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While SmB$_6$ attracts attention as a possible topological Kondo insulator, EuB$_6$ is known to host magnetic polarons that give rise to large magnetoresistive effects above its ferromagnetic order transition. Here we investigate single crystals of Sm$_{1-x}$Eu$_x$B$_6$ by magnetic and magnetotransport measurements to explore a possible interplay of these two intriguing phenomena, with focus on the Eu-rich substitutions. Sm$_{0.01}$Eu$_{0.99}$B$_6$ exhibits generally similar behavior as EuB$_6$. Interestingly, Sm$_{0.05}$Eu$_{0.95}$B$_6$ combines global antiferromagnetic order with local polaron formation. A pronounced hysteresis is found in the magnetoresistance of Sm$_{0.1}$Eu$_{0.9}$B$_6$ at low temperature ($T=$ 1.9 K) and applied magnetic fields between 2.3 $-$ 3.6 T. The latter is in agreement with a phenomenological model that predicts the stabilization of ferromagnetic polarons with increasing magnetic field within materials with global antiferromagnetic order., Comment: 9 pages, 10 figures

Reports of unconventional superconductivity in UBe13 in 1983 and soon thereafter of the possible coexistence of bulk superconductivity and spin fluctuations in UPt3 marked the beginning of a 40-year adventure in the study of strongly correlated quantum materials and phenomena at Los Alamos. The subsequent discovery and exploration of heavy-fermion magnetism, cuprates, Kondo insulators, Ce- and Pu-115 superconductors and, more broadly, quantum states of narrow-band systems provided challenges for the next 30 years. Progress was not made in a vacuum but benefitted from significant advances in the Americas, Asia and Europe as well as from essential collaborations, visitors and Los Alamos students and postdocs, many subsequently setting their own course in SCES. As often the case, serendipity played a role in shaping this history., Comment: Special session at the 2023 International Conference on Strongly Correlated Electron Systems

The ThIrSi superconductor, with $T_c = 6.5$ K, is expected to show unusual features in view of its noncentrosymmetric structure and the presence of heavy elements featuring a sizable spin-orbit coupling. Here, we report a comprehensive study of its electronic properties by means of local-probe techniques: muon-spin rotation and relaxation ({\textmu}SR) and nuclear magnetic resonance (NMR). Both the superfluid density $\rho_\mathrm{sc}(T)$ (determined via transverse-field {\textmu}SR) and the spin-lattice relaxation rate $T_1^{-1}(T)$ (determined via NMR) suggest a nodeless superconductivity. Furthermore, the absence of spontaneous magnetic fields below $T_c$, as evinced from zero-field {\textmu}SR measurements, indicates a preserved time-reversal symmetry in the superconducting state of ThIrSi. Temperature-dependent upper critical fields as well as field-dependent superconducting muon-spin relaxations suggest the presence of multiple superconducting gaps in ThIrSi., Comment: 8 pages, 8 figures

The proposed topological Kondo insulator SmB$_{6}$ hosts a bulk Kondo hybridization gap that stems from strong electronic correlations and a metallic surface state whose effective mass remains disputed. Thermopower and scanning tunneling spectroscopy measurements argue for heavy surface states that also stem from strong correlations, whereas quantum oscillation and angle-resolved photoemission measurements reveal light effective masses that would be consistent with a Kondo breakdown scenario at the surface. Here we investigate the evolution of the surface state via electrical and thermoelectric transport measurements under hydrostatic pressure, a clean symmetry-preserving tuning parameter that suppresses the Kondo gap and increases the valence of Sm from 2.6+ towards a 3+ magnetic metallic state. Electrical resistivity measurements reveal that the surface carrier density increases with increasing pressure, whereas thermopower measurements show an unchanged Fermi energy under pressure. As a result, the effective mass of the surface state charge carriers linearly increases with pressure as the Sm valence approaches 3+. Our results are consistent with the presence of correlation-driven surface states in SmB$_{6}$ and suggest that the surface Kondo effect persists under pressure to 2 GPa.

