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Brenden R. Ortiz

Publications and source records attributed to Brenden R. Ortiz.

At least 55 records · Page 3Linked to original sources
arXiv2024

Optical Manipulation of the Charge Density Wave state in RbV3Sb5

Broken time-reversal symmetry in the absence of spin order indicates the presence of unusual phases such as orbital magnetism and loop currents. The recently discovered family of kagome superconductors AV$_3$Sb$_5$ (A = K, Rb, or Cs), hosting an exotic charge-density wave (CDW) state, has emerged as a strong candidate for this phase. While initial experiments suggested that the CDW phase breaks time-reversal symmetry, this idea is being intensely debated due to conflicting experimental data. In this work we use laser-coupled scanning tunneling microscopy (STM) to study RbV$_3$Sb$_5$. STM data shows that the Fourier intensities of all three CDW peaks are different, implying that the CDW breaks rotational and mirror symmetries. By applying linearly polarized light along high-symmetry directions, we show that the relative intensities of the CDW peaks can be reversibly switched, implying a substantial electro-striction response, indicative of strong non-linear electron-phonon coupling. A similar CDW intensity switching is observed with perpendicular magnetic fields, which implies an unusual piezo-magnetic response that, in turn, requires time-reversal symmetry-breaking. We show that the simplest CDW that satisfies these constraints and reconciles previous seemingly contradictory experimental data is an out-of-phase combination of bond charge order and loop currents that we dub congruent CDW flux phase. Our laser-STM data opens the door to the possibility of dynamic optical control of complex quantum phenomenon in correlated materials.

cond-mat.str-el↗
arXiv2024

Quantifying magnetic field driven lattice distortions in kagome metals at the femto-scale using scanning tunneling microscopy

A wide array of unusual phenomena has recently been uncovered in kagome solids. The charge density wave (CDW) state in the kagome superconductor AV3Sb5 in particular intrigued the community -- the CDW phase appears to break the time-reversal symmetry despite the absence of spin magnetism, which has been tied to exotic orbital loop currents possibly intertwined with magnetic field tunable crystal distortions. To test this connection, precise determination of the lattice response to applied magnetic field is crucial, but can be challenging at the atomic-scale. We establish a new scanning tunneling microscopy based method to study the evolution of the AV3Sb5 atomic structure as a function of magnetic field. The method substantially reduces the errors of typical STM measurements, which are at the order of 1% when measuring an in-plane lattice constant change. We find that the out-of-plane lattice constant of AV3Sb5 remains unchanged (within 10^-6) by the application of both in-plane and out-of-plane magnetic fields. We also reveal that the in-plane lattice response to magnetic field is at most at the order of 0.05%. Our experiments provide further constraints on time-reversal symmetry breaking in kagome metals, and establish a new tool for higher-resolution extraction of the field-lattice coupling at the nanoscale applicable to other quantum materials.

cond-mat.str-el↗
arXiv2023

Nanoscale strain manipulation of smectic susceptibility in kagome superconductors

Exotic quantum solids can host electronic states that spontaneously break rotational symmetry of the electronic structure, such as electronic nematic phases and unidirectional charge density waves (CDWs). When electrons couple to the lattice, uniaxial strain can be used to anchor and control this electronic directionality. Here we reveal an unusual impact of strain on unidirectional "smectic" CDW orders in kagome superconductors AV3Sb5 using spectroscopic-imaging scanning tunneling microscopy. We discover local decoupling between the smectic electronic director axis and the direction of anisotropic strain. While the two are generally aligned along the same direction in regions of small CDW gap, the two become misaligned in regions where CDW gap is the largest. This in turn suggests nanoscale variations in smectic susceptibility, which we attribute to a combination of local strain and electron correlation strength. Overall, we observe an unusually high decoupling rate between the smectic electronic director of the 3-state Potts order and anisotropic strain, revealing weak smecto-elastic coupling in the CDW phase of kagome superconductors. This is phenomenologically different from the extensively studied nemato-elastic coupling in the Ising nematic phase of Fe-based superconductors, providing a contrasting picture of how strain can control electronic unidirectionality in different families of quantum materials.

