arXiv subjects

Zurab Guguchia

Publications and source records attributed to Zurab Guguchia.

At least 37 records · Page 2Linked to original sources

arXiv2024

Uniaxial strain tuning of charge modulation and singularity in a kagome superconductor

Tunable quantum materials hold great potential for applications. Of special interest are materials in which small lattice strain induces giant electronic responses. The kagome compounds AV3Sb5 (A = K, Rb, Cs) provide a testbed for such singular electronic states. In this study, through angle-resolved photoemission spectroscopy, we provide comprehensive spectroscopic measurements of the giant responses induced by compressive and tensile strains on the charge-density-wave (CDW) order parameter and high-order van Hove singularity (HO-VHS) in CsV3Sb5. We observe a tripling of the CDW gap magnitudes with ~1% strain, accompanied by the changes of both energy and mass of the saddle-point fermions. Our results reveal an anticorrelation between the unconventional CDW order parameter and the mass of a HO-VHS, and highlight the role of the latter in the superconducting pairing. The giant electronic responses uncover a rich strain tunability of the versatile kagome system in studying quantum interplays under lattice perturbations.

cond-mat.mtrl-sci↗

arXiv2024

Unveiling the nature of electronic transitions in RbV$_3$Sb$_5$ with Avoided Level Crossing $μ$SR

Kagome superconductors AV$_{3}$Sb$_{5}$ provide a unique platform for studying the interplay between a variety of electronic orders, including superconductivity, charge density waves, nematic phases and more. Understanding the evolution of the electronic state from the charge density wave to the superconducting transition is essential for unraveling the interplay of charge, spin, and lattice degrees of freedom giving rise to the unusual magnetic properties of these nonmagnetic metals. Previous zero-field and high-field $μ$SR studies revealed two anomalies in the muon spin relaxation rate, a first change at $T_{CDW} \sim 100$ K and a second steep increase at $T^{*}\sim 40$ K, further enhanced by an applied magnetic field, thus suggesting a contribution of magnetic origin. In this study, we use the avoided level crossing $μ$SR technique to investigate charge order in near-zero applied field. By tracking the temperature dependence of quadrupolar level-crossing resonances, we examined the evolution of the electric field gradient at V nuclei in the kagome plane. Our results show a significant rearrangement of the charge density starting at $T^{*}$ indicating a transition in the charge distribution, likely electronic in origin, well below $T_{CDW}$. These findings, combined with previous $μ$SR, STM, and NMR studies, emphasize the intertwined nature of proximate phases in these systems, with the charge rearrangement dominating the additional increase in $μ$SR relaxation rate below $T^{*}$.

cond-mat.str-el↗

arXiv2024

Vacancy-induced suppression of CDW order and its impact on magnetic order in kagome antiferromagnet FeGe

Two-dimensional (2D) kagome lattice metals are interesting because they display flat electronic bands, Dirac points, Van Hove singularities, and can have interplay between charge density wave (CDW), magnetic order, and superconductivity. In kagome lattice antiferromagnet FeGe, a short-range CDW order was found deep within an antiferromagnetically ordered state, interacting with the magnetic order. Surprisingly, post-growth annealing of FeGe at 560$^{\circ}$C can suppress the CDW order while annealing at 320$^{\circ}$C induces a long-range CDW order, with the ability to cycle between the states repeatedly by annealing. Here we perform transport, neutron scattering, scanning transmission electron microscopy (STEM), and muon spin rotation ($μ$SR) experiments to unveil the microscopic mechanism of the annealing process and its impact on magneto-transport, CDW, and magnetic properties of FeGe. We find that 560$^{\circ}$C annealing creates germanium vacancies uniformly distributed throughout the FeGe kagome lattice, which prevent the formation of Ge-Ge dimers necessary for the CDW order. Upon annealing at 320$^{\circ}$C, the system segregates into stoichiometric FeGe regions with long-range CDW order and regions with stacking faults that act as nucleation sites for the CDW. The presence or absence of CDW order greatly affects the anomalous Hall effect, incommensurate magnetic order, and spin-lattice coupling in FeGe, thus placing FeGe as the only known kagome lattice material with a tunable CDW and magnetic order, potentially useful for sensing and information transmission.

