arXiv subjects

P. F. S. Rosa

Publications and source records attributed to P. F. S. Rosa.

At least 55 records · Page 3Linked to original sources

arXiv2022

Slow crystalline electric field fluctuations in the Kondo lattice SmB$_{6}$

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 ($ΔH$) 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 $ΔH$ above $T\simeq$ 10 K gives rise to an excitation energy notably close to the hybridization gap of SmB$_{6}$ ($Δ\simeq$ 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.

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arXiv2022

Microscopic probe of magnetic polarons in antiferromagnetic Eu$_{5}$In$_{2}$Sb$_{6}$

Colossal magnetoresistance (CMR) emerges from intertwined spin and charge degrees of freedom in the form of ferromagnetic clusters also known as trapped magnetic polarons. As a result, CMR is rarely observed in antiferromagnetic materials. Here we use electron spin resonance (ESR) to reveal microscopic evidence for the formation of magnetic polarons in antiferromagnetic Eu$_{5}$In$_{2}$Sb$_{6}$. First, we observe a reduction of the Eu$^{2+}$ ESR linewidth as a function of the applied magnetic field consistent with ferromagnetic clusters that are antiferromagnetically coupled. Additionally, the Eu$^{2+}$ lineshape changes markedly below T' ~ 200 K, a temperature scale that coincides with the onset of CMR. The combination of these two effects provide strong evidence that magnetic polarons grow in size below T' and start influencing the macroscopic properties of the system.

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arXiv2021

Ground state of Ce$_{3}$Bi$_{4}$Pd$_{3}$ unraveled by hydrostatic pressure

Noncentrosymmetric Ce$_{3}$Bi$_{4}$Pd$_{3}$ has attracted a lot of attention as a candidate for strongly correlated topological material, yet its experimental ground state remains a matter of contention. Two conflicting scenarios have emerged from a comparison to prototypical Kondo insulator Ce$_{3}$Bi$_{4}$Pt$_{3}$: either Ce$_{3}$Bi$_{4}$Pd$_{3}$ is a spin-orbit-driven topological semimetal or a Kondo insulator with smaller Kondo coupling than its Pt counterpart. Here we determine the ground state of Ce$_{3}$Bi$_{4}$Pd$_{3}$ via electrical resistivity measurements under hydrostatic pressure, which is a clean symmetry-preserving tuning parameter that increases hybridization but virtually preserves spin-orbit coupling. Ce$_{3}$Bi$_{4}$Pd$_{3}$ becomes more insulating under pressure, which is a signature of Ce-based Kondo insulating materials. Its small zero-pressure gap increases quadratically with pressure, similar to the behavior observed in the series Ce$_{3}$Bi$_{4}$(Pt$_{1-x}$Pd$_{x}$)$_{3}$, which indicates that Pt substitution and applied pressure have a similar effect. Our result not only demonstrates that Kondo coupling, rather than spin-orbit coupling, is the main tuning parameter in this class of materials, but it also establishes that Ce$_{3}$Bi$_{4}$Pd$_{3}$ has a narrow-gap Kondo insulating ground state.

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arXiv2021

Single-component superconducting state in UTe2 at 2 K

UTe2 is a newly-discovered unconventional superconductor wherein multicomponent topological superconductivity is anticipated based on the presence of two superconducting transitions and time-reversal symmetry breaking in the superconducting state. The observation of two superconducting transitions, however, remains controversial. Here we demonstrate that UTe2 single crystals displaying an optimal superconducting transition temperature at 2 K exhibit a single transition and remarkably high quality supported by their small residual heat capacity in the superconducting state and large residual resistance ratio. Our results shed light on the intrinsic superconducting properties of UTe2 and bring into question whether UTe2 is a multicomponent superconductor at ambient pressure.

cond-mat.supr-con↗

arXiv2021

Bi kagome sublattice distortions by quenching and flux pinning in superconducting RbBi$_2$

The properties of RbBi$_2$, a 4.15 K superconductor, were investigated using magnetic field, pressure and neutron diffraction. Under hydrostatic pressure, an almost 50 % reduction of T$_c$ is observed, linked to a low Debye temperature estimated at 165 K. The resistivity and magnetic susceptibility were measured on quenched and slow-cooled polycrystalline samples. The resistivity follows a low temperature power-law dependence in both types of samples, while the diamagnetic susceptibility, $χ$, is dependent on the sample cooling history. Slow-cooled samples have a $χ= -1$ while quenched samples have $χ< -1$ due to grain size differences. Evidence of the effects of the cooling rate is also discerned from the local structure, obtained by neutron diffraction and the pair density function analysis. Slow-cooled samples have structurally symmetric Bi hexagons, in contrast to quenched samples in which disorder is manifested in periodic distortions of the Bi hexagonal rings of the kagome sublattice. Disorder may lead to flux pinning that reduces vortex mobility, but T$_c$ remains unaffected by the cooling rate.