This work reports on the temperature dependence of the electron spin resonance (ESR) of Gd$^{3+}$-doped SmB$_{6}$ single crystals at X- and Q-band microwave frequencies in different crystallographic directions. We found an anomalous inhomogeneous broadening of the Gd$^{3+}$ ESR linewidth ($\Delta H$) within 5.3 K $\leq T \leq$ 12.0 K which is attributed to slow crystalline electric field (CEF) fluctuations, slower than the timescale of the ESR microwave frequencies used ($\sim$10 GHz). This linewidth inhomogeneity may be associated to the coupling of the Gd$^{3+}$ $S$-states to the breathing mode of the SmB$_{6}$ cage, and can be simulated by a random distribution of the 4$^{th}$ CEF parameter, $b_4$, that strikingly takes negative and positive values. The temperature at which this inhomogeneity sets in, is related to the onset of a continuous insulator-to-metal phase transition. In addition, based on the interconfigurational fluctuation relaxation model, the observed exponential $T$-dependence of $\Delta H$ above $T\simeq$ 10 K gives rise to an excitation energy notably close to the hybridization gap of SmB$_{6}$ ($\Delta\simeq$ 60 K). This charge fluctuation scenario provides important ingredients to the physical properties of SmB$_{6}$. We finally discuss the interplay between charge and valence fluctuations under the view of slow CEF fluctuations in SmB$_{6}$ by coupling the Gd$^{3+}$ ions to the breathing phonon mode via a dynamic Jahn-Teller-like mechanism.

The interplay between non-trivial topological states of matter and strong electronic correlations is one of the most compelling open questions in condensed matter physics. Due to experimental challenges, there is an increasing desire to find more microscopic techniques to complement the results of more traditional experiments. In this work, we locally explore the Kondo insulator Sm$_{1-x}$Gd$_{x}$B$_{6}$ by means of electron spin resonance (ESR) of Gd$^{3+}$ ions at low temperatures. Our analysis reveals that the Gd$^{3+}$ ESR line shape shows an anomalous evolution as a function of temperature, wherein for highly dilute samples (x $\approx$ 0.0002) the Gd$^{3+}$ ESR line shape changes from a localized ESR local moment character to a diffusive-like character. Upon manipulating the sample surface with a focused ion beam we demonstrate, in combination with electrical resistivity measurements, that the localized character of the Gd$^{3+}$ ESR line shape is recovered by increasing the penetration of the microwave in the sample. This provides compelling evidence for the contribution of surface or near-surface excitations to the relaxation mechanism in the Gd$^{3+}$ spin dynamics. Our work brings new insights into the importance of non-trivial surface excitations in ESR, opening new routes to be explored both theoretically and experimentally., Comment: 10 pages, 5 figures, to appear in Phys. Rev. Research

We investigate flux-grown Sm-deficient Sm$_x$B$_6$ ($x < 1$) by global and local tools, including X-ray diffraction (XRD), electronic transport, and scanning tunneling microscopy (STM) and spectroscopy (STS). All these tools indicate a remarkable persistence of the SmB$_6$ local structure in the flux-grown samples even for nominal Sm concentrations as low as $x=0.75$. As a consequence, the overall electronic properties of Sm$_x$B$_6$, and particularly the surface conductance at low temperature, is only affected locally by the Sm-deficiency., Comment: 9 pages, 8 figures

We show that the resistivity plateau of SmB$_6$ at low temperature, typically taken as a hallmark of its conducting surface state, can systematically be influenced by different surface treatments. We investigate the effect of inflicting an increasing number of hand-made scratches and microscopically defined focused ion beam-cut trenches on the surfaces of flux-grown Sm$_{1-x}$Gd$_x$B$_6$ with $x =$ 0, 0.0002. Both treatments increase the resistance of the low-temperature plateau, whereas the bulk resistance at higher temperature largely remains unaffected. Notably, the temperature at which the resistance deviates from the thermally activated behavior decreases with cumulative surface damage. These features are more pronounced for the focused ion beam treated samples, with the difference likely being related to the absence of microscopic defects like subsurface cracks. Therefore, our method presents a systematic way of controlling the surface conductance., Comment: 7 pages,5 figures

The predicted interplay between Kondo physics and non-trivial topology in SmB$_{6}$ has stimulated many experimental reports, some of which are in apparent contradiction. The origin of the dispute may lie on the fragility of the Kondo insulating phase in the presence of Sm vacancies (Kondo holes) and/or natural impurities, such as Gd$^{3+}$. In this work, we locally investigated this fragility for Al-flux grown Sm$_{1-x}$Gd$_{x}$B$_{6}$ single crystals (0 $\leq$ $x$ $\leq$ 0.02) by combining electron spin resonance (ESR) and complementary bulk measurements. The Gd$^{3+}$ ESR spectra in a highly dilute regime ($x$ $\sim 0.0004$) display the features of an insulating cubic environment. Remarkably, a metallic ESR lineshape is observed for more concentrated samples ($x$ $\geq$ 0.004), even though these systems are still in a reasonably dilute regime and show insulating $dc$ electrical resistivity. Our data indicate that the Kondo insulating state is destroyed locally around impurities before a global percolation occurs. This result not only explains the discrepancy between $dc$ and $ac$ conductivity, but also provides a scenario to explain the presence of quantum oscillations in magnetization in the absence of quantum oscillations in electrical resistivity., Comment: 8 pages, 4 figures, 1 supplementary information. To be published in Phys. Rev. Research

