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The interplay between strong electron interactions and band topology is a new frontier in the search for exotic quantum phases. The Kondo insulator SmB6has emerged as a promising platform because its correlation-driven bulk gap is predicted to host topological surface modes entangled with felectrons, spawning heavy Dirac fermions1–4. Unlike the conventional surface states of non-interacting topological insulators, heavy Dirac fermions are expected to harbour spontaneously generated quantum anomalous Hall states5, non-Abelian quantum statistics6,7, fractionalization8and topological order6–8. However, the small energy scales required to probe heavy Dirac fermions have complicated their experimental realization. Here we use high-energy-resolution spectroscopic imaging in real and momentum space on SmB6. On cooling below 35?K, we observe the opening of an insulating gap that expands to 14?meV at 2?K. Within the gap, we image the formation of linearly dispersing surface states with effective masses reaching 410?±?20?me(where meis the mass of the electron). Our results demonstrate the presence of correlation-driven heavy surface states in SmB6, in agreement with theoretical predictions1–4. Their high effective mass translates to a large density of states near zero energy, which magnifies their susceptibility to the anticipated novel orders and their potential utility.

SmB6is a strongly correlated mixed-valence Kondo insulator with a newly discovered surface state, proposed to be of non-trivial topological origin. However, the surface state dominates electrical conduction only below T∗≈ 4 K (ref. ), limiting its scientific investigation and device application. Here, we report the enhancement of T∗in SmB6under the application of tensile strain. With 0.7% tensile strain we report surface-dominated conduction at up to a temperature of 240 K, persisting even after the strain has been removed. This can be explained in the framework of strain-tuned temporal and spatial fluctuations of f-electron configurations, which might be generally applied to other mixed-valence materials. We note that this amount of strain can be induced in epitaxial SmB6films via substrate in potential device applications.

We report the polarized optical reflectance of quasi-two-dimensional η-Mo4O11in a perpendicular magnetic field up to 30 T at 4.2 K. The in-field measurements are done on the conducting bc plane covering the 6000-26000 cm-1energy range. The absolute reflectance shows a free carrier response in the near infrared and two electronic transitions in the optical regime : an intraband transition (d → d) near 12000 cm-1and a charge transfer (oxygen p → molybdenum d) near 30000 cm-1. In the magnetic field, no change in the normalized reflectance is observed in the intraband transition range through the 20 T Landau transition, whereas a notable deviation from unity is observed on the low-energy side of the charge transfer transition along both b and c polarizations. We discuss the results in terms of the effect of the magnetic field on the electronic structure and quantum limit response of η-Mo4O11.

We report on a 170Yb Mössabauer spectroscopy investigation of the low temperature magnetic properties of the cubic heavy electron material YbAuCu4. In the antiferromagnetically ordered phase (TN=1 K), we measure a 20 % reduction of the Yb+3saturated spontaneous moment with respect to that of the crystal field Γ7ground state and we evidence a sizeable anisotropy of the interionic magnetic interaction. We interpret these features in terms of 4f electron - conduction electron hybridisation, which accounts for the heavy electron properties. The energy scale kBT0of hybridisation is estimated to be 0.3 K. The Mössbauer spectra in the ordered and paramagnetic phases show inhomogeneous line broadenings which are interpreted as arising from a distribution of local strains along the 〈111〉 crystal axes, of mean magneto-elastic energy 0.2 – 0.4 K.

We report a detailed characterization of BaFe2As2single crystals grown by a metallic In-flux technique, an alternative to well-established growth routes using FeAs self- or Sn-flux. Electrical resistivity, magnetic susceptibility, nuclear magnetic resonance, and energy dispersive spectroscopy measurements showed no evidence of flux incorporation. More importantly, our results demonstrate that BaFe2As2single crystals grown by In-flux have extremely high quality. To explore the efficiency of the In-flux growth method, we have also prepared nearly optimally doped superconducting samples of Ba(Fe1 −xMx)2As2(M= Co, Cu, Ni, and Ru). Among other interesting features, this alternative chemical substitution method has led to enhancement of the maximum Tcfor most dopings.

