arXiv subjects

Stephen D. Wilson

Publications and source records attributed to Stephen D. Wilson.

At least 127 records · Page 7Linked to original sources

arXiv2022

Defect control in the Heisenberg-Kitaev candidate material NaRuO$_2$

The combination of geometric frustration, extended hopping, spin-orbit coupling, and a disordered magnetic ground state make NaRuO$_{2}$ an attractive Heisenberg-Kitaev candidate material. Historically, NaRuO$_2$ has been a challenging material to produce, even in polycrystalline form. Here we present synthetic efforts that identify a propensity for Na$_\text{Ru}$ defects to form in NaRuO$_2$, revealing a full solid-solution between NaRuO$_{2}$ and disordered Na$_2$RuO$_3$. We report the synthesis of alloys along the Na$_{3+x}$Ru$_{3-x}$O$_6$ solid solution and characterize changes in the bulk magnetization and electron transport as a function of Na-loading. Our results highlight the importance of stoichiometry control in NaRuO$_{2}$ when investigating and interpreting this material's physical properties.

cond-mat.str-el↗

arXiv2022

Novel magnetic ordering in LiYbO2 probed by muon spin relaxation

The stretched diamond lattice material LiYbO2 has recently been reported to exhibit two magnetic transitions ($T_{N1} = 1.1 K$, $T_{N2} = 0.45 K$) via specific heat, magnetization, and neutron scattering measurements [Bordelon et al., Phys. Rev. B 103, 014420 (2021)]. Here we report complementary magnetic measurements down to T = 0.28 K via the local probe technique of muon spin relaxation. While we observe a rapid increase in the zero-field muon depolarization rate at $T_{N1}$, we do not observe any spontaneous muon precession for $T < T_{N1}$, which is typically associated with long-range magnetic ordering. The depolarization rate in the ordered state shows a surprising sensitivity to magnetic fields applied along the initial spin polarization direction. Using a simple one-dimensional model, we show that these results are consistent with the unusual random-phase bipartite incommensurate magnetic structure proposed by Bordelon et al. for the intermediate temperature range $T_{N2} < T < T_{N1}$. We also find evidence for temperature-independent magnetic fluctuations persisting to our lowest temperatures, but no obvious signature of the transition or spontaneous muon precession at and below TN2, respectively. This result is suggestive of quantum dynamics within a highly degenerate ground state.

cond-mat.str-el↗

arXiv2022

Incommensurate charge-stripe correlations in the kagome superconductor CsV$_3$Sb$_{5-x}$Sn$_x$

We track the evolution of charge correlations in the kagome superconductor CsV$_3$Sb$_5$ as its parent, long-ranged charge density order is destabilized. Upon hole-doping doping, interlayer charge correlations rapidly become short-ranged and their periodicity is reduced by half along the interlayer direction. Beyond the peak of the first superconducting dome, the parent charge density wave state vanishes and incommensurate, quasi-1D charge correlations are stabilized in its place. These competing, unidirectional charge correlations demonstrate an inherent electronic rotational symmetry breaking in CsV$_3$Sb$_5$, independent of the parent charge density wave state and reveal a complex landscape of charge correlations across the electronic phase diagram of this class of kagome superconductors. Our data suggest an inherent 2$k_f$ charge instability and the phenomenology of competing charge instabilities is reminiscent of what has been noted across several classes of unconventional superconductors.

cond-mat.supr-con↗

arXiv2022

Electronic and structural properties of RbCeX$_2$ (X$_2$: O$_2$, S$_2$, SeS, Se$_2$, TeSe, Te$_2$)