We compare STM investigations on two hexaboride compounds, SmB$_6$ and EuB$_6$, in an effort to provide a comprehensive picture of their surface structural properties. The latter is of particular importance for studying the nature of the surface states in SmB$_6$ by surface-sensitive tools. Beyond the often encountered atomically rough surface topographies of {\it in situ}, low-temperature cleaved samples, differently reconstructed as well as B-terminated and, more rarely, rare-earth terminated areas could be found. With all the different surface topographies observed on both hexaborides, a reliable assignment of the surface terminations can be brought forward., Comment: 8 pages, 10 figures

The search for topological states in strongly correlated electron systems has renewed the interest in the Kondo insulator SmB6. One of the most intriguing previous results was an anomalous electron spin resonance spectrum in Gd-doped SmB6. This spectrum was attributed to Gd2+ ions because it could be very well decribed by a model considering a change in the valence from Gd3+ to Gd2+, a dynamic Jahn-Teller effect and a 4f7 5d1 ground state in the Hamiltonian. In our work, we have revisited this scenario using electron spin resonance and energy dispersive X-ray spectroscopy measurements. Our results suggest that the resonance is produced by Gd2+ ions; however the resonance stems from an extrinsic oxide impurity phase that lies on the surface of the crystal., Comment: Accepted in JPS Conference Proceedings

We report electrical transport measurements on CaMn$_{2}$Bi$_{2}$ single crystals under applied pressure. At ambient pressure and high temperatures, CaMn$_{2}$Bi$_{2}$ behaves as a single-band semimetal hosting N\'{e}el order at $T_{N}=150$~K. At low temperatures, multi-band behavior emerges along with an activated behavior typical of degenerate semiconductors. The activation gap is estimated to be $\Delta \sim 20$~K. Applied pressure not only favors the antiferromagnetic order at a rate of 0.40(2)~K/kbar, but also enhances the activation gap at $20$~kbar by about $70$~\%. This gap enhancement is typical of correlated narrow-gap semiconductors such as FeSi and Ce$_{3}$Bi$_{4}$Pt$_{3}$, and places CaMn$_{2}$Bi$_{2}$ as a Mn-based hybridization-gap semiconductor candidate. \textit{Ab initio} calculations based on the density functional theory are shown to be insufficient to describe the ground state of CaMn$_{2}$Bi$_{2}$., Comment: 7 pages, 5 figures

$^{115}$In Nuclear magnetic resonance data are presented for a series of Ce$_{1-x}$La$_x$CoIn$_5$ crystals with different La dilutions, $x$. Multiple In(1) sites associated with different numbers of nearest-neighbor cerium atoms exhibit different Knight shifts and spin lattice relaxation rates. Analysis of the temperature dependence of these sites reveals both an evolution of the heavy electron coherence as a function of dilution, as well as spatial inhomogeneity associated with a complete suppression of antiferromagnetic fluctuations in the vicinity of the La sites. Quantum critical fluctuations persist within disconnected Ce clusters with dilution levels up to 75%, despite the fact that specific heat shows Fermi liquid behavior in dilute samples., Comment: 9 pages, 10 figures

Kondo insulators are primary candidates in the search for strongly correlated topological quantum phases, which may host topological order, fractionalization, and non-Abelian statistics. Within some Kondo insulators, the hybridization gap is predicted to protect a nontrivial topological invariant and to harbor emergent heavy Dirac fermion surface modes. We use high-energy-resolution spectroscopic imaging in real and momentum space on the Kondo insulator, SmB$_6$. On cooling through $T^*_{\Delta}\approx$ 35 K we observe the opening of an insulating gap that expands to $\Delta\approx$ 10 meV at 2 K. Within the gap, we image the formation of linearly dispersing surface states with effective masses reaching $m^* = (410\pm20)m_e$. We thus demonstrate existence of a strongly correlated topological Kondo insulator phase hosting the heaviest known Dirac fermions.