Local environment of Gd3+ and Eu2+ 4f7 ions, S = 7/2, in Ca1–xEuxB6 (0.0001 ≤ x ≤ 0.30) and CaxGdxB6 (0.0001 ≤ x ≤ 0.01) is investigated by means of electron spin resonance (ESR). For x ≤ 0.001 the spectra show resolved fine structures due to the cubic crystal electric field and, in the case of Eu, the hyperfine structure due to the nuclear hyperfine field is also observed. The resonances have Lorentzian line shape, indicating insulating host for the Gd3+ and Eu2+ ions. As x increases, the ESR lines broaden due to local distortions caused by the Ca/Gd,Eu ions substitution. For Gd (x ≈ 0.001) and Eu (x ≈ 0.02), the spectra present superposition of Lorentzian and Dysonian resonances, suggesting a coexistence of insulating and metallic hosts for the Gd3+ and Eu2+ ions. The Gd3+ and Eu2+ fine structures are still observable up to x ≈ 0.003 for Gd and x ≈ 0.15 for Eu. For larger values of x the fine and hyperfine structures are no longer observed, the line width increases, and the line shape becomes pure Dysonian anticipating the metallic and semimetallic character of GdB6 and EuB6, respectively. These results clearly show that in the low concentration regime the Ca1–xRxB6 (R = Gd, Eu) systems are intrinsically inhomogeneous. No evidence of weak ferromagnetism (WF) was found in the ESR spectra of either metallic or insulating phases of these compounds, suggesting that, if WF is present in these materials, the Gd3+ and Eu2+ 4f7-electrons are shielded from the WF field. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Single crystals of Ce5Pb3O have been synthesized by flux growth. This compound adopts the tetragonal I4/mcm space group with lattice parameters of a = b = 8.5870(2) Å and c = 14.3870(5) Å, Z = 4. The crystal structure of Ce5Pb3O consists of a network of interpenetrating corner-sharing Ce-centered Pb6 octahedra and O-centered Ce4 tetrahedra. The tetrahedra are linked to each other by Pb trigonal antiprisms. Magnetization measurements show that Ce5Pb3O orders ferrimagnetically at Tc = 46 K, the second-highest known ordering temperature for any Ce-based intermetallic compound.

CeRhIn5is known to show an unusual transition at a critical pressure of ∼15 kbar. Specific-heat data show a gradual change in the zero-field “magnetic” specific-heat anomaly from one typical of antiferromagnetic ordering at ambient pressure to one more characteristic of a Kondo singlet ground state at 21 kbar. However, at 15 kbar there is a discontinuous change from an antiferromagnetic ground state to a superconducting ground state, and evidence of a weak thermodynamic first-order transition. Above the critical pressure, the low-energy excitations are characteristic of superconductivity with line nodes in the energy gap, and, at intermediate pressures, of extended gaplessness.

Abstract:: EuB6, a ferromagnet with a Curie temperature of T C ∼ 15 K, exhibits very large magnetoresistive effects at temperatures both above and below T C . We present our magneto-optical investigations, including measurements of the reflectivity and Kerr rotation, covering the spectral range from the infrared up to the ultraviolet, as a function of temperature between 1.5 and 20 K and in external magnetic fields from 0 to 10 T. The Kerr rotation at high fields and low temperatures is enormous. The spectra reflect aspects of the large magnetoresistance and are shown to discriminate between the spectroscopic response of localized and itinerant electronic states in the ferromagnetic state.