Triangular lattice delafossite compounds built from magnetic lanthanide ions are a topic of recent interest due to their frustrated magnetism and realization of quantum disordered magnetic ground states. Here we report the evolution of the structure and electronic ground states of RbCe$X_2$ compounds, built from a triangular lattice of Ce$^{3+}$ ions, upon varying their anion character ($X_2$= O$_2$, S$_2$, SeS, Se$_2$, TeSe, Te$_2$). This includes the discovery of a new member of this series, RbCeO$_2$, that potentially realizes a quantum disordered ground state analogous to NaYbO$_2$. Magnetization and susceptibility measurements reveal that all compounds manifest mean-field antiferromagnetic interactions and, with the exception of the oxide, possess signatures of magnetic correlations onset below 1 K. The crystalline electric field level scheme is explored via neutron scattering and \textit{ab initio} calculations in order to model the intramultiplet splitting of the $J=5/2$ multiplet. In addition to the two excited doublets expected within the $J=5/2$ manifold, we observe one extra, local mode present across the sample series. This added mode shifts downward in energy with increasing anion mass and decreasing crystal field strength, suggesting a long-lived anomalous mode endemic to anion motion about the Ce$^{3+}$ sites.

cond-mat.str-el↗

arXiv2022

Traversing the pyrochlore stability diagram; microwave-assisted synthesis and discovery of mixed B-site Ln$_2$InSbO$_7$ family

The lanthanide pyrochlore oxides Ln$_2$B$_2$O$_7$ are one of the most intensely studied classes of materials within condensed matter physics, firmly centered as one of the pillars of frustrated magnetism. The extensive chemical diversity of the pyrochlores, coupled with their innate geometric frustration, enables realization of a wide array of exotic and complex magnetic ground states. Thus, the discovery of new pyrochlore compositions has been a persistent theme that continues to drive the field in exciting directions. The recent focus on the mixed B-site pyrochlores offers a unique route towards tuning both local coordination chemistry and sterics, while maintaining a nominally pristine magnetic sublattice. Here, we present a broad overview of the pyrochlore stability field, integrating recent synthetic efforts in mixed B-site systems with the historically established Ln$_2$B$_2$O$_7$ families. In parallel, we present the discovery and synthesis of the entire Ln$_2$InSbO$_7$ family (Ln: La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) located near the boundary of the pyrochlore stability field using a rapid, hybrid mechanicochemical/microwave-assisted synthesis technique. Magnetic characterization on the entire class of compounds draws striking parallels to the stannate analogs, suggesting that these compounds may host a breadth of exotic magnetic ground states.

cond-mat.str-el↗

arXiv2022

Spin-orbit excitons and electronic configuration of the $5d^4$ insulator Sr$_3$Ir$_2$O$_7$F$_2$

Here we report on the low-energy excitations within the paramagnetic spin-orbit insulator Sr$_3$Ir$_2$O$_7$F$_2$ studied via resonant inelastic X-ray scattering, \textit{ab initio} quantum chemical calculations, and model-Hamiltonian simulations. This material is a unique $d^{4}$ Ir$^{5+}$ analog of Sr$_3$Ir$_2$O$_7$ that forms when F ions are intercalated within the SrO layers spacing the square lattice IrO$_{6}$ bilayers of Sr$_3$Ir$_2$O$_7$. Due to the large distortions about the Ir$^{5+}$ ions, our computations demonstrate that a large single-ion anisotropy yields an $S$=1 ($L{\approx}$1, $J{\approx}$0) ground state wave function. Weakly coupled, excitonic modes out of the $S_z$=0 ground state are observed and are well-described by a phenomenological spin-orbit exciton model previously developed for $3d$ and $4d$ transition metal ions. The implications of our results regarding the interpretation of previous studies of hole-doped iridates close to $d^{4}$ fillings are discussed.

cond-mat.str-el↗

arXiv2022

Tuning charge-density wave order and superconductivity in the kagome metals KV$_3$Sb$_{5-x}$Sn$_x$ and RbV$_3$Sb$_{5-x}$Sn$_x$

The family of AV$_3$Sb$_5$ (A = K, Rb, Cs) kagome metals exhibit charge density wave (CDW) order, proposed to be chiral, followed by a lower temperature superconducting state. Recent studies have proposed the importance of band structure saddle points proximal to the Fermi energy in governing these two transitions. Here we show the effects of hole-doping achieved via chemical substitution of Sn for Sb on the CDW and superconducting states in both KV$_3$Sb$_5$ and RbV$_3$Sb$_5$, and generate a phase diagram. Hole-doping lifts the $Γ$ pocket and van Hove singularities (vHs) toward $E_F$ causing the superconducting $T_C$ in both systems to increase to about 4.5 K, while rapidly suppressing the CDW state.

