arXiv subjects

Brenden R. Ortiz

Publications and source records attributed to Brenden R. Ortiz.

At least 19 recordsLinked to original sources

arXiv2026

Complex magnetic phase diagrams in Tb$_2$IrAl$_4$Ge$_2$ and Er$_2$IrAl$_4$Ge$_2$

We report the synthesis and comprehensive investigation of single-crystalline Tb$_2$IrAl$_4$Ge$_2$ and Er$_2$IrAl$_4$Ge$_2$, the first Ir-based members of the $Ln_2M$Al$_4$Ge$_2$ family. Both compounds crystallize in the tetragonal Tb$_2$NiAl$_4$Ge$_2$-type structure (space group $I4/mmm$) and exhibit easy-axis magnetic anisotropy with the crystallographic $c$ axis as the preferred magnetization direction. Tb$_2$IrAl$_4$Ge$_2$ undergoes antiferromagnetic ordering below $T_N = 25$ K and develops a sequence of field-induced metamagnetic transitions accompanied by pronounced magnetotransport anomalies, including a nonlinear Hall response. In contrast, Er$_2$IrAl$_4$Ge$_2$ exhibits three successive zero-field magnetic transitions and a substantially richer low-temperature magnetic phase diagram comprising multiple competing field-induced phases confined to relatively modest magnetic fields. The differences between the Tb- and Er-based compounds demonstrate the wide range of magnetic behavior that can be realized within the \ce{Tb2NiAl4Ge2}-type structure. More broadly, the successful realization of Ir-based $Ln_2M$Al$_4$Ge$_2$ compounds considerably expands the accessible chemical space of this family and opens a route toward the exploration of related $4d$- and $5d$-based aluminogermanides.

cond-mat.mtrl-sci↗

arXiv2026

Type-I superconductivity in a quasi-2D topologically nontrivial YbBi$_2$

Intrinsic superconductivity in stoichiometric materials with nontrivial electronic topology remains uncommon, limiting opportunities to investigate how these two phenomena coexist within a single electronic system. Here, we report bulk type-I superconductivity below $T_c$ $\sim$ 0.9~K in YbBi$_2$, a layered rare-earth compound with a nonsymmorphic crystal structure and a quasi-two-dimensional Fermi surface. Thermodynamic and transport measurements establish the superconducting ground state, while quantum oscillations reveal exceptionally light carriers, with a cyclotron mass as low as 0.07 $m_e$, and a nonzero Berry phase of approximately 0.82 $π$. The latter closely matches the calculated value of the corresponding Wilson phase 0.99 $π$ for the corresponding orbit near a symmetry-protected band degeneracy. ARPES measurements show good agreement with key features of the calculated electronic structure, providing complementary experimental constraints on the normal-state band structure. The combination of intrinsic type-I superconductivity, light quasi-two-dimensional carriers, and signatures of nontrivial electronic topology identifies YbBi$_2$ as a distinct stoichiometric platform for investigating superconductivity in a topologically nontrivial electronic environment.

cond-mat.supr-con↗

arXiv2026

Band-Saddle-Point Engineering in Mixed $A$-site $A$V$_3$Sb$_5$ Kagome Metals

$A$V$_3$Sb$_5$ compounds ($A$ = K, Rb, Cs) offer an experimental platform for probing the physics of kagome metals near ideal Van Hove filling. Here, we use optical spectroscopy on $A$V$_3$Sb$_5$ with mixed $A$-site compositions to reveal the effect of the alkali metal on the low-energy electronic structure. Supported by density-functional-theory calculations, we identify a band-saddle-point inversion as a chemical consequence of Cs substitution, revealing that $A$-site composition controls the orbital character of Van Hove singularities, Fermi surface topology, and electron-phonon interactions while leaving the kagome layer intact.

