N. Marcano

Weak-antilocalization signatures in the magnetotransport properties of individual electrodeposited Bi Nanowires

We study the electrical resistivity of individual Bi nanowires of diameter 100 nm fabricated by electrodeposition using a four-probe method in the temperature range 5–300 K with magnetic fields up to 90 kOe. Low-resistance Ohmic contacts to individual Bi nanowires are achieved using a focused ion beam to deposit W-based nanocontacts. Magnetoresistance measurements show evidence for weak antilocalization at temperatures below 10 K, with a phase-breaking length of 100 nm.

Fractalization drives crystalline states in a frustrated spin system

The fractalized Hofstadter butterfly energy spectrum predicted for magnetically confined fermions diffracted by a crystal lattice has remained beyond the reach of laboratory-accessible magnetic fields. We find the geometrically frustrated spin system SrCu2(BO3)2 to provide a sterling demonstration of a system in which bosons confined by a magnetic and lattice potential mimic the behavior of fermions in the extreme quantum limit, giving rise to a sequence of plateaus at all magnetization mz/msat = 1/q ratios 9 ≥ q ≥ 2 and p/q = 2/9 (msat is the saturation magnetization) in magnetic fields up to 85 T and temperatures down to 29 mK, within the sequence of previously identified plateaus at 1/8, 1/4, and 1/3 of the saturated magnetization. We identify this hierarchy of plateaus as a consequence of confined bosons in SrCu2(BO3)2 mimicking the high magnetic field fractalization predicted by the Hofstadter butterfly for fermionic systems. Such an experimental realization of the Hofstadter problem for interacting fermions has not been previously achieved in real materials, given the unachievably high magnetic flux densities or large lattice periods required. By a theoretical treatment that includes short-range repulsion in the Hofstadter treatment, stripe-like spin density-modulated phases are revealed in SrCu2(BO3)2 as emergent from a fluidic fractal spectrum.

Quantitative analysis of the weak anti-localization effect in ultrathin bismuth films

Magnetic-field dependence of conductivity in ultrathin Bi films is measured in applied magnetic fields up to 9 T, in both directions, perpendicular and parallel to the film plane, at temperatures down to 0.4 K, and analyzed in terms of the weak anti-localization theory in two-dimensional systems. With the reduction of film thickness, the classical magnetoresistance effect is completely suppressed, and only the weak anti-localization effect is observed. The parameters extracted from the analysis allow the study of the contribution of the different scattering mechanisms to the electronic transport properties in ultrathin Bi films. In particular, the thickness-independent spin-orbit scattering length indicates that the spin-orbit split surface states dominate the transport in the ultrathin-film limit.

Novel metallic states at low temperatures

We present an overview of unconventional metallic states arising close to magnetic quantum critical points with a focus on d-electron systems. The applicability and potential breakdowns of traditional self-consistent field theories of such materials are discussed as well as related phenomena in other systems.

Theoretical models for a complex magnetic system: The case of CeNi1-xCux

Special features in the phase diagram and in the low temperature hysteresis cycles have been experimentally observed in the complex CeNi1-xCux system. We present also here theoretical approaches, based firstly on a Kondo lattice model which describes the coexistence between a spin glass or a cluster glass state, the Kondo regime and the ferromagnetic ordering. The second model is a Monte Carlo simulation on a 3D lattice with clusters and random anisotropy and reproduces the existence of steps in the magnetizations cycles at very low temperatures. The theoretical results are compared with the experimental data of the very complex magnetic behaviour of CeNi1−xCux alloys. In particular, we can account for the existence of a cluster spin glass state which changes continuously into an inhomogeneous ferromagnetic phase at very low temperatures.

Transport and Thermodynamic Evidence for a Marginal Fermi Liquid State in ZrZn2

characterize the non-Fermi liquid state of the itinerant ferromagnet ZrZn2. We observe a T 5/3 temperature dependence of the electrical resistivity at zero field, which becomes T 2 like in an applied field of 9 T. In zero field we also measured the thermal conductivity, and we see a novel linear in T dependence of the difference between the thermal and electrical resistivities. Heat capacity measurements, also at zero field, reveal an upturn in the electronic contribution at low temperatures when the phonon term is subtracted. Taken together, we argue that these properties are consistent with a marginal Fermi liquid state which is predicted by a mean-field model of enhanced spin fluctuations on the border of ferromagnetism in three dimensions. We compare our data to quantitative predictions and establish this model as a compelling theoretical framework for understanding ZrZn2.

Complex magnetic states of the heavy fermion compound CeGe

The intermetallic compound CeGe exhibits unusual magnetic behavior owing to the interplay between Kondo and antiferromagnetic coupling. This system is interesting because the Kondo temperature is close to the Neel temperature, so there is a close competition between the low-temperature interactions, which can be tuned by varying external parameters such as pressure and applied magnetic field. Interestingly, magnetization measurements up to 12 kbar reveal that the Neel temperature is not affected by pressure. Measurements of the electrical resistivity show, however, that the sharp upturn below TN is sensitive to pressures up to 15 kbar. This suggests that pressure may change the complex antiferromagnetic spin structure. The validity of an explanation based on the magnetic superzones seen in the rare earths is discussed here.