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Topological insulators give rise to exquisite electronic properties because of their spin-momentum locked Dirac-cone-like band structure. Recently, it has been suggested that the required opposite parities between valence and conduction band along with strong spin-orbit coupling can be realized in correlated materials. Particularly, SmB6 has been proposed as candidate material for a topological Kondo insulator. Here we observe, by utilizing scanning tunnelling microscopy and spectroscopy down to 0.35 K, several states within the hybridization gap of about ±20 meV on well characterized (001) surfaces of SmB6. The spectroscopic response to impurities and magnetic fields allows to distinguish between dominating bulk and surface contributions to these states. The surface contributions develop particularly strongly below about 7 K, which can be understood in terms of a suppressed Kondo effect at the surface. Our high-resolution data provide insight into the electronic structure of SmB6, which reconciles many current discrepancies on this compound.

For any thorough investigation of complex physical properties, as encountered in strongly correlated electron systems, not only single crystals of highest quality but also a detailed knowledge of the structural properties of the material are pivotal prerequisites. Here, we combine physical and chemical investigations on the prototypical heavy fermion superconductors CeIrIn5 and CeCoIn5 on atomic and macroscopic length scale to gain insight into their precise structural properties. Our approach spans from enhanced resolution X-ray diffraction experiments to atomic resolution by means of scanning tunneling microscopy (STM) and reveal a certain type of local features (coexistence of minority and majority structural patterns) in the tetragonal HoCoGa 5-type structure of both compounds.

The unconventional nature of the quantum criticality in YbRh 2Si 2 is highlighted on the basis of magnetoresistivity and susceptibility measurements. Results obtained under chemical pressure realized by isoelectronic substitution on the rhodium site are presented. These results illustrate the position of the T*-line associated with a breakdown of the Kondo effect near the antiferromagnetic instability in the low-temperature phase diagram. Whereas at zero temperature the Kondo breakdown and the antiferromagnetic quantum critical point coincide in the proximity of the stoichiometric compound, they are seen to be detached under chemical pressure: For positive chemical pressure the magnetically ordered phase extends beyond the T*(B)-line. For sufficiently high negative pressure the T*(B)-line is separated from the magnetically ordered phase. From our detailed analysis we infer that the coincidence is retained at small iridium concentrations, i.e., at small negative chemical pressure. We outline further measurements which may help to clarify the detailed behavior of the two instabilities. © 2010 Springer Science+Business Media, LLC.

We report experimental and theoretical investigations of the Hall effect in YbRh2 Si2 and its reference compounds LuRh2 Si2 and YbIr2 Si2. Based on band-structure calculations we identify two bands dominating the Hall coefficient in all these compounds. For the case of LuRh2 Si2 -the nonmagnetic reference compound of YbRh2 Si2 -the temperature dependence of the Hall coefficient is described quantitatively to arise from two holelike bands. For YbIr2 Si2 and YbRh2 Si2, renormalized band calculations yield two bands of opposite character. In YbRh2 Si2 these two bands almost compensate each other. We present strong indications that sample dependences of the low-temperature Hall coefficient observed for YbRh2 Si2 arise from slight variations in the relative scattering rates of the two bands. Minute changes in the composition appear to be the origin. The results of our band-structure calculations reveal that a transition of the 4f electrons from localized to itinerant leads to a decrease in the Hall coefficient. © 2010 The American Physical Society.

The apparently inimical relationship between magnetism and superconductivity has come under increasing scrutiny in a wide range of material classes, where the free energy landscape conspires to bring them in close proximity to each other. Particularly enigmatic is the case when these phases microscopically interpenetrate, though the manner in which this can be accomplished remains to be fully comprehended. Here, we present combined measurements of elastic neutron scattering, magnetotransport, and heat capacity on a prototypical heavy fermion system, in which antiferromagnetism and superconductivity are observed. Monitoring the response of these states to the presence of the other, as well as to external thermal and magnetic perturbations, points to the possibility that they emerge from different parts of the Fermi surface. Therefore, a single 4f state could be both localized and itinerant, thus accounting for the coexistence of magnetism and superconductivity.