The structures of the new heavy fermion materials CeIrIn5and CeRhIn5have been determined by single-crystal neutron (R(F)=0.051) and X-ray (R(F)=0.056) diffraction, respectively. Both compounds adopt tetragonal structure, space group P4/mmm(No. 123), Z=1, a=b=4.674(1) Å, and c=7.501(5) Å for CeIrIn5, and a=b=4.656(2) Å, and c=7.542(1) Å for CeRhIn5. The possible presence of antisite disorder, a long-standing question on this type of structure, was excluded by both electron and-neutron or X-ray diffraction. The compounds are built by monolayers of face-sharing distorted cuboctahedra [CeIn3] and monolayers of edge-sharing rectangular parallelepipeds [RhIn2] or [IrIn2], stacked alternatively in the [001] direction. Therefore, they are new members of the inhomogeneous linear homologous series MmTnT′3m+2n. Because of their ordered intergrowth structure, the physical properties of the quasi-two-dimensional heavy electron systems CeIrIn5and CeRhIn5can be directly compared with the corresponding ones of their parent compound, the three-dimensional heavy fermion material CeIn3.

η-Mo4O11 has been of recent interest as both a charge density wave (CDW) and a quantum limit material. Indeed, previous low temperature far-infrared measurements have highlighted the unusual spectroscopic response, while transport studies have identified low-field quantum Hall behavior as well as a semimetal to semiconductor transition above 19 T. In order to explore the low-temperature electrodynamics in this prototype system, we have measured the unpolarized far-infrared response (30-400 cm-1) of η-Mo4O11 in an applied magnetic field (0-17 T). The reflectance ratio of η-Mo4O11 changes systematically at low fields, saturating above 7 T. Here, a small rise in ratio spectra is found near 120 cm-1, whereas the reflectance ratio drops below 70 cm-1. Combining this data with the low-temperature power reflectance, we have used a Kramers-Kronig analysis to calculate the frequency dependent conductivity in the magnetic field. The preliminary data shows that the main effect of the field is related to the CDW2 gap.Work at SUNY-Binghamton was supported by the Division of Materials Research, National Science Foundation under Grant No. DMR-9623221. The measurements at high magnetic field were performed at the NHMFL, in Tallahassee, Florida, which is supported by NSF Cooperative Agreement No. DMR-9527035 and by the State of Florida. We gratefully acknowledge C.C. Homes and A.W. McConnell for generous access to their polarized far-infrared data, and their helpful discussion.

We present results of an electrochemical intercalation study of Zn into β-ZrNCl, β-ZrNBr and β-HfNCl. For β-ZrNCl an intercalation reaction takes place, leading to the final compound Zn0.04ZrNCl with a superconducting transition temperature Tcof 15 K. A similar behaviour is observed for β-ZrNBr where the insertion reaction produces the superconducting compound Zn0.04ZrNBr with a Tcof 14 K, whereas attempts to obtain ZnxHfNCl superconductors applying the same conditions failed. In comparison to β-MNX compounds with monovalent inserted ions, superconductivity in Zn doped β-MNX occurs at much lower levels of electron doping, although the values of Tcare similar.

We report specific heat measurements in a broad temperature range between 0.06 and 20 K on single crystalline CeRu2Si2in magnetic fields applied parallel to the c-direction around the critical field BM, where the metamagnetic transition from the itinerant to the localized state takes place. In the critical field BM∥ c=7.8 T a distinct deviation from the usual Fermi- liquid behavior is found down to our lowest temperature. Below 1.8 K C/T varies as (1−aT), while for 1.8 K

We report far-infrared transmission measurements of @h-Mo"4O"1"1 at temperatures above and below the two charge-density-wave (CDW) transitions at 109 and 35 K. We analyze the exotic splitting patterns of the vibrational features in terms of the nature of the structural distortions. Data on @c-Mo"4O"1"1, is also shown to compare with the @h-phase material. Preliminary far-infrared reflectance spectra of @h-Mo"4O"1"1 taken in the presence of an external magnetic field is also presented.

We have observed a quantum Hall effect in the bulk quasi-two-dimensional conductor @h-Mo"4-O"1"1. The Hall resistance exhibits well defined plateaux, coincident with pronounced minima in the diagonal resistance. We present data for several different samples and contact geometries, and discuss a possible mechanism for the quantum Hall effect in this system. We also discuss the implications of these findings in the light of recent predictions concerning chiral metallic surface states in bulk quantum Hall systems.

