Atomic Diffusion in the Surface State of Mott Insulator NiS2

We present resistivity measurements of Mott insulator NiS2 single crystals after heat treatment. We find a strong increase of the low temperature resistivity that relaxes back towards the pristine behaviour over several days with a time constant of 45 h at room temperature. The low temperature resistivity has previously been shown to be dominated by surface conduction (T. Thio and J. Bennett, PRB 50 10574 1994). Consequently, the changes induced by heat treatment are attributed to changes to surface states. Our results suggest the creation of vacancies in the surface that re refilled from the bulk via atomic diffusion. We estimate a diffusion constant of $D \approx 10^{-10}$ m$^2$/s at room temperature. We identify sulphur vacancies as the most likely to form via oxidisation of sulphur forming volatile SO2 during heat treatment. Our results point towards these sulphur vacancies to be the source of surface state localisation in NiS2.Comment: 6 pages, 6 figures. Journal of Magnetism and Magnetic Materials (2015

Quantum criticality in Yb(Rh0.97Co0.03)2Si2 probed by low-temperature resistivity

Quantum criticality in Yb(Rh0.97Co0.03)2Si2 is investigated by means of resistivity and magnetoresistance. The partial substitution of Co leads to a stabilization of the magnetism as expected according to the application of chemical pressure for Yb systems. However, the signature of the Kondo-breakdown remains at the same position in the temperature-magnetic field phase diagram compared to stoichiometric YbRh2Si2. As a consequence, the Kondo-breakdown is situated within the antiferromagnetic phase. These results fit well within the global phase diagram under chemical pressure [1].Comment: 4 pages, 4 figures, submitted to ICM/SCES200

Break up of heavy fermions at an antiferromagnetic instability

We present results of high-resolution, low-temperature measurements of the Hall coefficient, thermopower, and specific heat on stoichiometric YbRh2Si2. They support earlier conclusions of an electronic (Kondo-breakdown) quantum critical point concurring with a field induced antiferromagnetic one. We also discuss the detachment of the two instabilities under chemical pressure. Volume compression/expansion (via substituting Rh by Co/Ir) results in a stabilization/weakening of magnetic order. Moderate Ir substitution leads to a non-Fermi-liquid phase, in which the magnetic moments are neither ordered nor screened by the Kondo effect. The so-derived zero-temperature global phase diagram promises future studies to explore the nature of the Kondo breakdown quantum critical point without any interfering magnetism.Comment: minor changes, accepted for publication in JPS

Anisotropic Zeeman Splitting in YbNi4P2

The electronic structure of heavy-fermion materials is highly renormalised at low temperatures with localised moments contributing to the electronic excitation spectrum via the Kondo effect. Thus, heavy-fermion materials are very susceptible to Lifshitz transitions due to the small effective Fermi energy arising on parts of the renormalised Fermi surface. Here, we study Lifshitz transitions that have been discovered in YbNi4P2 in high magnetic fields. We measure the angular dependence of the critical fields necessary to induce a number of Lifshitz transitions and find it to follow a simple Zeeman-shift model with anisotropic g-factor. This highlights the coherent nature of the heavy quasiparticles forming a renormalised Fermi surface. We extract information on the orientation of the Fermi surface parts giving rise to the Lifshitz transitions and we determine the anisotropy of the effective g-factor to be $\eta \approx 3.8$ in good agreement with the crystal field scheme of YbNi4P2.Comment: 10 pages, 5 figures, prepared for resubmission to SciPos

Incoherent multi-gap optical solitons in nonlinear photonic lattices

We demonstrate numerically that partially incoherent light can be trapped in the spectral band gaps of a photonic lattice, creating partially incoherent multi-component spatial optical solitons in a self-defocusing nonlinear periodic medium. We find numerically such incoherent multi-gap optical solitons and discuss how to generate them in experiment by interfering incoherent light beams at the input of a nonlinear periodic medium.Comment: 9 pages, 5 figure

Hall effect in heavy-fermion metals

The heavy fermion systems present a unique platform in which strong electronic correlations give rise to a host of novel, and often competing, electronic and magnetic ground states. Amongst a number of potential experimental tools at our disposal, measurements of the Hall effect have emerged as a particularly important one in discerning the nature and evolution of the Fermi surfaces of these enigmatic metals. In this article, we present a comprehensive review of Hall effect measurements in the heavy-fermion materials, and examine the success it has had in contributing to our current understanding of strongly correlated matter. Particular emphasis is placed on its utility in the investigation of quantum critical phenomena which are thought to drive many of the exotic electronic ground states in these systems. This is achieved by the description of measurements of the Hall effect across the putative zero-temperature instability in the archetypal heavy-fermion metal YbRh$_2$Si$_2$. Using the Ce$M$In$_5$ (with $M =$ Co, Ir) family of systems as a paradigm, the influence of (antiferro-)magnetic fluctuations on the Hall effect is also illustrated. This is compared to prior Hall effect measurements in the cuprates and other strongly correlated systems to emphasize on the generality of the unusual magnetotransport in materials with non-Fermi liquid behavior.Comment: manuscript accepted in Adv. Phy

El carnaval rural en el río Magdalena

El carnaval, tradición de origen occidental que evoca antiguos ritos de propiciación a dioses griegos y romanos anteriores al cristianismo, llegó al nuevo mundo con los europeos. Su asentamiento, su significado social y simbólico y las transformaciones que ha experimentado en las diversas comunidades rurales y urbanas de América Latina se han convertido en temas de estudios sociopolíticos, antropológicos y estéticos

Electrical resistivity ofYb(Rh1-xCox)2Si2 single crystals at low temperatures

We report low-temperature measurements of the electrical resistivity of Yb(Rh1-xCox)2Si2 single crystals with 0 <= x <= 0.12. The isoelectronic substitution of Co on the Rh site leads to a decrease of the unit cell volume which stabilizes the antiferromagnetism. Consequently, the antiferromagnetic transition temperature increases upon Co substitution. For x = 0.07 Co content a subsequent low-temperature transition is observed in agreement with susceptibility measurements and results on YbRh2Si2 under hydrostatic pressure. Above the Neel transition the resistivity follows a non-Fermi liquid behavior similar to that of YbRh2Si2.Comment: 4 pages, submitted to SCES0

Lifshitz transitions and quasiparticle de-renormalization in YbRh2_2Si2_2

We study the effect of magnetic fields up to 15 T on the heavy fermion state of YbRh$_2$Si$_2$ via Hall effect and magnetoresistance measurements down to 50 mK. Our data show anomalies at three different characteristic fields. We compare our data to renormalized band structure calculations through which we identify Lifshitz transitions associated with the heavy fermion bands. The Hall measurements indicate that the de-renormalization of the quasiparticles, {\it i.e} the destruction of the local Kondo singlets, occurs smoothly while the Lifshitz transitions occur within rather confined regions of the magnetic field.Comment: 7 pages, 5 figure

Comment on "Zeeman-Driven Lifshitz Transition: A Model for the Experimentally Observed Fermi-Surface Reconstruction in YbRh2Si2"

In Phys. Rev. Lett. 106, 137002 (2011), A. Hackl and M. Vojta have proposed to explain the quantum critical behavior of YbRh2Si2 in terms of a Zeeman-induced Lifshitz transition of an electronic band whose width is about 6 orders of magnitude smaller than that of conventional metals. Here, we note that the ultra-narrowness of the proposed band, as well as the proposed scenario per se, lead to properties which are qualitatively inconsistent with the salient features observed in YbRh2Si2 near its quantum critical point.Comment: 3 page