arXiv subjects

Priscila F. S. Rosa

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

At least 19 recordsLinked to original sources

arXiv2026

Tracking flat bands via phonon-mediated interband scattering

Flat-band (FB) systems offer a fertile platform for realizing exotic correlated and topological phases, yet central challenges remain in tuning FBs towards the Fermi level $E_\text{F}$ and in understanding their impact on electronic transport. Identifying how close a FB lies to $E_\text{F}$ normally requires advanced spectroscopic probes or extensive doping studies hindering fast screening of promising candidates. Here, we show that standard electrical resistivity measurements already contain a clear and universal fingerprint of the proximity of a FB to $E_\text{F}$: A characteristic curvature in the temperature-dependent resistivity at high temperatures, i.e., sublinear (superlinear) behavior when EF lies within (away from) the FB states. Within a phenomenological model, we can attribute this response to the rapid change in scattering phase space associated with interband transitions from dispersive to FB states. Notably, this characteristic emerges independently of microscopic details, such as the electron-phonon coupling strength, as confirmed by comparison with ab initio electron-phonon transport calculations. We validate this mechanism across several FB materials$-$including kagome, pyrochlore and Cu$_3$Au-type intermetallics$-$by combining high-temperature resistivity measurements with a modified Bloch-Grüneisen analysis. Our results establish a simple and broadly applicable method to locate FBs via electronic transport, enabling rapid screening of promising FB materials.

cond-mat.str-el↗

arXiv2026

High-quality single crystals of the kagome metals Ni$_3$In and Ni$_3$Sn grown from Pb flux

The bilayer kagome metal Ni$_3$In has recently attracted attention due to the presence of a flat band located near the Fermi level, which has been associated with unconventional thermodynamic and electronic transport properties [Ye et al., Nat. Phys. 20, 610-614 (2024)]. However, further investigation of the intrinsic properties of this system has been hindered by the lack of large, high-quality single crystals. Here, we report the successful growth of Ni$_3$(In, Sn) single crystals using a Pb-flux technique. By optimizing the growth conditions, competing binary phases can be effectively suppressed, enabling the synthesis of single crystals with dimensions reaching several millimeters. We compare the physical properties of our Pb-flux-grown crystals to previously reported samples prepared by iodine-assisted chemical vapor transport and molecular beam epitaxy as well as to first-principles resistivity calculations. We find a significantly lower electrical resistivity in our crystals, in excellent agreement with calculations of resistivity from electron-phonon scattering, a sizeable non-saturating magnetoresistance, and a reduced Sommerfeld coefficient and magnetic susceptibility compared to previous experimental findings, which are likely related to differences in the Fermi level position. Our results establish Pb-flux growth as a reliable route for obtaining large single crystals of the bilayer kagome metals Ni$_3$(In, Sn) that are suitable for further thermodynamic and spectroscopic investigations of their intrinsic electronic and magnetic properties.

cond-mat.mtrl-sci↗

arXiv2026

Directional Manipulation of a Staggered Charge Density Wave and Kondo Resonance in UTe2

UTe2 is a rare example of a correlated quantum material in which unconventional density wave orders, Kondo physics, spin-triplet pairing, and reentrant superconductivity coexist within the same electronic system. Its superconducting state develops out of a strongly correlated normal phase. The identification and control of competing or intertwined normal-state orders are thus central to elucidating the electronic landscape from which its superconductivity arises. Here, using scanning tunnelling microscopy (STM) in a vector magnetic field, we uncover a previously unreported staggered charge-density-wave (CDW) in high-quality UTe2 crystals and demonstrate its strong directional response to an external magnetic field: the staggered CDW is completely quenched by a modest 1.7 T field aligned with the quasi-one-dimensional uranium chain direction (a-axis), while remaining robust against fields along other crystallographic directions. This pronounced anisotropy is consistent with an orbital-driven mechanism that leads to a field-tuned quantum phase transition. Strikingly and counterintuitively, the same field orientation and strength concomitantly alter the hybridization gap and suppress the 5f Kondo resonance. Modelling indicates that this correlated evolution arises from a switch of the dominant hybridization channel from Te 5p- U 5f to U 6d- U 5f coupling, suggesting an intimate interplay between CDW and the Kondo effect. Our work establishes an effective tuning knob for the intertwined orders in UTe2 and provides evidence for orbital-selective Kondo hybridization, shedding light on its correlated normal state.