cond-mat.str-el↗

arXiv2026

Disorder-induced proximate quantum spin ice phase in Pr2Sn2O7

Magnetic pyrochlores with non-Kramers rare-earth ions provide a platform for exploring emergent gauge physics and quantum spin-ice behavior, yet the influence of structural disorder on their ground states remains insufficiently understood. Here we combine bulk characterization and single-crystal neutron-scattering measurements to investigate the non-Kramers pyrochlore Pr2Sn2O7. At temperatures below ~1 K, the system exhibits key hallmarks of quantum spin-ice physics, including anisotropic spin-ice correlations and two distinct dynamical timescales. Upon further cooling, however, we observe a complete spin-freezing transition at T_f ~ 0.15 K, accompanied by recovery of the full nuclear Schottky anomaly, the emergence of a gapped magnetic excitation, and the development of incipient (100) magnetic correlations. Comparison with related Pr-based pyrochlores places Pr2Sn2O7 near the spin-frozen boundary of a disorder-perturbed phase diagram. These results establish a disorder-driven framework for the evolution of quantum spin-ice behavior into frozen ground states, revealing how signatures of a proximate quantum spin liquid can persist despite disorder-induced spin freezing in non-Kramers pyrochlores.

cond-mat.str-el↗

arXiv2026

Fermiology of the kagome compound LuNb6Sn6 probed by de Haas-van Alphen oscillations

We report a detailed de Haas-van Alphen (dHvA) study of the recently discovered kagome metal LuNb6Sn6 using torque magnetometry, magnetization, and heat-capacity measurements. Temperature-dependent torque and heat-capacity data reveal a charge density wave (CDW) transition at T_CDW = 85 K. The thermal hysteresis observed in both measurements establishes the first-order nature of the transition. Quantum oscillation measurements identify two major dHvA frequencies: F_alpha ~ 20 T and F_beta ~ 200 T, and their angular dependence is consistent with ellipsoidal Fermi surface (FS) pockets. Landau fan diagram analysis reveals evidence for a nontrivial Berry phase associated with the F_alpha pocket, indicating possible nontrivial electronic topology in LuNb6Sn6. Analysis of the temperature and magnetic field dependence of the oscillations using the Lifshitz-Kosevich formula yields electronic parameters that indicate anisotropic quantum transport properties. First-principles calculations provide further insight into the electronic structure, revealing Dirac-like band crossings, a flat band, and multiple van Hove singularities near the Fermi level. Our calculations based on the pristine phase cannot fully reproduce the experimentally observed quantum oscillation frequencies, suggesting that CDW-induced FS reconstruction plays a crucial role in the ground-state electronic structure of LuNb6Sn6. These results provide new insight into the FS topology and electronic structure of LuNb6Sn6, enriching our understanding of the electronic properties of kagome materials.

cond-mat.str-el↗

arXiv2026

Anisotropic magnetism and Kondo-lattice behavior in the frustrated antiferromagnet Ce3MgBi5

We report the synthesis and physical characterization of single-crystalline Ce3MgBi5, a previously unexplored member of the Ce3MPn5 family. This compound crystallizes in the hexagonal P63/mcm structure, featuring an anisotropic Ce sublattice composed of zig-zag chains along the c axis and a distorted kagome-like network in the basal plane. Magnetization measurements reveal antiferromagnetic order below TN approximately 4.2 K, accompanied by strong magnetic anisotropy and multiple field-induced metamagnetic transitions for fields applied perpendicular to [001], leading to a dome-shaped H-T phase diagram. Electrical transport exhibits characteristic signatures of a Ce-based Kondo lattice, including broad resistivity maxima and pronounced field-dependent anomalies in the magnetoresistance and Hall response that track the magnetic phase boundaries. Specific-heat measurements confirm the magnetic transition and show that the full R ln 2 entropy expected for a Ce3+ Kramers doublet is recovered by 20 K, indicating an extended temperature range of magnetic fluctuations consistent with Kondo correlations. Our results establish Ce3MgBi5 as a platform within the Ce3MPn5 family for exploring the interplay of geometric frustration, magnetic anisotropy, and Kondo-lattice physics under applied magnetic fields.