cond-mat.supr-con↗
arXiv2023

AV$_3$Sb$_5$ Kagome Superconductors: Progress and Future Directions

The recent discovery of the AV$_3$Sb$_5$ (A=K, Rb, Cs) kagome superconductors launched a growing field of research investigating electronic instabilities in kagome metals. Specifically, the AV$_3$Sb$_5$ family naturally exhibits a Fermi level tuned to the Van Hove singularities associated with the saddle points formed from the prototypical kagome band structure. The charge density wave and superconducting states that form within the kagome networks of these compounds exhibit a number of anomalous properties reminiscent of theoretical predictions of exotic states in kagome metals tuned close to their Van Hove fillings. Here we provide an overview of the key structural and electronic features of AV$_3$Sb$_5$ compounds and review the status of investigations into their unconventional electronic phase transitions.

cond-mat.supr-con↗
arXiv2023

Resolving the discrepancy between MOKE measurements at 1550-nm wavelength on Kagome Metal CsV3Sb5

Kagome metals AV3Sb5 (A = K, Cs, Rb) provide a rich platform for intertwined orders such as the charge density wave (CDW) and a chiral order with time-reversal symmetry breaking (TRSB). While early reports of large optical polarization rotations have been interpreted as the magneto-optic Kerr effect (MOKE) and as evidence for TRSB, recent dedicated optical rotation and MOKE experiments have clarified that this large optical rotation originates instead from an unconventional specular rotation. Yet a critical discrepancy remains regarding the possible existence of a true spontaneous MOKE signal: in experiments performed after training with modest magnetic fields of up to 0.3 Tesla, no MOKE signal was detected above the noise floor of 30 nano-radians, while micro-radian-level signals were found in an experiment using higher training fields. This raises an intriguing possibility of different zero-field ground states with opposite time-reversal symmetry properties, because of different magnetic histories. To unambiguously determine whether a training-field-dependent spontaneous MOKE signal exists in CsV3Sb5, we conduct comprehensive MOKE measurements with two Sagnac interferometer setups capable of both low and high training fields of up to 9 Tesla, and perform careful analyses of contributions of signals from various optical components. We conclude that there is no observable spontaneous MOKE signal, hence no optical evidence for TRSB, regardless of the magnitude of training fields and the speed of temperature ramping.

cond-mat.str-el↗
arXiv2023

Pressure evolution of electron dynamics in the superconducting kagome metal CsV$_3$Sb$_5$

The coexistence of the charge-density wave (CDW) and superconducting phases and their tunability under external pressure remains one of the key points in understanding the electronic structure of $A$V$_3$Sb$_5$ ($A$ = K, Rb, Cs) kagome metals. Here, we employ synchrotron-based infrared spectroscopy assisted by density-functional calculations to study the pressure evolution of the electronic structure at room temperature up to 17 GPa experimentally. The optical spectrum of CsV$_3$Sb$_5$ is characterized by the presence of localized carriers seen as a broad peak at finite frequencies in addition to the conventional metallic Drude response. The pressure dependence of this low-energy peak reflects the re-entrant behavior of superconductivity and may be interpreted in terms of electron-phonon coupling, varying with the growth and shrinkage of the Fermi surface. Moreover, drastic modifications in the low-energy interband absorptions are observed upon the suppression of CDW. These changes are related to the upward shift of the Sb2 $p_x+p_y$ band that eliminates part of the Fermi surface around the $M$-point, whereas band saddle points do not move significantly. These observations shed new light on the mixed electronic and lattice origin of the CDW in CsV$_3$Sb$_5$.

cond-mat.str-el↗
arXiv2023

Revisiting spin ice physics in the ferromagnetic Ising pyrochlore Pr$_2$Sn$_2$O$_7$

Pyrochlore materials are characterized by their hallmark network of corner-sharing rare-earth tetrahedra, which can produce a wide array of complex magnetic ground states. Ferromagnetic Ising pyrochlores often obey the "two-in-two-out" spin ice rules, which can lead to a highly-degenerate spin structure. Large moment systems, such as Ho$_2$Ti$_2$O$_7$ and Dy$_2$Ti$_2$O$_7$, tend to host a classical spin ice state with low-temperature spin freezing and emergent magnetic monopoles. Systems with smaller effective moments, such as Pr$^{3+}$-based pyrochlores, have been proposed as excellent candidates for hosting a "quantum spin ice" characterized by entanglement and a slew of exotic quasiparticle excitations. However, experimental evidence for a quantum spin ice state has remained elusive. Here, we show that the low-temperature magnetic properties of Pr$_2$Sn$_2$O$_7$ satisfy several important criteria for continued consideration as a quantum spin ice. We find that Pr$_2$Sn$_2$O$_7$ exhibits a partially spin-frozen ground state with a large volume fraction of dynamic magnetism. Our comprehensive bulk characterization and neutron scattering measurements enable us to map out the magnetic field-temperature phase diagram, producing results consistent with expectations for a ferromagnetic Ising pyrochlore. We identify key hallmarks of spin ice physics, and show that the application of small magnetic fields ($μ_0 H_c \sim$0.75T) suppresses the spin ice state and induces a long-range ordered magnetic structure. Together, our work clarifies the current state of Pr$_2$Sn$_2$O$_7$ and encourages future studies aimed at exploring the potential for a quantum spin ice ground state in this system.