In this paper, a comprehensive study of the effects of Ni-doping on structural, electrical, thermal and magnetic properties of the NbB2 is presented. Low amounts (\leq 10 %) of Ni substitution on Nb sites cause structural distortions and induce drastic changes in the physical properties, such as the emergence of a bulk superconducting state with anomalous behaviors in the critical fields (lower and upper) and in the specific heat. Ni-doping at the 9 at.% level, for instance, is able to increase the critical temperature (TC) in stoichiometric NbB2 (< 1.3 K) to approximately 6.0 K. Bulk superconductivity is confirmed by magnetization, electronic transport, and specific heat measurements. Both Hc1 and Hc2 critical fields exhibit a linear dependence with reduced temperature (T/TC), and the specific heat deviates remarkably from the conventional exponential temperature dependence of the single-band BCS theory. These findings suggest multiband superconductivity in the composition range from 0.01 \leq x \leq 0.10., Comment: Keywords: Multiband superconductivity; NbB2 system; Ni substitution; Linear critical fields

The newly discovered noncentrosymmetric superconductor ThCoC$_{2}$ exhibits numerous unconventional behavior in the field dependent heat capacity data. Here we present the first measurement of the gap symmetry of ThCoC$_{2}$ by muon spin rotation/relaxation $(\mu$SR) measurements. Temperature dependence of the magnetic penetration depth measured using the transverse field $\mu$SR measurement reveal the evidence of nodal pairing symmetry. To understand these findings, we carry out the calculations of superconducting pairing eigenvalue and eigenfunction symmetry due to the spin-fluctuation mechanism, by directly implemented the {\it ab-initio} band structures. We find that the system possesses a single Fermi surface with considerable three-dimensionality, and hence a strong nesting along the $k_z$-direction. Such a nesting promotes a superconducting pairing with a $\cos{k_z}$-like symmetry with a prominent nodal line on the $k_z=\pm\pi/2$ plane. The result agrees well with the experimental data., Comment: 3 pages, 5 figures

SmB6 is a candidate topological Kondo insulator that displays surface conduction at low temperatures. Here, we perform torque magnetization measurements as a means to detect de Haas-van Alphen (dHvA) oscillations in SmB6 crystals grown by aluminum flux. We find that dHvA oscillations occur in single crystals containing embedded aluminum, originating from the flux used to synthesize SmB6. Measurements on a sample with multiple, unconnected aluminum inclusions show that aluminum crystallizes in a preferred orientation within the SmB6 cubic lattice. The presence of aluminum is confirmed through bulk susceptibility measurements, but does not show a signature in transport measurements.

Because of their complex Fermi surfaces, the identification of the physical phenomena contributing to electronic scattering in the Fe-based superconductors is a difficult task. Here, we report on the electrical resistivity, magnetoresistance, and Hall effect in two series of BaFe$_{2-x}$T$_x$As$_2$ (T = Co, Ni) crystals with different values of $x$. The T contents were chosen so that the majority of the investigated samples present an intermediate magnetically ordered state and a superconducting ground state. We interpret the obtained results in terms of scattering of charge carriers by magnetic excitations instead of describing them as resulting uniquely from effects related to multiple-band conduction. Our samples are single crystals from the structural point of view and their overall magnetotransport properties are dominated by a single magnetic state.

We present linear polarization-dependent soft x-ray absorption spectroscopy data at the Ce $M_{4,5}$ edges of Cd and Sn doped CeCoIn$_5$. The 4$f$ ground state wave functions have been determined for their superconducting, antiferromagnetic and paramagnetic ground states. The absence of changes in the wave functions in CeCo(In$_{1-x}$Cd$_x$)$_5$ suggests the 4$f$\,--\,conduction electron ($cf$) hybridization is not affected by globally Cd doping, thus supporting the interpretation of magnetic droplets nucleating long range magnetic order. This is contrasted by changes in the wave function due to Sn substitution. Increasing Sn in CeCo(In$_{1-y}$Sn$_y$)$_5$ compresses the 4$f$ orbitals into the tetragonal plane of these materials, suggesting enhanced $cf$ hybridization with the in-plane In(1) atoms and a homogeneous altering of the electronic structure. As these experiments show, the 4$f$ wave functions are a very sensitive probe of small changes in the hybridization of 4$f$ and conduction electrons, even conveying information about direction dependencies., Comment: 6 pages, 4 figures

We introduce a new resistance measurement method that is useful in characterizing materials with both surface and bulk conduction, such as three-dimensional topological insulators. The transport geometry for this new resistance measurement configuration consists of one current lead as a closed loop that fully encloses the other current lead on the surface, and two voltage leads that are both placed outside the loop. We show that in the limit where the transport is dominated by the surface conductivity of the material, the four-terminal resistance measured from such a transport geometry is proportional to $\sigma_b/\sigma_s^2$, where $\sigma_b$ and $\sigma_s$ are the bulk and surface conductivities of the material, respectively. We call this new type of measurement \textit{inverted resistance measurement}, as the resistance scales inversely with the bulk resistivity. We discuss possible implementations of this new method by performing numerical calculations on different geometries and introduce strategies to extract the bulk and surface conductivities. We also demonstrate inverted resistance measurements on SmB$_6$, a topological Kondo insulator, using both single-sided and coaxially-aligned double-sided Corbino disk transport geometries. Using this new method, we are able to measure the bulk conductivity, even at low temperatures, where the bulk conduction is much smaller than the surface conduction in this material.