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Magnetic properties of the Li‐doped cuprates La2Cu1-xLixO4(where x=0.01,\ 0.05,\ 0.10,\ 0.45, and 0.50) have been studied by μSR. For low Li concentrations (x\Eleq 0.10) we find a rapid suppression of TNas x increases, but little change in the magnitude and temperature dependence of the AFM order parameter. This indicates that Li doping effectively destroys AFM (similar to Sr doping, but different from Zn doping) without strongly affecting the onsite Cu moments and the shape of the spinwave excitation spectrum. For high Li concentrations we find magnetic clusters in about 15% of the sample volume; the remaining volume is non‐magnetic, suggesting possible singlet‐state formation.

Abstract: Zeeman splitting of Mossbauer spectra of57Co doped into Eu2CuO4 and Gd2CuO4 indicates magnetic ordering of Cu moments for temperatures below about 150 K in Eu2CuO4 and below about 200 K in Gd2CuO4.

The relaxation of theμ+SR signal has been investigated for the three compounds YRh4B4, ErRh4B4 and SmRh4B4. In the non-magnetic superconducting (Tc ≈ 11 K) YRh4B4, the data display a Kubo-Toyabe (gaussian) shape for zero (transverse) magnetic fields. ErRh4B4 (superconducting below 8.7 K and ferromagnetic below 1 K) shows a dominant signal with very slow relaxation. In contrast SmRh4B4 (superconducting below 2.7 K and antiferromagnetic-superconducting below 0.87 K) shows a change in relaxation from gaussian above ≈ 60 K to exponential between 1 K and 4 K to two exponential signals (fast and slow) belowTN=0.9 K. In the region 0.9 K

The heavy fermions are a subset of the f-electron intermetallic compounds straddling the magnetic/nonmagnetic boundary. Their low-temperature properties are characterized by an electronic energy scale of order 1-10 K. Among the low-temperature ground states observed in heavy fermion compounds are exotic superconductors and magnets, as well as unusual semiconductors. We review here the current experimental and theoretical understanding of these systems.

The thermal conductivity ? of the heavy-fermion superconductor UPt3 has been measured on two single crystals of different quality as a function of temperature and applied magnetic field for different orientations of the heat current relative to both the field and the crystalline axes. The temperature dependence of ? is far from exponential and nearly the same for both crystals, in which the heat current is, respectively, parallel and perpendicular to the hexagonalc axis, suggesting a gap structure with nodes in the basal plane and normal to it. The field dependence of ? is strongly anisotropic. In the best sample at low fields, where the scattering of heat carriers by vortices is thought to be important, ?(H) depends on the relative orientation of field and current. On the other hand, at high fields nearHc2 (in both samples), ?(H) depends on the relative orientation of field and crystalline axes, reflecting an anisotropy in the gap structure and in the Fermi velocities.

High-field magnetoresistance and Shubnikov-de Haas (SdH) effects were studied in the A15 compound Nb3Sb in fields up to 215 kG. A change in the field dependence of the magnetoresistance for certain field directions above about 150 kG appears to signal the onset of magnetic breakdown. Five sets of SdH frequencies were observed, four of them closely corresponding to de Haas-van Alphen (dHvA) frequencies observed by Arko et al. The fifth frequency had an extremely large amplitude, about 20% of the backgroud magnetoresistance, and it is suggested that this also is due to magnetic breakdown. The results are compared with the ab initio band calculations of van Kessel et al., which can explain many of the observed features of the dHvA and SdH frequencies.

Anisotropic behavior of the normal state thermoelectric power, electrical resistivity, and magnetic susceptibility is reported for the superconducting Co1-xNixZr2 system.

Abstract: The 1 K phase transition in UCu5, which shows up in specific heat data within the antiferromagnetic state belowT N =16 K, was investigated by zero field μSR. The data reveal a pronounced increase in the internal field spread at theμ + sites below 1.2 K while the average fields seem to be unaffected. These results are compatible with the onset of a charge density wave state below 1.2 K.