cond-mat.str-el↗
arXiv2023

Frustrated charge order and cooperative distortions in ScV6Sn6

Here we study the stability of charge order in the kagome metal ScV6Sn6. Synchrotron x-ray diffraction measurements reveal high-temperature, short-range charge correlations at the wave vectors along q=(1/3,1/3,1/2) whose inter-layer correlation lengths diverge upon cooling. At the charge order transition, this divergence is interrupted and long-range order freezes in along q=(1/3,1/3,1/3), as previously reported, while disorder enables the charge correlations to persist at the q=(1/3,1/3,1/2) wave vector down to the lowest temperatures measured. Both short-range and long-range charge correlations seemingly arise from the same instability and both are rapidly quenched upon the introduction of larger Y ions onto the Sc sites. Our results validate the theoretical prediction of the primary lattice instability at q=(1/3,1/3,1/2), and we present a heuristic picture for viewing the frustration of charge order in this compound.

cond-mat.str-el↗
arXiv2023

Three-state nematicity and magneto-optical Kerr effect in the charge density waves in kagome superconductors

The kagome lattice provides a fascinating playground to study geometrical frustration, topology and strong correlations. The newly-discovered kagome metals AV$_3$Sb$_5$ (where A can refer to K, Rb, or Cs) exhibit phenomena including topological band structure, symmetry-breaking charge-density waves and superconductivity. Nevertheless, the nature of the symmetry breaking in the charge-density wave phase is not yet clear, despite the fact that it is crucial in order to understand whether the superconductivity is unconventional. In this work, we perform scanning birefringence microscopy on all three members of this family and find that six-fold rotation symmetry is broken at the onset of the charge-density wave transition in all these compounds. We show that the three nematic domains are oriented at 120$^\circ$ to each other and propose that staggered charge-density wave orders with a relative $π$ phase shift between layers is a possibility that can explain these observations. We also perform magneto-optical Kerr effect and circular dichroism measurements. The onset of both signals is at the transition temperature, indicating broken time-reversal symmetry and the existence of the long-sought loop currents in that phase.

cond-mat.str-el↗
arXiv2023

Evolution of highly anisotropic magnetism in the titanium-based kagome metals LnTi$_3$Bi$_4$ (Ln: La...Gd$^{3+}$, Eu$^{2+}$, Yb$^{2+}$)

Here we present the family of titanium-based kagome metals of the form LnTi$_3$Bi$_4$ (Ln: La...Gd$^{3+}$, Eu$^{2+}$, Yb$^{2+}$). Single crystal growth methods are presented alongside detailed magnetic and thermodynamic measurements. The orthorhombic (Fmmm) LnTi$_3$Bi$_4$ family of compounds exhibit slightly distorted titanium-based kagome nets interwoven with zig-zag lanthanide-based (Ln) chains. Crystals are easily exfoliated parallel to the kagome sheets and angular resolved photoemission (ARPES) measurements highlight the intricacy of the electronic structure in these compounds, with Dirac points existing at the Fermi level. The magnetic properties and the associated anisotropy emerge from the quasi-1D zig-zag chains of Ln, and impart a wide array of magnetic ground states ranging from anisotropic ferromagnetism to complex antiferromagnetism with a cascade of metamagnetic transitions. Kagome metals continue to provide a rich direction for the exploration of magnetic, topologic, and highly correlated behavior. Our work here introduces the LnTi$_3$Bi$_4$ compounds to augment the continuously expanding suite of complex and interesting kagome materials.

cond-mat.mtrl-sci↗
arXiv2023

YbV$_3$Sb$_4$ and EuV$_3$Sb$_4$, vanadium-based kagome metals with Yb$^{2+}$ and Eu$^{2+}$ zig-zag chains

