses were found, and ultrasound propagation measurements are revealing new information about the behavior of quantum solid He-4.

☞  Superfluid He-3 A₁ Phase in High Magnetic Fields: Magnetic Fountain Effect, Super-Spin Pump and Spin-Entropy Wave

Phase diagram of liquid He-3 displayed at left shows that  superfluid A1 phase is produced by applying magnetic fields at very low temperatures.  In our experiments, we can attain magnetic fields up to 80,000 gauss at temperatures from 0.001 to 0.0028 Kelvin above absolute zero. Applying gradient in magnetic field to A1 phase produces fountain pressure through a superleak.  The fountain is created by a current of quantum spin current.   This is effect analogous to applying temperature gradient producing a fountain in superfluid He-4.   Quantum spin flow phenomena are studied with the magnetic fountain effect. Unique spin-entropy wave (analogous to second sound) can be propagated in A1 phase.  Superfluid density, spin diffusion, and anisotropic kinetic texture transformations are studied with this wave propagation. The left photo shows the ultra low temperature (T > 0.8 mK) high magnetic field (H < 15 Tesla) apparatus at the Institute for Solid State Physics located in Kashiwanoha, Chiba, Japan, where the collaborative experiments on magnetic fountain effect are carried out.

☞  Stress Driven Instability on He-4 Crystal Surface: Grinfeld Instability, equilibrium shape, quantum dot self-assembly, fracture and crack

Fig. 1 below shows a view of Giant’s Causeway in Northern Ireland.  The meter-scale geologic formation may have been produced by large stresses accumulated in molten larva during their rapid cooling.  Fig. 2 shows an AFM image of nano-meter scale dots of Si0.25Ge0.75 on Si substrate (C. Teichert, et al., Phys. Rev. B53, 16334(1996)).  The dots are thought to form by self-assembly driven by inherent stresses present at the interface between materials with different lattice constants.  Stress-driven instabilities are also thought to be related to formations of cracks and fractures.

Figure 1                                                                                               Figure 2

Controlled experiments to study the above effects are difficult.  Solid helium provides an ideal material to study stress-driven instabilities.   A cryogenic optical interferometer apparatus is used for observing the solid surface profile of helium crystal under externally applied stress.  Fig. 3 shows an interference pattern with no stress applied.  Fig. 4 shows an example when a large uniaxial stress is applied.  Note the appearance of dark patches which indicate sharp changes in local solid height profile.

Figure 3                                                                                                          Figure 4

☞  Electrons Confined in Two Dimensions:  Metal-Insulator Transition in Two Dimensional Electrons, Strongly correlated electrons

figure 1

figure 2 figure 3

Two dimensional conducting systems are of great importance to both technology and condensed matter physics.  There remain many fundamental questions on the behavior of two dimensional systems at low temperatures.   We study 2D electrons in Si-MOSFET inversion layer by making transport measurements (see fig. 1) using crossed field techniques in magnetic fields up to 5 tesla and down to 20 mK.  We investigate the interplay between electron-electron interaction and disorder.   From extensive measurements of Shubnikov-de Haas oscillations (fig. 2), renormalized magnetic susceptibility was measured as a function of density. The magnetic susceptibility increases rapidly near the metal-insulator crossover density.  See fig. 3.

☞  Single Bubble Sonoluminescence: Effects of mixing noble gases and low temperature

Shown at right is a glass flast filled with partially degassed water.  An acoustic resonance in the water is excited by piezoelectric transducers (not visible) at high amplitude at about 30 kHz and pressure amplitude of 1.1 bar.  A tiny gas bubble is trapped near the center of the flask and is emitting bluish light.  (Magnification may be helpful in making it visible on monitor.  Ignore the yellow/orange color patches which is caused by back light with incandescent light to make the outline of the flask visible.)  This unusual phenomenon creating flashes of light from an acoustically driven bubble, known as single bubble sonoluminescence, is not yet totally understood.  At Rutgers, we are studying the changes in light emission properties by mixing different combinations of noble gases and decreasing the temperature of water to near 0 ˚C.

 ☞  Quantum Critical Point: Nuclear Quadruple Resonance Studies of CeCu_5.9Au_0.1

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