The electrical resistivity ?(T) and the thermal conductivity ?(T) have been measured for UCu5 in the temperature range between 0.02 and 20 K. Two distinct anomalies in ?(T) are due to previously established phase transitions at approximately 15 and 1 K, respectively. They indicate considerable changes in the electronic structure of this compound, implying sizeable truncations of the Fermi surface with decreasing temperature at both transitions. In almost the entire covered temperature range the thermal conductivity is dominated by phonon contributions. Its temperature dependence is fairly well reproduced by a calculation considering phonon scattering by electrons and by point defects. At very low temperatures, asT approaches 0 K, the Wiedemann-Franz law ?e ?L0T, where ?e is the electronic part of ?(T) andL0 is the Lorenz number, is almost perfectly fulfilled.

The phase transition from the tetragonal to the orthorhombic structure in La2-xSrxCuO4+d has been studied combining elastic and inelastic neutron scattering and specific heat measurements on the same well characterized crystals. This analysis focusses on the comparison between undoped and doped compounds. The proportionality between the orderparameter and the orthorhombic strain, predicted by the Landau theory, can be confirmed in La2-xSrxCuO4+d in a wide concentration and temperature range. We observe different values for the critical exponent ß for doped superconducting and undoped insulating crystals. The temperature dependent study of the soft modes on a metallic superconducting crystal reveals an extremely large temperature range where this mode is strongly anharmonic. The comparison with previously published data shows that the frequencies of the soft modes at low temperatures decrease with doping, i.e. with increasing charge carrier concentration.

Through-focus electron holography and quantitative electron diffraction have been applied to determine the structure of the 21 K superconductor ThPdxB6 -2x with x ≈ 0.65. This compound adopts a cubic structure (a = 0.42nm; space group, Pm3m), which is an intermediate between the structures of the CaB6 and Cu3Au. Short-range ordering results in a superstructure as is evident from diffuse streaks in the electron diffraction pattern.

The application of modest pressures pto rare-earth and actinide-based heavy-electron / Kondo-lattice materials produces significant changes in both their temperature dependent electrical resistivity ϱ and electronic specific heat γ. For a given compound, γ(p, T=0) appears to scale inversely with a characteristic temperature associated with features in ϱ(p, T). These changes can be understood as arising from the strongly volume-dependent competition of interactions giving rise to the heavy-mass ground state. Similar behavior also may be found in transition-metal compounds, e.g., MnSi.

Experimental studies are discussed which shed light on the nature of the heavy fermion normal state, the occurrence of heavy fermion superconductivity and anomalous properties of the superconducting state: i) Magnetization and magnetoresistance of the Kondo lattice UBe13demonstrate the breakdown of a description in terms of a single energy scale TK. Here TKbecomes temperature dependent below ≃ 10 K, and the canonical relationship between R(H) and M(H) does not hold. ii) Superconductivity in UPt3occurs at the borderline to magnetism: spin-density wave-like instabilities are induced by a few percent of Th or Au, whereas mass enhancement and susceptibility are rapidly reduced by Ir substitution. This suggests that spin fluctuations, rather than phonons, are important in mediating superconducting pairing of heavy fermions in UPt3. iii) The possibilities of noncubic strain induced by anisotropic superconductivity in UBe13is investigated by ultrasound experiments. The suggestion of tetragonal strain in (U, Th)Be13is found to be not applicable, and observed frequency and amplitude dependent absorption anomalies at T⪡ Tcmight reflect domain wall motion.

9Be NMR spectra have been studied in the heavy-fermion compound UBe13as functions of spectrometer frequency, crystal orientation, and temperature. BeI-site isotropic Knight shift and BeII-site anisotropic shifts, corresponding hyperfine fields, and quadrupolar tensors have been determined. For T⪅ 10 K a typical heavy-fermion Knight shift anomaly is observed.

We present the results of an investigation of the de Haas-van Alphen (dHvA) effect in the heavy fermion superconductor UPt3.