cond-mat.str-el↗

arXiv2026

Dynamical moment fragmentation in the all-in all-out pyrochlore Nd2Sn2O7

We report single-crystal neutron spectroscopy and bulk characterization on hydrothermally grown Nd2Sn2O7, revealing a dynamical moment fragmentation embedded within the all-in-all-out ordered state. The spectra show a nearly flat band with pinch-point-like momentum dependence, accompanied by dispersive branches that generate half-moon features across multiple Brillouin zones. These defining signatures are captured quantitatively by a minimal dipolar-octupolar spin Hamiltonian, demonstrating excellent agreement between experiment and theory. The higher flat-mode energy helps account for the absence of dynamical interference in prior muon spin relaxation (muSR) studies, while the lack of any photon-like excitation imposes strict constraints on the proposed Coulombic antiferromagnet scenario. Our results extend dynamical moment fragmentation to Nd2Sn2O7 and identify it as a clean, tractable platform for quantitative exploration of emergent gauge-field physics and multipolar spin-wave dynamics in frustrated magnets.

cond-mat.str-el↗

arXiv2026

Synergistic doping and stabilization of magnetically tunable LnTi$_3$(Sb,Sn)$_4$ (Ln:Ce--Gd) kagome metals

Here we present our synthesis and characterization of the LnTi$_3$(Sb,Sn)$_4$ (Ln: Ce, Pr, Nd, Sm, Gd) family of cleavable kagome metals. While these materials are isostructural to the LnTi$_3$Bi$_4$ family, they only form as (Sb,Sn) solid-solutions with no corresponding LnTi$_3$Sb$_4$ or LnTi$_3$Sn$_4$ phases. We use a combination of first-principles density functional theory (DFT) and Crystal Orbital Hamilton Population (COHP) calculations to show that (Sb,Sn) alloying has a stabilizing effect on the structure by adjusting the Fermi level, filling bonding states, depopulating antibonding states, and adjusting the density-of-states (DOS) towards local minima, an effect we call ``synergistic doping.'' The tunable Fermi level also has a profound effect on the magnetism, which we demonstrate through a detailed characterization of the SmTi$_3$(Sb,Sn)$_4$ series. The series hosts multiple magnetic ground states resulting from competing magnetic interactions that are tunable by the (Sb,Sn) ratio. While the focus of this work is on SmTi$_3$(Sb,Sn)$_4$, we briefly comment on the (Sb,Sn) solubility range and the conferred magnetic tunability in the other rare-earths compounds (Ln: Ce, Pr, Nd, Gd) as well. Our work demonstrates how the (Sb,Sn) synergistic pair can be used to stabilize the LnTi$_3$(Sb,Sn)$_4$ structure while simultaneously providing a means to tune the magnetism, ultimately providing a potential route to develop new intermetallics with chemical, magnetic, and electronic tunability.

cond-mat.str-el↗

arXiv2026

Revealing Rotational Symmetry Breaking Charge-density Wave Order in Kagome Superconductor (Rb, K)V$_3$Sb$_5$ by Ultrafast Pump-probe Experiments

The recently discovered Kagome superconductor AV$_3$Sb$_5$ (where A refers to K, Rb, Cs) has stimulated widespread research interest due to its interplay of non-trivial topology and unconventional correlated physics including charge-density waves (CDW) and superconductivity. The essential prerequisite to understanding the microscopic mechanisms of this complex electronic landscape is to unveil the configuration and symmetry of the charge-density wave order. As to now, little consensus has been made on what symmetry is broken. Herein, we clarify the microscopic structure and symmetry breaking of the CDW phase in RbV$_3$Sb$_5$ and KV$_3$Sb$_5$ by ultrafast time-resolved reflectivity. Our approach is based on extracting coherent phonon spectra induced by three-dimensional CDW and comparing them to calculated phonon frequencies via density-functional theory. The combination of these experimental results and calculations provides compelling evidence that the CDW structure of both compounds prevailing up to T$_{\text{CDW}}$ is the 2 $\times$ 2 $\times$ 2 staggered inverse Star-of-David pattern with interlayer $π$ phase shift, in which the six-fold rotational symmetry is broken. These observations thus corroborate six-fold rotational symmetry breaking throughout the CDW phase of RbV$_3$Sb$_5$ and KV$_3$Sb$_5$.

