We study theoretically and experimentally the frequency and temperature dependence of resistivity noise in semiconductor heterostructures delta-doped by Mn. The resistivity noise is observed to be non-monotonous as a function of frequency. As a funct
ion of temperature, the noise increases by two orders of magnitude for a resistivity increase of about 50%. We study two possible sources of resistivity noise -- dynamic spin fluctuations and charge fluctuations, and find that dynamic spin fluctuations are more relevant for the observed noise data. The frequency and temperature dependence of resistivity noise provide important information on the nature of the magnetic interactions. In particular, we show how noise measurements can help resolve a long standing debate on whether the Mn-doped GaAs is an p-d Zener/RKKY or double exchange ferromagnet. Our analysis includes the effect of different kinds of disorder such as spin-glass type of interactions and a site-dilution type of disorder. We find that the resistivity noise in these structures is well described by a disordered RKKY ferromagnet model dynamics with a conserved order parameter.
We propose a probe based on nuclear relaxation and Knight shift measurements for the Kondo scenario for the 0.7 feature in semiconductor quantum point contact (QPC) devices. We show that the presence of a bound electron in the QPC would lead to a muc
h higher rate of nuclear relaxation compared to nuclear relaxation through exchange of spin with conduction electrons. Furthermore, we show that the temperature dependence of this nuclear relaxation is very non-monotonic as opposed to the linear-T relaxation from coupling with conduction electrons. We present a qualitative analysis for the additional relaxation due to nuclear spin diffusion (NSD) and study the extent to which NSD affects the range of validity of our method. The conclusion is that nuclear relaxation, in combination with Knight shift measurements, can be used to verify whether the 0.7 feature is indeed due to the presence of a bound electron in the QPC.
We propose a new method for dynamic nuclear polarisation in a quasi one-dimensional quantum wire utilising the spin-orbit interaction, the hyperfine interaction, and a finite source-drain potential difference. In contrast with current methods, our sc
heme does not rely on external magnetic or optical sources which makes independent control of closely placed devices much more feasible. Using this method, a significant polarisation of a few per cent is possible in currently available InAs wires which may be detected by conductance measurements. This may prove useful for nuclear-magnetic-resonance studies in nanoscale systems as well as in spin-based devices where external magnetic and optical sources will not be suitable.
We describe how a local non-equilibrium nuclear polarisation can be generated and detected by electrical means in a semiconductor quantum point contact device. We show that measurements of the nuclear spin relaxation rate will provide clear signature
s of the interaction mechanism underlying the 0.7 conductance anomaly. Our analysis illustrates how nuclear magnetic resonance methods, which are used extensively to study strongly-correlated electron phases in bulk materials, can be made to play a similarly important role in nanoscale devices.
Physicists have long debated whether the hidden order in URu2Si2 is itinerant or localized, and it remains inaccessible to direct external probes. Recent observation of an overdamped collective mode in this material (C. Weibe et al, Nature Physics 3,
96-100 (2007)), appears to resolve this outstanding issue.
