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SUMMARY:Microscopic Origin of the 0.7-Anomaly in Quantum Point Contacts - 
 Professor Jan von Delft\, Ludwig-Maximilians-Universität\, München
DTSTART:20141022T131500Z
DTEND:20141022T141500Z
UID:TALK55549@talks.cam.ac.uk
CONTACT:Teri Bartlett
DESCRIPTION:The conductance of a quantum point contact exhibits an unexpec
 ted shoulder at ~0.7 (2 e^2/h)\, known as the "0.7-anomaly"\, whose origin
  is still subject to debate. Proposed scenarios for explaining it have evo
 ked spontaneous spin polarization\, ferromagnetic spin coupling\, the form
 ation of a quasi-bound state leading to the Kondo effect\, Wigner crystall
 isation\, various treatments of inelastic scattering\, and a smeared van H
 ove peak in the local density of states.\n\nIn my talk\, I will argue that
  the 0.7-anomaly arises from "slow spin fluctuations" in the quantum point
  contact. The microscopic origin of these slow (ferromagnetic) spin fluctu
 ations is the presence of a smeared van Hove peak in the local density of 
 states at the bottom of the lowest one-dimensional subband of the point co
 ntact. This peak in the local density of states\, which reflects the fact 
 that electrons are being slowed down while they cross the 1D barrier const
 ituting the QPC\, amplifies interaction effects and enhances the magnetic 
 spin susceptibility and inelastic scattering rate. I will present theoreti
 cal calculations and experimental results that show good qualitative agree
 ment for the dependence of the conductance on gate voltage and magnetic fi
 eld\, including the behavior of the effective low-energy scale that govern
 s the strength of the magnetic response.\n\nFinally\, I will argue that "s
 low spin fluctuations" can be viewed as the common ground shared by severa
 l of the seemingly contradictory scenarios for explaining the 0.7-anomaly 
 that are currently on the market. In particular\, slow spin fluctuations a
 rise also in the scenarios evoking a quasi-bound state\, ferromagnetic spi
 n coupling and Wigner crystallization. Common ground can also be found wit
 h the spin polarization scenario if one is willing to reinterpret "spontan
 eous spin polarization" to mean a slowly fluctuating ferromagnetic spin co
 nfiguration that looks static on short time scales\, but averages to zero 
 over longer times.\n
LOCATION:Mott Seminar Room\, Cavendish Laboratory\, Department of Physics
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