cond-mat.supr-con↗

arXiv2022

Study of the electronic properties of topological kagome metals YV$_6$Sn$_6$ and GdV$_6$Sn$_6$

The synthesis and characterization of vanadium-based kagome metals YV$_6$Sn$_6$ and GdV$_6$Sn$_6$ are presented. X-ray diffraction, magnetization, magnetotransport, and heat capacity measurements reveal an ideal kagome network of V-ions coordinated by Sn and separated by triangular lattice planes of rare-earth ions. The onset of low-temperature, likely noncollinear, magnetic order of Gd spins is detected in GdV$_6$Sn$_6$, while V-ions in both compounds remain nonmagnetic. Density functional theory calculations are presented modeling the band structures of both compounds, which can be classified as $\mathbb{Z}_2$ topological metals in the paramagnetic state. Both compounds exhibit high mobility, multiband transport and present an interesting platform for controlling the interplay between magnetic order associated with the $R$-site sublattice and nontrivial band topology associated with the V-based kagome network. Our results invite future exploration of other $R$V$_6$Sn$_6$ ($R$=rare earth) variants where this interplay can be tuned via $R$-site substitution.

cond-mat.str-el↗

arXiv2022

Charge density wave order in kagome metal AV$_3$Sb$_5$ (A= Cs, Rb, K)

We employ polarization-resolved electronic Raman spectroscopy and density functional theory to study the primary and secondary order parameters, as well as their interplay, in the charge density wave (CDW) state of the kagome metal AV3Sb5. Previous x-ray diffraction data at 15K established that the CDW order in CsV3Sb5 comprises of a 2x2x4 structure: one layer of inverse-star-of-David and three consecutive layers of star-of-David pattern. We analyze the lattice distortions based the 2x2x4 structure at 15K, and find that U lattice distortion is the primary order parameter while M and L distortions are secondary order parameters for Vanadium displacements. This conclusion is confirmed by the calculation of bare susceptibility that shows a broad peak at around qz=0.25 along the hexagonal Brillouin zone face central line (U-line). We also identify several phonon modes emerging in the CDW state, which are lattice vibration modes related to V and Sb atoms as well as alkali atoms. The detailed temperature evolution of these modes' frequencies, HWHM, and integrated intensities support a phase diagram with two successive structural phase transitions in CsV3Sb5: the first one with a primary order parameter appearing at TS=94K and the second isostructural one appearing at around 70K.

cond-mat.str-el↗

arXiv2022

Band Structure and Polarization Effects in Photothermoelectric Spectroscopy of a Bi2Se3 Device

Bi2Se3 is a prototypical topological insulator which has a small band gap (~0.3 eV) and topologically protected conducting surface states. This material exhibits quite strong thermoelectric effects. Here we show in a mechanically exfoliated thick (~100 nm) nanoflake device that we can measure the energy dependent optical absorption through the photothermoelectric effect. Spectral signatures are seen for a number of optical transitions between the valence and conduction bands, including a broad peak at 1.5 eV which is likely dominated by bulk band-to-band optical transitions but is at the same energy as the well known optical transition between the two topologically protected conducting surface states. We also observe a surprising linear polarization dependence in the response of the device that reflects the influence of the metal contacts.

cond-mat.mtrl-sci↗

arXiv2022

Phonon Chirality Induced by Vibronic-Orbital Coupling

The notion that phonons can carry pseudo-angular momentum has become popular in the last decade, with recent research efforts highlighting phonon chirality, Berry curvature of phonon band structure, and the phonon Hall effect. When a phonon is resonantly coupled to a crystal electric field excitation, a so-called vibronic bound state forms. Here, we observe angular momentum transfer of $δ$Jz = $\pm$1$\hbar$ between phonons and an orbital state in a vibronic bound state of a candidate quantum spin liquid. This observation has profound implications for the engineering of phonon band structure topology through chiral quasiparticle interactions.

cond-mat.mes-hall↗

arXiv2022

Role of Sb in the superconducting kagome metal CsV$_3$Sb$_5$ revealed by its anisotropic compression