Several technical issues and challenges are identified and investigated for the planar tunneling spectroscopy of the topological Kondo insulator SmB$_6$. Contrasting behaviors of the tunnel junctions prepared in two different ways are analyzed and explained in detail. The conventional approach based on an AlO$_\text{x}$ tunnel barrier results in unsatisfactory results due to the inter-diffusion between SmB$_6$ and deposited Al. On the contrary, plasma oxidation of SmB$_6$ crystals produces high-quality tunnel barriers on both (001) and (011) surfaces. Resultant conductance spectra are highly reproducible with clear signatures for the predicted surface Dirac fermions and the bulk hybridization gap as well. The surface states are identified to reside on two or one {\it distinguishable} Dirac cone(s) on the (001) and (011) surface, respectively, in good agreement with the recent literature. However, their topological protection is found to be limited within the low energy region due to their inevitable interaction with the bulk excitations, called spin excitons, consistent with a recent theoretical prediction. Implications of our findings on other physical properties in SmB$_6$ and also other correlated topological materials are remarked., Comment: 11 pages, 11 figures, published in Phys. Rev. B

We have carried out bulk-sensitive hard x-ray photoelectron spectroscopy (HAXPES) measurements on in-situ cleaved and ex-situ polished SmB6 single crystals. Using the multiplet-structure in the Sm 3d core level spectra, we determined reliably that the valence of Sm in bulk SmB6 is close to 2.55 at ~5 K. Temperature dependent measurements revealed that the Sm valence gradually increases to 2.64 at 300 K. From a detailed line shape analysis we can clearly observe that not only the J=0 but also the J=1 state of the Sm 4f 6 configuration becomes occupied at elevated temperatures. Making use of the polarization dependence, we were able to identify and extract the Sm 4f spectral weight of the bulk material. Finally, we revealed that the oxidized or chemically damaged surface region of the ex-situ polished SmB6 single crystal is surprisingly thin, about 1 nm only., Comment: 11 pages, 8 figures

Unlike the widely studied $s$-type two-gap superconductor MgB$_2$, the chemically similar compounds ZrB$_2$ and HfB$_2$ do not superconduct above 1 K. Yet, it has been shown that small amounts of self- or extrinsic doping (in particular with vanadium), can induce superconductivity in these materials. Based on results of different macro- and microscopic measurements, including magnetometry, nuclear magnetic resonance (NMR), resistivity, and muon-spin rotation ($\mu$SR), we present a comparative study of Zr$_{0.96}$V$_{0.04}$B$_2$ and Hf$_{0.97}$V$_{0.03}$B$_2$. Their key magnetic and superconducting features are determined and the results are considered within the theoretical framework of multiband superconductivity proposed for MgB$_2$. Detailed Fermi surface (FS) and electronic structure calculations reveal the difference between MgB$_2$ and transition-metal diborides., Comment: 10 pages, 8 figures

We report on electrical resistivity, magnetoresistance (MR) and Hall effect measurements in four non-superconducting BaFe$_{2-x}$TM$_x$As$_2$ (TM = Mn, Cu and Ni) single crystals with small values of the chemical substitution $x$. The spin density wave (SDW) ordering that occurs in these systems at temperatures $T\sim$ (120 - 140) K, in close vicinity to a tetragonal/orthorhombic transition, produces significant modifications in their magneto-transport properties. While in the magnetically ordered phase the MR is positive and its magnitude increases with decreasing temperatures, in the paramagnetic regime the MR becomes vanishingly small. Above the spin density wave transition temperature ($T_{\text{SDW}}$) the Hall coefficient $R_H$ is negative, small and weakly temperature dependent, but a remarkable change of slope occurs in the $R_H$ versus $T$ curves at $T = T_{\text{SDW}}$. The Hall coefficient amplitude, while remaining negative, increases steadily and significantly as the temperature is decreased below $T_{\text{SDW}}$ and down to $T =$ 20 K. The qualitative behavior of both MR and Hall coefficient is weakly dependent on the chemical substitution in the studied limit. The experiments provide strong evidence that scattering of charge carriers by magnetic excitations has to be taken into account to explain the behavior of the resistivity, magnetoresistance and Hall effect in the ordered phase of the studied compounds. Effects of multiple band conduction also must be considered for a complete interpretation of the results.