cond-mat.str-el↗

arXiv2026

Random singlet physics in exchange disordered 2D triangular YbCu$_{1.14}$Se$_2$

Quantum spin liquid (QSL) phases exist in theory, but real candidate QSL materials are often extraordinarily sensitive to structural defects which disrupt the ground state. Here, we investigate candidate triangular QSL material YbCu$_{1.14}$Se$_2$ and discover the absence of magnetic order, but also no compelling evidence of a QSL ground state due to significant structural disorder. We instead look at the results through a lens of a 2-dimensional (2D) random singlet phase. We are able to match thermodynamic measurements using a phenomenological model of a distribution of singlet formation. YbCu$_{1.14}$Se$_2$ behaves strikingly similar to other disordered triangular lattice materials, suggesting universal behavior of random singlet formation in 2D frustrated systems.

cond-mat.str-el↗

arXiv2025

UTe$_2$: a narrow band superconductor

We investigate the nature of the 5$f$ electrons in the unconventional odd-parity superconductor UTe$_2$, focusing on the degree of covalency, localization versus itinerancy, and dominant electronic configuration. This is achieved using density functional theory (DFT) in combination with dynamical mean-field theory (DMFT) calculations. A key aspect of our approach is the material-specific tuning of the double-counting correction parameter, $μ_{\rm dc}$, within the DFT+DMFT part. This tuning is guided by the energy dependence of photo-ionization cross-sections in valence band photoelectron spectroscopy. The reliability of the parameters is confirmed by the accurate reproduction of the angle-resolved valence-band photoemission spectra and the U 4$f$ core-level data. The DFT+DMFT model reveals that in UTe$_2$ U 5$f^n$ configurations with n=1 to 4 contribute to the ground state, with the 5$f^2$ configuration being most prevalent and an average 5$f$ shell fillings close to 2.5. The model further suggests that the 5$f$ electrons form narrow bands and that charge fluctuations due to degeneracy play a role in addition to coherent valence dynamics arising from hybridization with the conduction bath. Additionally, the significance of the U 6$d$ states in UTe$_2$ is discussed.

cond-mat.str-el↗

arXiv2025

Quantum oscillation and topology change of the uncondensed Landau Fermi surface in superconducting CeCoIn5

Metals typically have multiple Fermi surface sheets, and when they enter the superconducting state, some electrons on these sheets may remain uncondensed, or their superconducting pairs can be rapidly destroyed by a magnetic field. Detecting uncondensed electrons within the superconducting state provides key information about the underlying electronic structure; however, this task remains a significant experimental challenge. Here we demonstrate quantum oscillations from the uncondensed electrons in the heavy-fermion superconductor CeCoIn5, observed through thermal conductivity measurements with a magnetic field rotating within the tetragonal a-b plane. We detect a fine structure in thermal conductivity, characterized by multiple small resonances (oscillations) in a rotating magnetic field. Remarkably, the phase of these resonances shifted by as much as π for a field above 9.7 T where spin-density wave (SDW) order emerges and coexists with superconductivity. This phase shift is naturally explained by a change in the Berry phase of the uncondensed Fermi surface, driven by the Fermi surface reconstruction associated with the onset of SDW order. Our work unambiguously shows the existence of uncondensed electrons in the superconducting state of CeCoIn5, thus resolving a longstanding debate on this issue.

cond-mat.supr-con↗

arXiv2025

Putative excitonic insulating state in narrow-gap semiconductor La$_3$Cd$_2$As$_6$

Excitonic insulators are electronically-driven phases of matter characterized by the spontaneous condensation of electron-hole pairs. Here we show that La$_3$Cd$_2$As$_6$ undergoes a transition at $T_{0}=278$ K to a highly insulating state with no accompanying structural transition. We observe quasi-two-dimensional electrical transport and charge fluctuations consistent with an electronic transition enabled by enhanced Coulomb interactions. Density functional theory calculations are unable to replicate the insulating ground state. Our results support the opening of a gap by excitonic effects at $T_{0}$, placing La$_3$Cd$_2$As$_6$ as a rare example of a bulk excitonic insulator.

