The 0.7 (2e 2 /h) conductance anomaly is studied in strongly confined, etched GaAs/GaAlAs quantum point contacts, by measuring the differential conductance as a function of source-drain and gate bias as well as a function of temperature. We investigate in detail how, for a given gate voltage, the differential conductance depends on the finite bias voltage and find a so-called self-gating effect, which we correct for. The 0.7 anomaly at zero bias is found to evolve smoothly into a conductance plateau at 0.85 (2e 2 /h) at finite bias. Varying the gate voltage the transition between the 1.0 and the 0.85 (2e 2 /h) plateaus occurs for definite bias voltages, which defines a gate voltage dependent energy difference ∆. This energy difference is compared with the activation temperature Ta extracted from the experimentally observed activated behavior of the 0.7 anomaly at low bias. We find ∆ = kBTa which lends support to the idea that the conductance anomaly is due to transmission through two conduction channels, of which the one with its subband edge ∆ below the chemical potential becomes thermally depopulated as the temperature is increased.PACS 73.61.-r, 73.23.-b
The excitation spectrum of a model magnetic system, LiHoF4, was studied with the use of neutron spectroscopy as the system was tuned to its quantum critical point by an applied magnetic field. The electronic mode softening expected for a quantum phase transition was forestalled by hyperfine coupling to the nuclear spins. We found that interactions with the nuclear spin bath controlled the length scale over which the excitations could be entangled. This generic result places a limit on our ability to observe intrinsic electronic quantum criticality.
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