Applied pressure drives the heavy-fermion antiferromagnet CeRhIn$_{5}$ towards a quantum critical point that becomes hidden by a dome of unconventional superconductivity. Magnetic fields suppress this superconducting dome, unveiling the quantum phase transition of local character. Here, we show that $5\%$ magnetic substitution at the Ce site in CeRhIn$_{5}$, either by Nd or Gd, induces a zero-field magnetic instability inside the superconducting state. This magnetic state not only should have a different ordering vector than the high-field local-moment magnetic state, but it also competes with the latter, suggesting that a spin-density-wave phase is stabilized in zero field by Nd and Gd impurities - similarly to the case of Ce$_{0.95}$Nd$_{0.05}$CoIn$_{5}$. Supported by model calculations, we attribute this spin-density wave instability to a magnetic-impurity driven condensation of the spin excitons that form inside the unconventional superconducting state.

Topological insulators give rise to exquisite electronic properties due to 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, SmB$_6$ has been proposed as candidate material for a topological Kondo insulator. By utilizing scanning tunneling microscopy and spectroscopy measurements down to 0.35 K, we observed several states within the hybridization gap of about $\pm$20 meV on well characterized (001) surfaces of SmB$_6$. The spectroscopic response to impurities and magnetic fields allows to distinguish between dominating bulk and surface contributions to these states. The surface contributions develop particularly strongly below about 7 K which can be understood in terms of a breakdown of the Kondo effect at the surface. Our high-resolution data provide detailed insight into the electronic structure of SmB$_6$, which will reconcile many current discrepancies on this compound.

We investigated the ground state symmetry of the cubic hidden order compound CeB$_6$ by means of core level non-resonant inelastic x-ray scattering (NIXS). The information is obtained from the directional dependence of the scattering function that arises from higher than dipole transitions. Our new method confirms that the ground state is well described using a localized crystal-field model assuming a $\Gamma_8$ quartet ground state., Comment: 5 pages, 4 figures

SmB$_6$ exhibits a small (15-20 meV) bandgap at low temperatures due to hybridized $d$ and $f$ electrons, a tiny (3 meV) transport activation energy $(E_{A})$ above 4 K, and surface states accessible to transport below 2 K. We study its magnetoresistance in 60-T pulsed fields between 1.5 K and 4 K. The response of the nearly $T$-independent surface states (which show no Shubnikov-de Haas oscillations) is distinct from that of the activated bulk. $E_{A}$ shrinks by 50% under fields up to 60 T. Data up to 93 T suggest that this trend continues beyond 100 T, in contrast with previous explanations. It rules out emerging theories to explain observed exotic magnetic quantum oscillations., Comment: 12 pages, 3 figures

We have studied the isothermal magnetization $M(H)$ of CeCo(In$_{1-x}$Cd$_x$)$_5$ with $x$ = 0.0075 and 0.01 down to 50 mK. Pronounced field-history dependent phenomena occur in the coexistence regime of the superconducting and antiferromagnetic phases. At low-fields, a phenomenological model of magnetic-flux entry well explains $M(H)$ implying the dominance of bulk pinning effect. However, unless crystallographic quenched disorder is hysteretic, the asymmetric peak effect (ASPE) which appears at higher fields cannot be explained by the pinning of vortices due to material defects. Also the temperature dependence of the ASPE deviates from the conventional scenario for the peak effect. Comparison of our thermodynamic phase diagrams with those from previous neutron scattering and magnetoresistance experiments indicates that the pinning of vortices takes place at the field-history dependent antiferromagnetic domain boundaries., Comment: 13 pages,4 figures, to be published in New Journal of Physics

The Kondo insulator SmB$_6$ has long been known to display anomalous transport behavior at low temperatures, T$<5$ K. In this temperatures range, a plateau is observed in the dc resistivity, contrary to the exponential divergence expected for a gapped system. Recent theoretical calculations suggest that SmB$_6$ may be the first topological Kondo insulator (TKI) and propose that the residual conductivity is due to topological surface states which reside within the Kondo gap. Since the TKI prediction many experiments have claimed to observe high mobility surface states within a perfectly insulating hybridization gap. Here, we investigate the low energy optical conductivity within the hybridization gap of single crystals of SmB$_6$ via time domain terahertz spectroscopy. Samples grown by both optical floating zone and aluminum flux methods are investigated to probe for differences originating from sample growth techniques. We find that both samples display significant 3D bulk conduction originating within the Kondo gap. Although SmB$_6$ may be a bulk dc insulator, it shows significant bulk ac conduction that is many orders of magnitude larger than any known impurity band conduction. The nature of these in-gap states and their coupling with the low energy spin excitons of SmB$_6$ is discussed. Additionally, the well defined conduction path geometry of our optical experiments allows us to show that any surface states, which lie below our detection threshold if present, must have a sheet resistance of R$/\square \ge$ 1000 $\Omega$., Comment: 10 pages, 3 figures