cond-mat.str-el↗

arXiv2026

Common sublattice-pure van Hove singularities in the kagome superconductors $\textit{A}$V$_{3}$Sb$_{5}$ ($\textit{A}$ = K, Rb, Cs)

Kagome materials offer a versatile platform for exploring correlated and topological quantum states, where van Hove singularities (VHSs) play a pivotal role in driving electronic instabilities, exhibiting distinct behaviors depending on electron filling and interaction settings. In the recently discovered kagome superconductors $\textit{A}$V$_{3}$Sb$_{5}$ ($\textit{A}$ = K, Rb, Cs), unconventional charge density wave order, superconductivity, and electronic chirality emerge, yet the nature of VHSs near the Fermi level ($\textit{E}$$_{F}$) and their connection to these exotic orders remain elusive. Here, using high-resolution polarization-dependent angle-resolved photoemission spectroscopy, we uncover a universal electronic structure across $\textit{A}$V$_{3}$Sb$_{5}$ that is distinct from density-functional theory predictions that show noticeable discrepancies. We identify multiple common sublattice-pure VHSs near $\textit{E}$$_{F}$, arising from strong V-$\textit{d}$/Sb-$\textit{p}$ hybridization, which significantly promote bond-order fluctuations and likely drive the observed charge density wave order. These findings provide direct spectroscopic evidence for hybridization-driven VHS formation in kagome metals and establish a unified framework for understanding the intertwined electronic instabilities in $\textit{A}$V$_{3}$Sb$_{5}$.

cond-mat.supr-con↗

arXiv2025

Observation of multiple flat bands and van Hove singularities in the distorted kagome metal NdTi3Bi4

Kagome materials have attracted enormous research interest recently owing to their diverse topological phases and manifestation of electronic correlation. Here, we present the electronic structure of a distorted ferromagnetic kagome metal, NdTi3Bi4, exhibiting a transition temperature of 9 K. Our investigation employs a combination of angle-resolved photoemission spectroscopy (ARPES) measurements and density functional theory (DFT) calculations. We discover the presence of two flat bands which are found to originate from the kagome structure formed by Ti atoms with major contribution from Ti dxy and Ti dx2-y2 orbitals. We also observed multiple van Hove singularities (VHSs) in its electronic structure, with one VHS lying near the Fermi level. The ARPES data reveals the existence of Dirac cone at the K point, a finding which is corroborated by our DFT calculations. These findings present detailed electronic structure capable of hosting correlation-driven phenomenon in this novel ferromagnetic kagome metal.

cond-mat.mtrl-sci↗

arXiv2025

Interplay between charge correlations and superconductivity across the superconducting domes of CsV$_{3}$Sb$_{5-x}$Sn$_x$

The kagome metal CsV$_3$Sb$_5$ shows an unconventional interplay between charge density wave (CDW) order and superconductivity. Tuning the band filling is known to rapidly suppress long-range CDW order and drive the formation of two superconducting ``domes" upon increasing hole concentration. Here we determine the detailed evolution of charge correlations across this phase diagram and resolve their interplay with the superconducting state. Upon light hole-doping, the suppression of a metastable $2\times 2\times 4$ CDW state coincides with the suppression of superconducting fluctuations present in the parent CsV$_3$Sb$_5$ compound. Continued doping suppresses long-range $2\times 2\times 2$ CDW order, leaving remnant short-range, quasi-1D correlations that persist across the second superconducting dome. These higher temperature charge correlations are seemingly essential to the lower temperature superconducting state, as charge correlations vanish coincident with superconductivity as a function of hole-doping. A multidomain model of short-range V-V dimer formation within the kagome plane is proposed in the second superconducting dome, where rotational and translational symmetry remain locally broken even in the absence of long-range CDW order.