cond-mat.supr-con↗

arXiv2021

Spatially inhomogeneous superconductivity in UTe2

Newly-discovered superconductor UTe$_2$ is a strong contender for a topological spin-triplet state wherein a multi-component order parameter arises from two nearly-degenerate superconducting states. A key issue is whether both of these states intrinsically exist at ambient pressure. Through thermal expansion and calorimetry, we show that UTe$_2$ at ambient conditions exhibits two detectable transitions only in some samples, and the size of the thermal expansion jump at each transition varies when the measurement is performed in different regions of the sample. This result indicates that the two transitions arise from two spatially separated regions that are inhomogeneously mixed throughout the volume of the sample, each with a discrete superconducting transition temperature (T$_c$). Notably, samples with higher T$_c$ only show a single transition at ambient pressure. Above 0.3 GPa, however, two transitions are invariably observed in ac calorimetry. Our results not only point to a nearly vertical line in the pressure-temperature phase diagram but also provide a unified scenario for the sample dependence of UTe$_{2}$.

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arXiv2021

Narrow-gap semiconducting behavior in antiferromagnetic Eu$_{11}$InSb$_9$

Here we investigate the thermodynamic and electronic properties of Eu$_{11}$InSb$_9$ single crystals. Electrical transport data show that Eu$_{11}$InSb$_9$ has a semiconducting ground state with a relatively narrow band gap of $320$~meV. Magnetic susceptibility data reveal antiferromagnetic order at low temperatures, whereas ferromagnetic interactions dominate at high temperature. Specific heat, magnetic susceptibility, and electrical resistivity measurements reveal three phase transitions at $T_{N1}=9.3$~K, $T_{N2} =8.3$~K, and $T_{N3} =4.3$~K. Unlike Eu$_{5}$In$_{2}$Sb$_6$, a related europium-containing Zintl compound, no colossal magnetoresistance (CMR) is observed in Eu$_{11}$InSb$_9$. We attribute the absence of CMR to the smaller carrier density and the larger distance between Eu ions and In-Sb polyhedra in Eu$_{11}$InSb$_9$. Our results indicate that Eu$_{11}$InSb$_9$ has potential applications as a thermoelectric material through doping or as a long-wavelength detector due to its narrow gap.

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arXiv2021

Surface excitations relaxation in the Kondo insulator Sm$_{1-x}$Gd$_{x}$B$_{6}$

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.

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arXiv2021

Fingerprinting Triangular-Lattice Antiferromagnet by Excitation Gaps

CeCd$_3$As$_3$ is a rare-earth triangular-lattice antiferromagnet with large inter-layer separation. Our field-dependent heat capacity measurements at dilution fridge temperatures allow us to trace the field-evolution of the spin-excitation gaps throughout the antiferromagnetic and paramagnetic regions. The distinct gap evolution places strong constraints on the microscopic pseudo-spin model, which, in return, yields a close {\it quantitative} description of the gap behavior. This analysis provides crucial insights into the nature of the magnetic state of CeCd$_3$As$_3$, with a certainty regarding its stripe order and low-energy model parameters that sets a compelling paradigm for exploring and understanding the rapidly growing family of the rare-earth-based triangular-lattice systems.

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arXiv2021

Electron-beam Floating-zone Refined UCoGe

The interplay between unconventional superconductivity and quantum critical ferromagnetism in the U-Ge compounds represents an open problem in strongly correlated electron systems. Sample quality can have a strong influence on both of these ordered states in the compound UCoGe, as is true for most unconventional superconductors. We report results of a new approach at UCoGe crystal growth using a floating-zone method with potential for improvements of sample quality and size as compared with traditional means such as Czochralski growth. Single crystals of the ferromagnetic superconductor UCoGe were produced using an ultra-high vacuum electron-beam floating-zone refining technique. Annealed single crystals show well-defined signatures of bulk ferromagnetism and superconductivity at $T_c \sim$2.6 K and $T_s \sim$0.55 K, respectively, in the resistivity and heat capacity. Scanning electron microscopy of samples with different surface treatments shows evidence of an off-stoichiometric uranium rich phase of UCoGe collected in cracks and voids that might be limiting sample quality.

cond-mat.supr-con↗

arXiv2021

Competition between electronic correlations and hybridization in CaMn$_{2}$Bi$_{2}$

We study the interplay between electronic correlations and hybridization in the low-energy electronic structure of CaMn$_2$Bi$_2$, a candidate hybridization-gap semiconductor. Utilizing a DFT+$U$ approach we find both the antiferromagnetic Néel order and band gap in good agreement with the corresponding experimental values. We further find that, under hydrostatic pressure, the band gap is mainly governed by magnetic correlations, whereas hybridization has a greater impact on states at higher band energies. This result suggests that CaMn$_2$Bi$_2$ is more closely related to the high-temperature superconducting cuprates and iron pnictides than the heavy fermion class of materials. Finally, we also find the antiferromagnetic CaMn$_2$Bi$_2$ to be topologically trivial for all pressures studied.