SmB6 is 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 limiting its scientific investigation and device application. Here, we report the enhancement of T * in SmB6 under 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 indued in epitaxial SmB6 films via substrate in potential device applications., Comment: to appear in Nature Materials

We investigate single crystalline samples of Ce$_{1-x}$Nd$_{x}$RhIn$_{5}$ by means of X-ray diffraction, microprobe, magnetic susceptibility, heat capacity, and electrical resistivity measurements. Our data reveal that the antiferromagnetic transition temperature of CeRhIn$_{5}$, $T_{N}^{\mathrm{Ce}} = 3.8$ K, is linearly suppressed with $x_{\mathrm{Nd}}$, by virtue of the "Kondo hole" created by Nd substitution. The extrapolation of $T^{\mathrm{Ce}}_{N}$ to zero temperature, however, occurs at $x_{c} \sim 0.3$, which is below the 2D percolation limit found in Ce$_{1-x}$La$_{x}$RhIn$_{5}$. This result strongly suggests the presence of crystal-field frustration effects. Near $x_{\mathrm{Nd}} \sim 0.2$, the Ising AFM order from Nd ions is stabilized and $T^{\mathrm{Nd}}_{N}$ increases up to $11$ K in pure NdRhIn$_{5}$. Our results shed light on the effects of magnetic doping in heavy-fermion antiferromagnets and stimulate the study of such systems under applied pressure., Comment: 6 pages, 6 figures

The mixed valent compound SmB6 is of high current interest as the first candidate example of topologically protected surface states in a strongly correlated insulator and also as a possible host for an exotic bulk many-body state that would manifest properties of both an insulator and a metal. Two different de Haas van Alphen (dHvA) experiments have each supported one of these possibilities, while angle resolved photoemission spectroscopy (ARPES) for the (001) surface has supported the first, but without quantitative agreement to the dHvA results. We present new ARPES data for the (110) surface and a new analysis of all published dHvA data and thereby bring ARPES and dHvA into substantial consistency around the basic narrative of two dimensional surface states., Comment: 5 pages, 4 figures + supplement (12 pages, 6 figures)

SmB$_6$, a so called Kondo insulator, is recently discussed as a candidate material for a strong topological insulator. We present detailed atomically resolved topographic information on the (001) surface from more than a dozen SmB$_6$ samples. Atomically flat, {\it in situ} cleaved surfaces often exhibit B- and Sm-terminated surfaces as well as reconstructed and non-reconstructed areas {\it coexisting} on different length scales. The terminations are unambiguously identified. In addition, electronic inhomogeneities are observed which likely result from the polar nature of the (001) surface and may indicate an inhomogeneous Sm valence at the surface of SmB$_6$. In addition, atomically resolved topographies on a (110) surface are discussed., Comment: 8 pages, 6 figures

Radio frequency tunable oscillators are vital electronic components for signal generation, characterization, and processing. They are often constructed with a resonant circuit and a 'negative' resistor, such as a Gunn-diode, involving complex structure and large footprints. Here we report that a piece of SmB6, 100 micron in size, works as a current-controlled oscillator in the 30 MHz frequency range. SmB6 is a strongly correlated Kondo insulator that was recently found to have a robust surface state likely to be protected by the topology of its electronics structure. We exploit its non-linear dynamics, and demonstrate large AC voltage outputs with frequencies from 20 Hz to 30 MHz by adjusting a small DC bias current. The behaviors of these oscillators agree well with a theoretical model describing the thermal and electronic dynamics of coupled surface and bulk states. With reduced crystal size we anticipate the device to work at higher frequencies, even in the THz regime. This type of oscillator might be realized in other materials with a metallic surface and a semiconducting bulk., Comment: v3 to appear in Physical Review Letters

