Condensed Matter Physics Seminar

2 p.m., Thursday, February 17, 2005
Room 1201, Physics Building

 What’s new in quantum Hall effect?

Wei Pan

(Sandia National Laboratories)

Abstract:  In the first part of talk, I will discuss recent results in the second Landau level (LL). The data was obtained at very low temperatures (sample temperatures as low as 9 mK) for a very high mobility 2D electron system. We observe well-quantized FQHE states at the LL filling ν= 2+1/2, 2+1/3, and ν= 2+2/3 in coexistence with the reentrant integer quantum Hall states, and a new FQHE state at ν= 2+2/5. The origin of the 2+2/5 state is not clear and the numerical results strongly suggest that it is not a conventional FQHE state but a parafermionic state. There is also evidence for a second even-denominator FQHE state at ν= 2+3/8.

In the second part, I will discuss the resistivity rule, i.e., Rxx ~ B x dRxy/dB. This empirical rule was observed twenty years ago. Yet, today we have only a relatively complex model that addresses the origin of this rule. What’s more interesting is that in the second LL, since Rxy switches back and forth between FQHE and IQHE values several times as the filling factor varies from ν =4 to ν = 2, this non-monotonic Rxy leads to regions of negative B x dRxy/dB, which cannot find an equivalent in Rxx, a positive definite, thus apparently violating the empirical rule. However, in a more detailed examination, we found, surprisingly, a new resistivity rule in the second LL. The regular, positive parts of B x dRxy/dB are well reflected in Rxx(+B), whereas the irregular negative going sections of B x dRxy/dB closely match the inverted Rxx(-B) trace, where -B refers to the opposite magnetic field direction of +B. It is unclear whether our observations of an expanded resistivity rule reinforces or refutes the present model of its origin.

Host:  Das Sarma
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