cond-mat.supr-con↗

arXiv2025

Pervasive electronic nematicity as the parent state of kagome superconductors

Kagome superconductors $A$V$_3$Sb$_5$ ($A$ = Cs, K, Rb) have developed into an exciting playground for realizing and exploring exotic solid state phenomena. Abundant experimental evidence suggests that electronic structure breaks rotational symmetry of the lattice, but whether this may be a simple consequence of the symmetry of the underlying 2 $\times$ 2 charge density wave phase or an entirely different mechanism remains intensely debated. We use spectroscopic imaging scanning tunneling microscopy to explore the phase diagram of the prototypical kagome superconductor CsV$_3$Sb$_5$ as a function of doping. We intentionally suppress the charge density wave phase with chemical substitutions selectively introduced at two distinct lattice sites, and investigate the resulting system. We discover that rotational symmetry breaking of the electronic structure -- now present in short-range nanoscale regions -- persists in all samples, in a wide doping range long after all charge density waves have been suppressed. As such, our experiments uncover ubiquitous electronic nematicity across the $A$V$_3$Sb$_5$ phase diagram, unrelated to the 2 $\times$ 2 charge density wave. This further points towards electronic nematicity as the intrinsic nature of the parent state of kagome superconductors, under which other exotic low-temperature phenomena subsequently emerge.

cond-mat.str-el↗

arXiv2025

Cluster spin glass correlations and dynamics in Zn$_{0.5}$Mn$_{0.5}$Te

We present a magnetometry, muon spin relaxation ($μ$SR), and neutron scattering study of the insulating face-centered-cubic spin glass Zn$_{0.5}$Mn$_{0.5}$Te. The magnetometry and $μ$SR results confirm a spin freezing transition around $T_f \approx 23$ K, with the spin fluctuation rate decreasing gradually and somewhat inhomogeneously through the sample volume as the temperature decreases toward $T_f$. Characteristic spin correlation times well above $T_f$ are on the order of 10$^{-10}$ s, in line with expectations for a cluster spin glass. Using magnetic pair distribution function (mPDF) analysis and reverse Monte Carlo (RMC) modeling of the magnetic diffuse neutron scattering data, we show that the spin-glass ground state consists of clusters of spins exhibiting short-range-ordered type-III antiferromagnetic correlations, with a locally ordered moment of 3.1(1) $μ_{\mathrm{B}}$ between nearest-neighbor spins. The type-III correlations decay exponentially as a function of spin separation distance with a correlation length of approximately 5 Å. The diffuse magnetic scattering and corresponding mPDF show no significant changes across $T_f$, indicating that the dynamically fluctuating short-range spin correlations in the paramagnetic state retain the same basic type-III configuration; the only change apparent from the neutron scattering data is a gradual reduction of the correlation length and locally ordered moment with increasing temperature. Taken together, these results paint a unique and detailed picture of the local magnetic structure and dynamics in Zn$_{0.5}$Mn$_{0.5}$Te and show that this material is best described as a cluster spin glass. In addition, this work showcases a statistical method for extracting diffuse scattering signals from neutron powder diffraction data.

cond-mat.mtrl-sci↗

arXiv2025

Magnetic dilution in the triangular lattice antiferromagnet NaYb$_{1-x}$Lu$_{x}$O$_2$

The delafossite-like compound NaYbO$_2$ hosts a triangular lattice of Yb$^{3+}$ moments and is a promising candidate for the realization of a quantum spin liquid ground state -- an exotic, quantum-disordered magnetic phase featuring long-range entanglement of spins. Tuning this system away from this quantum-disordered regime toward classical order or spin freezing is a powerful approach to shed light on the nature of the parent ground state. Here we leverage the substitution of nonmagnetic Lu$^{3+}$ onto the Yb$^{3+}$ sites to study the effects of magnetic disorder in NaYbO$_2$ using low-temperature ac susceptibility, heat capacity, and muon spin relaxation ($μ$SR) measurements. Our $μ$SR measurements reveal resilient, correlated magnetic fluctuations that persist to at least 15\% dilution, precluding conventional spin freezing and magnetic inhomogeneity. Heat capacity and magnetic susceptibility resolve a rapid suppression of the field-induced ``up-up-down'' magnetic order upon dilution and a crossover in the power-law behavior of the low-temperature magnetic excitations associated with the zero-field quantum disordered ground state. Taken together, these results support the notion of a robust network of entangled moments in NaYbO$_2$, and provides experimental validation of several models of a Heisenberg triangular lattice antiferromagnet in the presence of disorder.