Pressure evolution of the superconducting kagome metal CsV$_3$Sb$_5$ is studied with single-crystal x-ray diffraction and density-functional band-structure calculations. A highly anisotropic compression observed up to 5 GPa is ascribed to the fast shrinkage of the Cs-Sb distances and suppression of Cs rattling motion. This prevents Sb displacements required to stabilize the three-dimensional charge-density-wave (CDW) order and elucidates the disappearance of the CDW already at 2 GPa despite only minor changes in the electronic structure of the normal state. At higher pressures, vanadium bands still change only marginally, whereas antimony bands undergo a major reconstruction caused by the gradual formation of the interlayer Sb-Sb bonds. Our results exclude pressure tuning of vanadium kagome bands as the main mechanism for the non-trivial evolution of superconductivity in real-world kagome metals. Concurrently, we establish the central role of Sb atoms in the stabilization of a three-dimensional CDW and Fermi surface reconstruction.

cond-mat.supr-con↗

arXiv2022

Fermi level tuning and double-dome superconductivity in the kagome metals CsV$_3$Sb$_{5-x}$Sn$_x$

The recently reported \textit{A}V$_3$Sb$_5$ (\textit{A} = K, Rb, Cs) family of kagome metals are candidates for unconventional superconductivity and chiral charge density wave (CDW) order; both potentially arise from nested saddle points in their band structures close to the Fermi energy. Here we use chemical substitution to introduce holes into CsV$_3$Sb$_{5}$ and unveil an unconventionalcoupling of the CDW and superconducting states. Specifically, we generate a phase diagram for CsV$_3$Sb$_{5-x}$Sn$_{x}$ that illustrates the impact of hole-doping the system and lifting the nearest vHs toward and above $E_F$. Superconductivity exhibits a non-monotonic evolution with the introduction of holes, resulting in two "domes" peaked at 3.6\,K and 4.1\,K and the rapid suppression of three-dimensional CDW order. The evolution of CDW and superconducting order is compared with the evolution of the electronic band structure of CsV$_3$Sb$_{5-x}$Sn$_x$, where the complete suppression of superconductivity seemingly coincides with an electron-like band comprised of Sb $p_z$ orbitals pushed above E$_F$.

cond-mat.supr-con↗

arXiv2022

Coexistence of Tri-Hexagonal and Star-of-David Pattern in the Charge Density Wave of the Kagome Superconductor AV$_3$Sb$_5$

The recently discovered layered kagome metals AV$_3$Sb$_5$(A=K, Rb, Cs) have attracted much attention because of their unique combination of superconductivity, charge density wave (CDW) order, and nontrivial band topology. The CDW order with an in-plane 2x2 reconstruction is found to exhibit exotic properties, such as time-reversal symmetry breaking and rotational symmetry breaking. However, the nature of the CDW, including its dimensionality, structural pattern, and effect on electronic structure, remains elusive despite intense research efforts. Here, using angle-resolved photoemission spectroscopy, we unveil for the first time characteristic double-band splittings and band reconstructions, as well as the band gap resulting from band folding, in the CDW phase. Supported by density functional theory calculations, we unambiguously show that the CDW in AV$_3$Sb$_5$ originates from the intrinsic coexistence of Star-of-David and Tri-Hexagonal distortions. The alternating stacking of these two distortions naturally leads to three-dimensional 2x2x2 or 2x2x4 CDW order. Our results provide crucial insights into the nature and distortion pattern of the CDW order, thereby laying down the basis for a substantiated understanding of the exotic properties in the family of AV$_3$Sb$_5$ kagome metals.

cond-mat.supr-con↗

arXiv2021

Realizing Kagome Band Structure in Two-Dimensional Kagome Surface States of $RV_{6}Sn_{6}$ ($R$=Gd,Ho)

We report angle resolved photoemission experiments on a newly discovered family of kagome metals $RV_{6}Sn_{6}$ ($R$=Gd, Ho). Intrinsic bulk states and surface states of the vanadium kagome layer are differentiated from those of other atomic sublattices by the real-space resolution of the measurements with a small beam spot. Characteristic Dirac cone, saddle point and flat bands of the kagome lattice are observed. Our results establish the two-dimensional (2D) kagome surface states as a new platform to investigate the intrinsic kagome physics.