High-quality single crystals of the heavy fermion superconductors CeCoIn5 and CeIrIn5 have been studied by means of low-temperature scanning tunneling microscopy. Methods were established to facilitate in situ sample cleaving. Spectroscopy in CeCoIn5 reveals a gap which persists to above Tc, possibly evidencing a precursor state to superconductivity. Atomically resolved topographs show a rearrangement of the atoms at the crystal surface. This modification at the surface might influence the surface properties as detected by tunneling spectroscopy. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The Hall effect of LuRh2Si2 - the non-magnetic homologue of the heavy-fermion material YbRh2Si2 - is studied with two different setups: in the conventional single-field geometry, the field dependence is analyzed in terms of the differential Hall coefficient. Beyond that, the recently developed crossed-field experiment allows to examine the linear-response Hall coefficient as a function of magnetic field. The results reveal the expected analogy between both experiments which corroborates the equivalent findings in YbRh2Si2. This emphasizes the feasibility to investigate field-induced quantum critical points with both methods. Setup of the crossed-field Hall-effect experiment allows to examine the linear-response (with respect to the magnetic field B1) Hall coefficient as a function of tuning field B2. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We present measurements of the low temperature Hall effect and magnetoresistance in the heavy fermion superconductor CeCo(In 0.925Cd0.075)5 as a function of applied magnetic field. The Hall resistivity ρxy is observed to be negative indicating electron dominated transport. Features corresponding to the possible destabilization of the antiferromagnetic ground state are observed. Magnetic field sweeps show that the resistivity ρxx(H) is hysteretic in this regime, indicating that these transitions could be first order in nature. © 2009 IOP Publishing Ltd.

We present Hall effect and magnetoresistance measurements in the heavy fermion superconductor CeIrIn(5). At low temperature, a Kondo coherent state is established. Deviations from Kohler's rule and a quadratic temperature dependence of the cotangent of the Hall angle are reminiscent of properties observed in the high-temperature superconducting cuprates. A striking observation pertains to the presence of a precursor state--characterized by a change in the Hall mobility--that precedes the superconductivity in this material, in similarity to the pseudogap in the cuprate superconductors.

We report Hall Effect measurements under pressure in the normal state of CeCoIn5, from 60 to 355 mK and for fields exceeding the superconducting upper critical field Hc 2, with the field oriented parallel to [0 0 1]. At low pressures, the field dependence of the Hall coefficient exhibits a scaling consistent with the one reported in the normal state at higher temperatures, but at odds with the Δ H / T scaling expected near a field tuned quantum critical point. The breakdown of this scaling at higher pressures, concomitant with the suppression of spin fluctuations, suggests that it is a hallmark of the spin fluctuations.

We report on Hall effect measurements on YbRh2Si2 single crystals with different residual resistivity and on LuRh2Si2 single crystal. The temperature dependence of the linear-response Hall coefficient of YbRh2Si2 is described by the anomalous Hall effect and the normal contribution incorporating multi-band effects. Sample dependencies at low T are found and explained by slight changes in the charge-carrier concentrations. © 2007 Elsevier B.V. All rights reserved.

We conducted Hall effect measurements on the heavy-fermion superconductor CeCoIn5 for temperatures 0.05-5 K and for pressures up to 1.2 GPa. A scaling of the magnetic field H is introduced for the differential Hall coefficient, RHd = ∂ ρxy (T, H) / ∂ H resulting in a single generic curve for RHd (H) curves obtained at different T. We argue that the peak feature apparent in this generic curve corresponds to the crossover from non-Fermi liquid to Landau Fermi liquid behavior. © 2007 Elsevier B.V. All rights reserved.

We present highly sensitive Hall effect measurements of the heavy fermion compound CeCoIn5 down to temperatures of 55 mK. A pronounced dip in the differential Hall coefficient | partial differential rho(xy)/ partial differential H| at low temperature and above the upper critical field of superconductivity, H(c2), is attributed to critical spin fluctuations associated with the departure from Landau Fermi liquid behavior. This identification is strongly supported by a systematic suppression of this feature at elevated pressures. The resulting crossover line in the field-temperature phase diagram favors a field induced quantum critical point at mu(0)H(qc) approximately 4.1 T below H(c2)(T=0) suggesting related, yet separate, critical fields.

Hall effect measurements have been conducted on high-quality single crystals of the heavy-fermion superconductor CeCoIn5. The anomalous Hall contribution is negligible in the investigated temperature range from 0.05 to 5 K. The measured Hall resistivities ρxy show a noticeable change in slope between the low-field (initial Hall coefficient) and the high-field region. In the superconducting regime, T < 2.3 K, Hall measurements are restricted to high magnetic fields H > Hc 2, the upper critical field of superconductivity. The high-field Hall coefficient is almost constant for temperatures down to 250 mK. At T {less-than or slanted equal to} 250 mK, an additional change in curvature of ρxy vs. H is observed. © 2006 Elsevier B.V. All rights reserved.