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arXiv2020

Metallic islands in the Kondo insulator SmB$_{6}$

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.

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arXiv2020

Electronic and magnetic properties of stoichiometric CeAuBi$_{2}$

We report the electronic and magnetic properties of stoichiometric CeAuBi$_{2}$ single crystals. At ambient pressure, CeAuBi$_{2}$ orders antiferromagnetically below a Néel temperature ($T_{N}$) of 19 K. Neutron diffraction experiments revealed an antiferromagnetic propagation vector $\hatτ = [0, 0, 1/2]$, which doubles the paramagnetic unit cell along the $c$-axis. At low temperatures several metamagnetic transitions are induced by the application of fields parallel to the $c$-axis, suggesting that the magnetic structure of CeAuBi$_{2}$ changes as a function of field. At low temperatures, a linear positive magnetoresistance may indicate the presence of band crossings near the Fermi level. Finally, the application of external pressure favors the antiferromagnetic state, indicating that the 4$f$ electrons become more localized.

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arXiv2020

Colossal magnetoresistance in a nonsymmorphic antiferromagnetic insulator

Here we investigate antiferromagnetic Eu$_{5}$In$_{2}$Sb$_{6}$, a nonsymmorphic Zintl phase. Our electrical transport data show that Eu$_{5}$In$_{2}$Sb$_{6}$ is remarkably insulating and exhibits an exceptionally large negative magnetoresistance, which is consistent with the presence of magnetic polarons. From {\it ab initio} calculations, the paramagnetic state of Eu$_{5}$In$_{2}$Sb$_{6}$ is a topologically nontrivial semimetal within the generalized gradient approximation (GGA), whereas an insulating state with trivial topological indices is obtained using a modified Becke-Johnson potential. Notably, GGA+U calculations suggest that the antiferromagnetic phase of Eu$_{5}$In$_{2}$Sb$_{6}$ may host an axion insulating state. Our results provide important feedback for theories of topological classification and highlight the potential of realizing clean magnetic narrow-gap semiconductors in Zintl materials.

cond-mat.str-el↗

arXiv2020

Evidence for a pressure-induced antiferromagnetic quantum critical point in intermediate valence UTe2

UTe$_2$ is a recently discovered unconventional superconductor that has attracted much interest due to its many intriguing properties - a large residual density-of-states in the superconducting state, re-entrant superconductivity in high magnetic fields, and potentially spin-triplet topological superconductivity. Our ac calorimetry, electrical resistivity, and x-ray absorption study of UTe$_2$ under applied pressure reveals key new insights on the superconducting and magnetic states surrounding pressure-induced quantum criticality at P$_{c1}$ = 1.3 GPa. First, our specific heat data at low pressures, combined with a phenomenological model, show that pressure alters the balance between two closely competing superconducting orders. Second, near 1.5 GPa we detect two bulk transitions that trigger changes in the resistivity which are consistent with antiferromagnetic order, rather than ferromagnetism. The presence of both bulk magnetism and superconductivity at pressures above P$_{c2}$ = 1.4 GPa results in a significant temperature difference between resistively and thermodynamically determined transitions into the superconducting state, which indicates a suppression of the superconducting volume fraction by magnetic order. Third, the emergence of magnetism is accompanied by an increase in valence towards a U$^{4+}$ (5f2) state, which indicates that UTe$_2$ exhibits intermediate valence at ambient pressure. Our results suggest that antiferromagnetic fluctuations may play a more significant role on the superconducting state of UTe$_2$ than previously thought.

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arXiv2020

Revisiting the Possible 4f7 5d1 Ground State of Gd Impurities in SmB6 by Electron Spin Resonance

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.

cond-mat.str-el↗

arXiv2019

Nematic state in CeAuSb$_{2}$

At ambient pressure and zero field, tetragonal CeAuSb$_{2}$ hosts stripe antiferromagnetic order at $T_{N} = 6.3$ K. Here we first show via bulk thermodynamic probes and x-ray diffraction measurements that this magnetic order is connected with a structural phase transition to a superstructure which likely breaks $C_{4}$ symmetry, thus signaling nematic order. The temperature-field-pressure phase diagram of CeAuSb$_{2}$ subsequently reveals the emergence of additional ordered states under applied pressure at a multicritical point. Our phenomenological model supports the presence of a vestigial nematic phase in CeAuSb$_{2}$ akin to iron-based high-temperature superconductors; however, superconductivity, if present, remains to be discovered.

cond-mat.str-el↗

arXiv2019

Putative hybridization gap in CaMn$_{2}$Bi$_{2}$ under applied pressure

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 Néel 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 $Δ\sim 20$~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$_{2}$.

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