cond-mat.str-el↗

arXiv2025

Hydrostatic pressure studies on non-superconducting UTe2

We report the temperature-pressure phase diagram of a UTe$_2$ single crystal that does not undergo a bulk superconducting transition but shows filamentary superconductivity with a critical transition temperature of 1 K at ambient pressure. Electrical-resistivity measurements reveal that the evolution of the filamentary superconducting state under pressure resembles the behavior observed in previous reports on bulk superconducting samples. AC calorimetry, however, does not show evidence for either bulk superconductivity or magnetism for pressures up to 1.6 GPa. Our results highlight the role of inhomogeneity in chemical-vapor-transport-grown UTe$_2$ samples and serve as a cautionary tale when probing electrical resistivity alone.

cond-mat.str-el↗

arXiv2025

Observation of Persistent Zero Modes and Superconducting Vortex Doublets in UTe$_2$

Superconducting vortices can reveal electron pairing details and nucleate topologically protected states. Yet, vortices of bulk spin-triplet superconductors have never been visualized. Recently, UTe$_2$ has emerged as a nominative spin-triplet superconductor, but its superconducting order parameter is elusive, and whether time-reversal symmetry is broken remains unsettled. Here, we visualize superconducting vortices on the (011) surface of ultra-clean UTe$_2$ single crystals ($T_c=2.1 K$) using scanning tunneling microscopy (STM). We introduce $\frac{d^2 I}{dV^2}$ imaging as an effective method for vortex visualization in superconductors with substantial residual zero-energy density of states (DOS), as in UTe$_2$. Anisotropic single-flux-quantum vortices, with coherence lengths of $\sim$ 12 nm (4 nm) parallel (perpendicular) to the a-axis, form a triangular vortex lattice (VL) under small out-of-plane magnetic fields. The vortex size decreases at higher fields, suggesting multi-band superconductivity. The robustness of vortex structures and VL against magnetic field polarity and cooling history strongly supports time-reversal invariant superconductivity. Spatially non-splitting and spectroscopically narrow zero-bias peaks (ZBPs) of differential conductance at vortex cores (VCs) persist up $8T$, well beyond the Pauli limit ($3.9T$), consistent with Majorana zero modes (MZMs) in a topological vortex line protected by a combination of mirror, time-reversal, and particle-hole symmetries. Close examination of vortex structures reveals a mirror-asymmetric doublet $--$ one with a singular core and another with an enhanced energy gap. Such unconventional vortices in a non-chiral $p$-wave superconductor could originate from the induction of a subdominant order parameter close to the VCs.

cond-mat.supr-con↗

arXiv2025

Realizing a topological diode effect on the surface of a topological Kondo insulator

Introducing the concept of topology into material science has sparked a revolution from classic electronic and optoelectronic devices to topological quantum devices. The latter has potential for transferring energy and information with unprecedented efficiency. Here, we demonstrate a topological diode effect on the surface of a three-dimensional material, SmB6, a candidate topological Kondo insulator. The diode effect is evidenced by pronounced rectification and photogalvanic effects under electromagnetic modulation and radiation at radio frequency. Our experimental results and modeling suggest that these prominent effects are intimately tied to the spatially inhomogeneous formation of topological surface states (TSS) at the intermediate temperature. This work provides a manner of breaking the mirror symmetry (in addition to the inversion symmetry), resulting in the formation of pn-junctions between puddles of metallic TSS. This effect paves the way for efficient current rectifiers or energy-harvesting devices working down to radio frequency range at low temperature, which could be extended to high temperatures using other topological insulators with large bulk gap.

cond-mat.str-el↗

arXiv2024

Evidence for incommensurate antiferromagnetism in nonsymmorphic UPd$_{0.65}$Bi$_2$