cond-mat.mtrl-sci↗

arXiv2021

Evolution of superconductivity and charge order in pressurized RbV$_3$Sb$_5$

The kagome metals $A$V$_3$Sb$_5$ ($A=$ K, Rb, Cs) under ambient pressure exhibit an unusual charge order, from which superconductivity emerges. In this work, by applying hydrostatic pressure using a liquid pressure medium and carrying out electrical resistance measurements for RbV$_3$Sb$_5$, we find the charge order becomes suppressed under a modest pressure $p_{\rm c}$ ($1.4<p_{\rm c}<1.6$ GPa), while the superconducting transition temperature $T_{\rm c}$ is maximized. $T_{\rm c}$ is then gradually weakened with further increase of pressure and reaches a minimum around 14.3 GPa, before exhibiting another maximum around 22.8 GPa, signifying the presence of a second superconducting dome. Distinct normal state resistance anomalies are found to be associated with the second superconducting dome, similar to KV$_3$Sb$_5$. Our findings point to qualitatively similar temperature-pressure phase diagrams in KV$_3$Sb$_5$ and RbV$_3$Sb$_5$, and suggest a close link between the second superconducting dome and the high-pressure resistance anomalies.

cond-mat.supr-con↗

arXiv2021

Fermi surface mapping and the nature of charge density wave order in the kagome superconductor CsV$_3$Sb$_5$

The recently discovered family of AV$_3$Sb$_5$ (A: K, Rb Cs) kagome metals possess a unique combination of nontrivial band topology, superconducting ground states, and signatures of electron correlations manifest via competing charge density wave order. Little is understood regarding the nature of the charge density wave (CDW) instability inherent to these compounds and the potential correlation with the accompanying onset of a large anomalous Hall response. To understand the impact of the CDW order on the electronic structure in these systems, we present quantum oscillation measurements on single crystals of CsV$_3$Sb$_5$. Our data provides direct evidence that the CDW invokes a substantial reconstruction of the Fermi surface pockets associated with the vanadium orbitals and the kagome lattice framework. In conjunction with density functional theory modeling, we are able to identify split oscillation frequencies originating from reconstructed pockets built from vanadium orbitals and Dirac-like bands. Complementary diffraction measurements are further able to demonstrate that the CDW instability has a correlated phasing between neighboring V$_3$Sb$_5$ planes. These results provide critical insights into the underlying CDW instability in AV$_3$Sb$_5$ kagome metals and support minimal models of CDW order arising from within the vanadium-based kagome lattice.

cond-mat.str-el↗

arXiv2021

Twofold van Hove singularity and origin of charge order in topological kagome superconductor CsV3Sb5

The layered vanadium antimonides AV3Sb5 (A = K, Rb, Cs) are a recently discovered family of topological kagome metals with a rich phenomenology of strongly correlated electronic phases including charge order and superconductivity. Understanding how the singularities inherent to the kagome electronic structure are linked to the observed many-body phases is a topic of great interest and relevance. Here, we combine angle-resolved photoemission spectroscopy and density functional theory to reveal multiple kagome-derived van Hove singularities (vHs) coexisting near the Fermi level of CsV3Sb5 and analyze their contribution to electronic symmetry breaking. Intriguingly, the vHs in CsV3Sb5 have two distinct flavors - p-type and m-type - which originate from their pure and mixed sublattice characters, respectively. This twofold vHs is unique property of the kagome lattice, and its flavor critically determines the pairing symmetry and ground states emerging in AV3Sb5 series. We establish that, among the multiple vHs in CsV3Sb5, the m-type vHs of the dxz/dyz kagome band and the p-type vHs of the dxy/dx2-y2 kagome band cross the Fermi level to set the stage for electronic symmetry breaking. The former band exhibits pronounced Fermi surface nesting, while the latter contributes via higher-order vHs. Our work reveals the essential role of kagome-derived vHs for the collective phenomena realized in the AV3Sb5 family, paving the way to a deeper understanding of strongly correlated topological kagome systems.

cond-mat.str-el↗