Here we present YbV$_3$Sb$_4$ and EuV$_3$Sb$_4$, two new compounds exhibiting slightly distorted vanadium-based kagome nets interleaved with zig-zag chains of divalent Yb$^{2+}$ and Eu$^{2+}$ ions. Single crystal growth methods are reported alongside magnetic, electronic, and thermodynamic measurements. YbV$_3$Sb$_4$ is a nonmagnetic metal with no collective phase transitions observed between 60mK and 300K. Conversely, EuV$_3$Sb$_4$ is a magnetic kagome metal exhibiting easy-plane ferromagnetic-like order below $T_\text{C}$=32K with signatures of noncollinearity under low field. Our discovery of YbV$_3$Sb$_4$ and EuV$_3$Sb$_4$ demonstrate another direction for the discovery and development of vanadium-based kagome metals while incorporating the chemical and magnetic degrees of freedom offered by a rare-earth sublattice.

cond-mat.mtrl-sci↗
arXiv2023

Electron-hole asymmetry in the phase diagram of carrier-tuned CsV$_3$Sb$_5$

Here we study the effect of electron doping the kagome superconductor CsV$_3$Sb$_5$. Single crystals and powders of CsV$_3$Sb$_{5-x}$Te$_x$ are synthesized and characterized via magnetic susceptibility, nuclear quadrupole resonance, and x-ray diffraction measurements, where we observe a slight suppression of the charge density wave transition temperature and superconducting temperature with the introduction of electron dopants. In contrast to hole-doping, both transitions survive relatively unperturbed up to the solubility limit of Te within the lattice. A comparison is presented between the electronic phase diagrams of electron- and hole-tuned CsV$_3$Sb$_5$.

cond-mat.supr-con↗
arXiv2023

Electronic nematicity without charge density waves in titanium-based kagome metal

Layered crystalline materials that consist of transition metal atoms on a kagome network have emerged as a versatile platform to study unusual electronic phenomena. For example, in the vanadium-based kagome superconductors AV3Sb5 (where A can stand for K, Cs, or Rb) there is a parent charge density wave phase that appears to simultaneously break both the translational and the rotational symmetry of the lattice. Here, we show a contrasting situation where electronic nematic order - the breaking of rotational symmetry without the breaking of translational symmetry - can occur without a corresponding charge density wave. We use spectroscopic-imaging scanning tunneling microscopy to study the kagome metal CsTi3Bi5 that is isostructural to AV3Sb5 but with a titanium atom kagome network. CsTi3Bi5 does not exhibit any detectable charge density wave state, but comparison to density functional theory calculations reveals substantial electronic correlation effects at low energies. Comparing the amplitudes of scattering wave vectors along different directions, we discover an electronic anisotropy that breaks the six-fold symmetry of the lattice, arising from both in-plane and out-of-plane titanium-derived d orbitals. Our work uncovers the role of electronic orbitals in CsTi3Bi5, suggestive of a hexagonal analogue of the nematic bond order in Fe-based superconductors.

cond-mat.str-el↗
arXiv2023

High Resolution Polar Kerr Effect Studies of CsV${}_3$Sb${}_5$: Tests for Time Reversal Symmetry Breaking Below the Charge Order Transition

We report high resolution polar Kerr effect measurements on CsV${}_3$Sb${}_5$ single crystals in search for signatures of spontaneous time reversal symmetry breaking below the charge order transition at $T^* \approx 94$ K. Utilizing two different versions of zero-area loop Sagnac interferometers operating at 1550 nm wavelength, each with the fundamental attribute that without a time reversal symmetry breaking sample at its path, the interferometer is perfectly reciprocal, we find no observable Kerr effect to within the noise floor limit of the apparatus at 30 nanoradians. Simultaneous coherent reflection ratio measurements confirm the sharpness of the charge order transition in the same optical volume as the Kerr measurements. At finite magnetic field we observe a sharp onset of a diamagnetic shift in the Kerr signal at $T^*$, which persists down to the lowest temperature without change in trend. Since 1550 nm is an energy that was shown to capture all features of the optical properties of the material that interact with the charge order transition, we are led to conclude that it is highly unlikely that time reversal symmetry is broken in the charge ordered state in CsV${}_3$Sb${}_5$.

cond-mat.str-el↗
arXiv2023

Effect of nonhydrostatic pressure on the superconducting kagome metal CsV$_3$Sb$_5$