The intersection between nonsymmorphic symmetry and electronic correlations has emerged as a platform for topological Kondo semimetallic states and unconventional spin textures. Here we report the synthesis of nonsymmorphic UPd$_{0.65}$Bi$_2$ single crystals and their structural, electronic, magnetic, and thermodynamic properties. UPd$_{0.65}$Bi$_2$ orders antiferromagnetically (AFM) below $T_N\simeq$ 161 K as evidenced by a sharp cusp in magnetic susceptibility, a second-order phase transition in specific heat, and an upturn in electrical resistivity, which suggests an incommensurate AFM structure that deviates from the A-type magnetism typically observed in this class of materials. Across $T_N$, Hall effect measurements reveal a change from electron-dominated to hole-dominated transport, which points to a sharp reconstruction in the electronic structure at $T_N$. Upon further cooling, a first-order transition is observed at $T_1 \simeq 30 $K in magnetic susceptibility and heat capacity but not in electrical resistivity or Hall measurements, which indicates a small change in the AFM structure that does not affect the electronic structure. Our specific heat data reveal a small Sommerfeld coefficient ($γ\simeq$13 mJmol$^{-1}$K$^{-2}$), consistent with localized 5$f$ electrons. Our results indicate that UPd$_{0.65}$Bi$_2$ hosts weak electronic correlations and is likely away from a Kondo semimetallic state. Low-temperature magnetization measurements show that the AFM structure is remarkably stable to 160 kOe and does not undergo any field-induced transitions. Neutron diffraction and magnetization experiments at higher fields would be valuable to probe the presence of unconventional spin textures.

cond-mat.str-el↗

arXiv2024

Magnetic and electronic inhomogeneity in Sm$_{1-x}$Eu$_x$B$_6$

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.

cond-mat.str-el↗

arXiv2024

Thermodynamic evidence for polaron stabilization inside the antiferromagnetic order of Eu$_5$In$_2$Sb$_6$

Materials exhibiting electronic inhomogeneities at the nanometer scale have enormous potential for applications. Magnetic polarons are one such type of inhomogeneity which link the electronic, magnetic and lattice degrees of freedom in correlated matter and often give rise to colossal magnetoresistance. Here, we investigate single crystals of Eu$_5$In$_2$Sb$_6$ by thermal expansion and magnetostriction along different crystallographic directions. These data provide compelling evidence for the formation of magnetic polarons in Eu$_5$In$_2$Sb$_6$ well above the magnetic ordering temperature. More specifically, our results are consistent with anisotropic polarons with varying extent along the different crystallographic directions. A crossover revealed within the magnetically ordered phase can be associated with a surprising stabilization of ferromagnetic polarons within the global antiferromagnetic order upon decreasing temperature. These findings make Eu$_5$In$_2$Sb$_6$ a rare example of such coexisting and competing magnetic orders and, importantly, shed new light on colossal magnetoresistive behavior beyond manganites.

cond-mat.str-el↗

arXiv2024

Magnetic edge fields in UTe$_2$ near zero background fields

Chiral superconductors are theorized to exhibit spontaneous edge currents. Here, we found magnetic fields at the edges of UTe$_2$, a candidate odd-parity chiral superconductor, that seem to agree with predictions for a chiral order parameter. However, we did not detect the chiral domains that would be expected, and recent polar Kerr and muon spin relaxation data in nominally clean samples argue against chiral superconductivity. Our results show that hidden sources of magnetism must be carefully ruled out when using spontaneous edge currents to identify chiral superconductivity.

cond-mat.supr-con↗

arXiv2024

Magnetic Phase Diagram of ErB$_4$ as Explored by Neutron Scattering

The tetragonal $4f$-electron intermetallic ErB$_4$ is characterized by strong Ising anisotropy along the tetragonal $c$ axis. The magnetic moments on the erbium sites can be mapped onto a Shastry-Sutherland (SSL) lattice resulting in geometrical frustration. At zero magnetic field ErB$_4$ exhibits collinear columnar antiferromagnetic (CAFM) order below $T_\text{N} = 15.4$ K. In the presence of a magnetic field parallel to the $c$ axis, ErB$_4$ exhibits a plateau at $1/2$ of the saturation magnetization $M_\text{S}$, which arises at a spin flip transition at $H_1$ $=$ 1.9 T. Fractional magnetization plateaus and other exotic spin phases are a well-established characteristic feature of frustrated spin systems. Monte Carlo simulations propose that ErB$_4$ is an ideal candidate to feature a spin supersolid phase in close vicinity of $H_1$ between the CAFM and $M/M_\text{S}=1/2$ plateau (HP) phase. Here we combine single-crystal neutron diffraction and inelastic neutron scattering to study the magnetic phase diagram and the crystal electric field (CEF) ground state of ErB$_4$. Our measurements as a function of magnetic field find no signature of the spin supersolid phase but allow us to determine the magnetic structure of the HP phase to be of the uuud type consistent with an Ising material. The magnetic moment $μ_{\mathrm{CEF}}$ $=$ 8.96 $μ_B$ expected from the CEF configuration determined by our inelastic neutron scattering measurements is also consistent with the ordered moment observed in neutron diffraction showing that the moments are fully ordered and close to the Er$^{3+}$ free ion moment (9.6 $μ_B$).