Heavy fermion (HF) materials exhibit a rich array of phenomena due to the strong Kondo coupling between their localized moments and itinerant electrons. A central question in their study is to understand the interplay between magnetic order and charge transport, and its role in stabilizing new quantum phases of matter. Particularly promising in this regard is a family of tetragonal intermetallic compounds Ce{$TX$}$_2$ ($T=$ transition metal, $X=$ pnictogen), that includes a variety of HF compounds showing $T$-linear electronic specific heat $\bf{C_e \sim \gamma T}$, with $\gamma\sim$ 20-500 mJ$\cdot$mol$^{-1}$~K$^{-2}$, reflecting an effective mass enhancement ranging from small to modest. Here, we study the low-temperature field-tuned phase diagram of high-quality CeAgBi$_2$ using magnetometry and transport measurements. We find an antiferromagnetic transition at ${T_{N} = 6.4}$~K with weak magnetic anisotropy and the easy axis along the $c$-axis, similar to previous reports (${T_{N} = 6.1}$~K). This scenario, along with the presence of two anisotropic Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions, leads to a rich field-tuned magnetic phase diagram, consisting of five metamagnetic transitions of both first and second order. In addition, we unveil an anomalous Hall contribution for fields $H<54$ kOe which is drastically altered when $H$ is tuned through a trio of transitions at 57, 78, and 84~kOe, suggesting that the Fermi surface is reconstructed in a subset of the metamagnetic transitions., Comment: (*equal contribution)

Motivated by the presence of competing magnetic interactions in the heavy fermion family Ce$TX_2$ ($T$ = transition metal, $X$ = pnictogen), here we study the novel parent compound CeCd$_{1-\delta}$Sb$_{2}$ by combining magnetization, electrical resistivity, and heat-capacity measurements. Contrary to the antiferromagnetic (AFM) ground state observed in most members of this family, the magnetic properties of our CeCd$_{1-\delta}$Sb$_{2}$ single crystals revealed a ferromagnetic (FM) ordering at $T_{\rm c}$ = 3 K with an unusual soft behavior. By using a mean field model including anisotropic nearest-neighbors interactions and the tetragonal crystalline electric field (CEF) Hamiltonian, a systematic analysis of our macroscopic data was obtained. Our fits allowed us to extract a simple but very distinct CEF scheme, as compared to the AFM counterparts. As in the previously studied ferromagnet CeAgSb$_{2}$, a pure $|\pm 1/2 \rangle$ ground state is realized, hinting at a general trend within the ferromagnetic members. We propose a general scenario for the understanding of the magnetism in this family of compounds based on the subtle changes of dimensionality in the crystal structure., Comment: 6 pages, 4 figures

Topological insulators (TIs) have the singular distinction of being electronic insulators while harboring metallic, conductive surfaces. In ordinary materials, defects such as cracks and deformations are barriers to electrical conduction, intuitively making the material more electrically resistive. Peculiarly, 3D TIs should become better conductors when they are cracked because the cracks themselves, which act as conductive topological surfaces, provide additional paths for the electrical current. Significantly, for a TI material, any surface or extended defect harbors such conduction. In this letter, we demonstrate that small subsurface cracks formed within the predicted 3D TI samarium hexaboride (SmB$_{6}$) via systematic scratching or sanding results in such an increase in the electrical conduction. SmB$_{6}$ is in a unique position among TIs to exhibit this effect because its single-crystals are thick enough to harbor cracks, and because it remarkably does not appear to suffer from conduction through bulk impurities. Our results not only strengthen the building case for SmB$_{6}$'s topological nature, but are relevant to all TIs with cracks, including TI films with grain boundaries., Comment: 10 pages, 2 figures

Experimental contributors to the field of Superconducting Materials share their informal views on the subject., Comment: Epilogue to Physica C Special Issue on Superconducting Materials, Volume 514 (2015)

The effects of electron-electron correlations on the low-energy electronic structure and their relationship with unconventional superconductivity are central aspects in the research on the iron-based pnictide superconductors. Here we use soft X-ray angle-resolved photoemission spectroscopy (SX-ARPES) to study how electronic correlations evolve in different chemically substituted iron pnictides. We find that correlations are intrinsically related to the effective filling of the correlated orbitals, rather than to the filling obtained by valence counting. Combined density functional theory (DFT) and dynamical mean-field theory (DMFT) calculations capture these effects, reproducing the experimentally observed trend in the correlation strength. The occupation-driven trend in the electronic correlation reported in our work supports the recently proposed connection between cuprate and pnictides phase diagrams., Comment: 8 pages, 6 figures

We have used ion-irradiation to damage the (001) surfaces of SmB_6 single crystals to varying depths, and have measured the resistivity as a function of temperature for each depth of damage. We observe a reduction in the residual resistivity with increasing depth of damage. Our data are consistent with a model in which the surface state is not destroyed by the ion-irradiation, but instead the damaged layer is poorly conducting and the initial surface state is reconstructed below the damage. This behavior is consistent with a surface state that is topologically protected., Comment: 5 pages, 3 figures