cond-mat.str-el↗

arXiv2024

Chiral kagome superconductivity modulations with residual Fermi arcs in KV3Sb5 and CsV3Sb5

Superconductivity involving finite momentum pairing can lead to spatial gap and pair density modulations, as well as Bogoliubov Fermi states within the superconducting gap. However, the experimental realization of their intertwined relations has been challenging. Here, we detect chiral kagome superconductivity modulations with residual Fermi arcs in KV3Sb5 and CsV3Sb5 by normal and Josephson scanning tunneling microscopy down to 30mK with resolved electronic energy difference at microelectronvolt level. We observe a U-shaped superconducting gap with flat residual in-gap states. This gap exhibits chiral 2 by 2 spatial modulations with magnetic field tunable chirality, which align with the chiral 2 by 2 pair density modulations observed through Josephson tunneling. These findings demonstrate a chiral pair density wave (PDW) that breaks time-reversal symmetry. Quasiparticle interference imaging of the in-gap zero-energy states reveals segmented arcs, with high-temperature data linking them to parts of the reconstructed V d-orbital states within the charge order. The detected residual Fermi arcs can be explained by the partial suppression of these d-orbital states through an interorbital 2 by 2 PDW and thus serve as candidate Bogoliubov Fermi states. Additionally, we differentiate the observed PDW order from impurity-induced gap modulations. Our observations not only uncover a chiral PDW order with orbital-selectivity, but also illuminate the fundamental space-momentum correspondence inherent in finite momentum paired superconductivity.

cond-mat.supr-con↗

arXiv2024

Coexistence of local magnetism and superconductivity in the heavy-fermion CeRh$_2$As$_2$ revealed by $μ$SR studies

The superconducting (SC) state ($T_\mathrm{c}$ = 0.3 K) of the heavy-fermion compound CeRh$_2$As$_2$, which undergoes an unusual field-induced transition to another high-field SC state, emerges from an unknown ordered state below $T_\mathrm{o}$ = 0.55 K. While an electronic multipolar order of itinerant Ce-4$f$ states was proposed to account for the $T_\mathrm{o}$ phase, the exact order parameter has not been known to date. Here, we report on muon spin relaxation ($μ$SR) studies of the magnetic and SC properties in CeRh$_2$As$_2$ single crystals at low temperatures. We reveal a magnetic origin of the $T_\mathrm{o}$ order by identifying a spontaneous internal field below $T_\mathrm{o}$ = 0.55 K. Furthermore, we find evidence of a microscopic coexistence of local magnetism with bulk superconductivity. Our findings open the possibility that the $T_\mathrm{o}$ phase involves both dipole and higher order Ce-4$f$ moment degrees of freedom and accounts for the unusual non-Fermi liquid behavior.

cond-mat.str-el↗

arXiv2024

Ti4Ir2O a time-reversal-invariant fully gapped unconventional superconductor

Here we report muon spin rotation (muSR) experiments on the temperature and field dependence of the effective magnetic penetration depth (lambda) in the eta-carbide-type suboxide Ti4Ir2O, a superconductor with an considerably high upper critical field. Temperature dependence of penetration depth, obtained from transverse-field (TF)-muSR measurements, is in perfect agreement with an isotropic fully gaped superconducting state. Furthermore, our ZF muSR results confirm that the time-reversal symmetry is preserved in the superconducting state. We find, however, a notably low ratio of 1.22 between the superconducting critical temperature and the superfluid density. This value is close to most unconventional superconductors, showing that a very small superfluid density is present in the superconducting state of Ti4Ir2O. The presented results will pave the way for further theoretical and experimental investigations to obtain a microscopic understanding of the origin of such a high upper critical field in an isotropic single gap superconducting system.