cond-mat.str-el↗

arXiv2024

Mixed-valence state in the dilute-impurity regime of La-substituted SmB$_6$

Homogeneous mixed-valence (MV) behaviour is one of the most intriguing phenomena of $f$-electron systems. Despite extensive efforts, a fundamental aspect which remains unsettled is the determination of the limiting cases for which MV emerges. Here we address this question for SmB$_6$, a prototypical MV system characterized by two nearly-degenerate Sm$^{2+}$ and Sm$^{3+}$ configurations. By combining angle resolved photoemission spectroscopy (ARPES) and x-ray absorption spectroscopy (XAS), we track the evolution of the mean Sm valence, $v_{Sm}$, in the Sm$_x$La$_{1-x}$B$_6$ series. Upon substitution of Sm ions with trivalent La, we observe a linear decrease of valence fluctuations to an almost complete suppression at $x$$\,$=$\,$0.2, with $v_{Sm}$$\,$$\sim$$\,$2; surprisingly, by further reducing $x$, a re-entrant increase of $v_{Sm}$ develops, approaching the value of $v_{imp}$$\,$$\sim$$\,$2.35 in the dilute-impurity limit. Such observation departs from a monotonic evolution of $v_{Sm}$ across the whole series, as well as from the expectation of its convergence to an integer value for $x$$\,$$\rightarrow$$\,$0. Our ARPES and XAS results, complemented by a phenomenological model, demonstrate an unconventional evolution of the MV character in the Sm$_x$La$_{1-x}$B$_6$ series, paving the way to further theoretical and experimental considerations on the concept of MV itself, and its influence on the macroscopic properties of rare-earth compounds in the dilute-to-intermediate impurity regime.

cond-mat.str-el↗

arXiv2024

Normal Fermi Surface in the Nodal Superconductor CeCoIn$_5$ Revealed via Thermal Conductivity

The thermal conductivity of heavy-fermion superconductor CeCoIn$_5$ was measured with a magnetic field rotating in the tetragonal a-b plane, with the heat current in the anti-nodal direction, $J$ || [100]. We observe a sharp resonance in thermal conductivity for the magnetic field at an angle $θ$ $\sim$ 12$^{\circ}$, measured from the heat current direction [100]. This resonance corresponds to the reported resonance at an angle $θ'$ $\sim$ 33$^{\circ}$ from the direction of the heat current applied along the nodal direction, $J$ || [110]. Both resonances, therefore, occur when the magnetic field is applied in the same crystallographic orientation in the two experiments, regardless of the direction of the heat current, proving conclusively that these resonances are due to the structure of the Fermi surface of CeCoIn$_5$. We argue that the uncondensed Landau quasiparticles, emerging with field, are responsible for the observed resonance. We support our experimental results with density-functional-theory model calculations of the density of states in a rotating magnetic field. Our calculations, using a model Fermi surface of CeCoIn$_5$, reveal several sharp peaks as a function of the field direction. Our study demonstrates that the thermal-conductivity measurement in rotating magnetic field can probe the normal parts of the Fermi surface deep inside the superconducting state.

cond-mat.supr-con↗

arXiv2024

Stabilization of U 5$f^2$ configuration in UTe$_2$ through U 6d dimers in the presence of Te2 chains

We investigate the topological superconductor candidate UTe$_2$ using high-resolution valence-band resonant inelastic x-ray scattering at the U $M_{4,5}$-edges. We observe atomic-like low-energy excitations that support the correlated nature of this unconventional superconductor. These excitations originate from the U $5f^2$ configuration, which is unexpected since the short Te2-Te2 distances exclude Te2 being 2-. By utilizing the photoionization cross-section dependence of the photoemission spectra in combination with band structure calculations, we infer that the stabilization of the U $5f^2$ configuration is due to the U $6d$ bonding states in the U-dimers acting as a charge reservoir. Our results emphasize that the description of the physical properties should commence with a $5f^2$ $ansatz$.

cond-mat.str-el↗