cond-mat.str-el↗

arXiv2025

Supercurrent interference and its transfer in a kagome superconductor

Superconductivity represents a macroscopic quantum state notable for its rich manifestations of electronic coherence and collective behavior. Kagome materials AV3Sb5 (A= K, Cs, Rb) possess cascade intertwined quantum phases including superconductivity, symmetry-breaking charge orders, nematic orders and topological states, making them attractive materials for exploring exotic superconducting states. However, the superconducting properties and the Cooper pairing behaviors have not been fully explored and understood. In this work, by studying both the magnetoresistance and critical current behaviors in KV3Sb5 ring and pristine flakes, we reveal the charge 2e paring in KV3Sb5 although anomalous oscillations with smaller periodicity were observed, and report the intrinsic superconducting phase coherence in KV3Sb5 flakes. The former is demonstrated by the careful verification of the Little-Parks oscillations in differential resistance colormaps, and the latter indicates the existence of superconducting domains in KV3Sb5. Moreover, we observed a special phenomenon: the transfer of supercurrent interference patterns between the superconducting ring and the superconducting flake, which demonstrates the global critical current effect of the superconducting phase coherence. These findings provide new insights into the Cooper pairing behaviors in KV3Sb5 and highlight the importance of global effect of superconducting phase coherence in the understanding of the superconducting behaviors.

cond-mat.supr-con↗

arXiv2025

Coherent Phonon Pairs and Rotational Symmetry Breaking of Charge Density Wave Order in the Kagome Superconductor CsV$_3$Sb$_5$

In this work, we perform ultrafast time-resolved reflectivity measurements to study the symmetry breaking in the charge-density wave (CDW) phase of CsV$_3$Sb$_5$. By extracting the coherent phonon spectrum in the CDW phase of CsV$_3$Sb$_5$, we discover close phonon pairs near 1.3 THz and 3.1 THz, as well as a new mode at 1.84 THz. The 1.3 THz phonon pair and the 1.84 THz mode are observed up to the CDW transition temperature. Combining density-functional theory calculations, we point out these phonon pairs arise from the coexistence of Star-of-David and inverse Star-of-David distortions combined with six-fold rotational symmetry breaking. An anisotropy in the magnitude of transient reflectivity change is also revealed at the onset of CDW order. Our results thus indicate broken six-fold rotational symmetry in the charge-density wave state of CsV$_3$Sb$_5$, along with the absence of nematic fluctuation above T$_{\text{CDW}}$. Meanwhile, the measured coherent phonon spectrum in the CDW phase of CsV$_3$Sb$_{5-\text{x}}$Sn$_\text{x}$ with x = 0.03-0.04 matches with staggered inverse Star-of-David with interlayer $π$ phase shift. This CDW structure contrasts with undoped CsV$_3$Sb$_5$ and explains the evolution from phonon pair to a single mode at 1.3 THz by x = 0.03-0.04 Sn-doping.

cond-mat.mtrl-sci↗

arXiv2025

Observation of Anomalous Thermal Hall Effect in a Kagome Superconductor

Broken time-reversal symmetry (TRS) in superconductors can induce not only spontaneous magnetization by the finite angular momentum of Cooper pairs, but also anomalous thermal Hall effects (ATHEs), whose detection has been extremely challenging. Here we report the successful observation of an ATHE developing below the superconducting transition temperature at zero magnetic field in the kagome-lattice superconductor CsV3Sb5. This finding is verified by the absence of a signal in a conventional type-II superconductor using the same setup and by ruling out the trapped-vortex effects through micro-Hall array measurements. Remarkably, both the temperature dependence and the magnitude of the observed anomalous thermal Hall conductivity are quite different from those expected for the quantized thermal edge current of an intrinsic ATHE, but consistent with extrinsic impurity-induced ATHEs in chiral superconductivity. Our study of ATHE offers an alternative approach to probe TRS breaking in the superconducting states.

cond-mat.supr-con↗