High-pressure single-crystal x-ray diffraction experiments reveal that the superconducting kagome metal CsV$_3$Sb$_5$ transforms from hexagonal ($P6/mmm$) to monoclinic ($C2/m$) symmetry above 10 GPa if nonhydrostatic pressure conditions are created in a diamond anvil cell with silicon oil as pressure-transmitting medium. This is contrary to the behavior of CsV$_3$Sb$_5$ under quasi-hydrostatic conditions in neon, with the hexagonal symmetry retained up to at least 20 GPa. Monoclinic distortion leaves the kagome planes almost unchanged but deforms honeycomb nets of the Sb atoms. While the onset of the distortion almost coincides with the reentrance of superconductivity, our \textit{ab initio} density-functional calculations reveal only minor changes in the electronic structure compared to the quasi-hydrostatic case. In particular, Fermi surface reconstruction driven by the formation of interlayer Sb-Sb bonds is observed in both monoclinic and hexagonal CsV$_3$Sb$_5$ structures at high pressures and comes out as the likely cause for the reentrant behavior.

cond-mat.supr-con↗
arXiv2023

Unconventional Optical Rotation in the Charge Ordered State of Kagome Metal CsV3Sb5

Kagome metals AV3Sb5 (A = K, Cs, Rb) provide a rich platform for intertwined orders, where evidence for time-reversal symmetry breaking, likely due to the long-sought loop currents, has emerged in STM and muon spin relaxation experiments. An isotropic component in the spontaneous optical rotation has also been reported, and it was interpreted as the magneto-optic Kerr effect arising from time-reversal symmetry breaking. Intriguingly, the observed rotations differ by five orders of magnitude between different wavelengths and samples, suggesting more intricate physics. Here we report optical rotation and polar Kerr measurements in CsV3Sb5 crystals at the same wavelength. We observe large isotropic components of 1 milliradian in the optical rotation that do not respond to applied magnetic fields but flip sign between samples, while the spontaneous Kerr signal is less than 20 nanoradian. Our results prove unambiguously that the reported isotropic rotation is not related to time-reversal symmetry breaking but instead indicates a new intertwined order that onsets at the same temperature.

cond-mat.str-el↗
arXiv2023

Structural evolution of the kagome superconductors $A$V$_3$Sb$_5$ ($A$ = K, Rb, and Cs) through charge density wave order

The kagome superconductors KV$_3$Sb$_5$, RbV$_3$Sb$_5$, and CsV$_3$Sb$_5$ are known to display charge density wave (CDW) order which impacts the topological characteristics of their electronic structure. Details of their structural ground states and how they evolve with temperature are revealed here using single crystal X-ray crystallographic refinements as a function of temperature, carried out with synchrotron radiation. The compounds KV$_3$Sb$_5$ and RbV$_3$Sb$_5$ present 2$\times$2$\times$2 superstructures in the $Fmmm$ space group with a staggered tri-hexagonal deformation of vanadium layers. CsV$_3$Sb$_5$ displays more complex structural evolution, whose details have been unravelled by applying machine learning methods to the scattering data. Upon cooling through the CDW transition, CsV$_3$Sb$_5$ displays a staged progression of ordering from a 2$\times$2$\times$1 supercell and a 2$\times$2$\times$2 supercell into a final 2$\times$2$\times$4 supercell that persists to $T$ = 11 K and exhibits an average structure where vanadium layers display both tri-hexagonal and Star of David patterns of deformations. Diffraction from CsV$_3$Sb$_5$ under pulsed magnetic fields up to $μ_0H$ = 28 T suggest the real component of the CDW state is insensitive to external magnetic fields.

cond-mat.supr-con↗
arXiv2023

Order-disorder Peierls instability in the kagome metal (Cs,Rb)V$_3$Sb$_5$

The nature of the charge density wave phases in the kagome metal compound AV$_3$Sb$_5$ has raised many questions and their origin is still under debate. Here, we combine diffuse scattering (DS) and inelastic x-ray scattering (IXS) to identify a 3-dimensional (3D) precursor of the charge order with propagation vector (0.5 0 0.5), which condenses into a 3D-CDW through a first order phase transition. The quasi-elastic critical scattering indicates that the dominant contribution to the diffuse precursor is the elastic central peak without phonon softening. However, the inelastic spectra show a small broadening of the Einstein-type phonon mode on approaching the phase transition. Our results point to the situation where the Fermi surface instability at the \textit{L} point is of order-disorder type with a critical growth of quasi-static domains. The results go beyond the classical weak-coupling Peierls transition dynamics and are discussed within models including strong-electron phonon coupling and non-adiabaticity.

cond-mat.str-el↗