cond-mat.supr-con↗

arXiv2024

Tuning of Charge Order by Uniaxial Stress in a Cuprate Superconductor

Strongly correlated electron materials are often characterized by competition and interplay of multiple quantum states. For example, in high-temperature cuprate superconductors unconventional superconductivity, spin- and charge-density wave orders coexist. A key question is whether competing states coexist on the atomic scale or if they segregate into distinct 'islands'. Using X-ray diffraction, we investigate the competition between charge order and superconductivity in the archetypal cuprate La(2-x)BaxCuO4, around the x = 1/8-doping, where uniaxial stress restores optimal 3D superconductivity at approximately 0.06 GPa. We find that the charge order peaks and the correlation length along the stripe are strongly reduced up to the critical stress, above which they stay constant. Simultaneously, the charge order onset temperature only shows a modest decrease. Our findings suggest that optimal 3D superconductivity is not linked to the absence of charge stripes but instead requires their arrangement into smaller 'islands'. Our results provide insight into the length scales over which the interplay between superconductivity and charge order takes place.

cond-mat.supr-con↗

arXiv2023

Emergence of flat bands and their impact on superconductivity of Mo$_5$Si$_{3-x}$P$_x$

The first-principles calculations and measurements of the magnetic penetration depths, the upper critical field, and the specific heat were performed for a family of Mo$_5$Si$_{3-x}$P$_x$ superconducotrs. First-principles calculations suggest the presence of a flat band dispersion, which gradually shifts to the Fermi level as a function of phosphorus doping $x$. The flat band approaches the Fermi level at $x\simeq 1.3$, thus separating Mo$_5$Si$_{3-x}$P$_x$ between the purely steep band and the steep band/flat band superconducting regimes. The emergence of flat bands lead to an abrupt change of nearly all the superconducting quantities. In particular, a strong reduction of the coherence length $ξ$ and enhancement of the penetration depth $λ$ result in nearly factor of three increase of the Ginzburg-Landau parameter $κ=λ/ξ$ (from $κ\simeq 25$ for $x\lesssim 1.2$ to $κ\simeq 70$ for $x\gtrsim 1.4$) thus initiating the transition of Mo$_5$Si$_{3-x}$P$_x$ from a moderate to an extreme type-II superconductivity.

cond-mat.supr-con↗

arXiv2023

In-plane magnetic penetration depth in Sr$_2$RuO$_4$: muon-spin rotation/relaxation study

We report on measurements of the in-plane magnetic penetration depth ($λ_{\rm ab}$) in single crystals of Sr$_2$RuO$_4$ down to $\simeq 0.015$ K by means of muon-spin rotation/relaxation. The linear temperature dependence of $λ^{-2}_{\rm ab}$ for $T\lesssim 0.7$ K suggests the presence of nodes in the superconducting gap. This statement is further substantiated by observation of the Volovik effect, $i.e.$ the reduction of $λ_{ab}^{-2}$ as a function of the applied magnetic field. The experimental zero-field and zero-temperature value of $λ_{\rm ab}=124(3)$ nm agrees with $λ_{\rm ab}\simeq 130$ nm, calculated based on results of electronic structure measurements reported in [Phys. Rev X 9, 021048 (2019)]. Our analysis reveals that a simple nodal superconducting energy gap, described by the lowest possible harmonic of a gap function, does not capture the dependence of $λ_{\rm ab}^{-2}$ on $T$, so the higher angular harmonics of the energy gap function need to be introduced.

cond-mat.supr-con↗

arXiv2023

Phonon promoted charge density wave in topological kagome metal ScV$_{6}$Sn$_{6}$

Charge density wave (CDW) orders in vanadium-based kagome metals have recently received tremendous attention due to their unique properties and intricate interplay with exotic correlated phenomena, topological and symmetry-breaking states. However, the origin of the CDW order remains a topic of debate. The discovery of ScV$_{6}$Sn$_{6}$, a vanadium-based bilayer kagome metal exhibiting an in-plane $\sqrt{3}$ x $\sqrt{3} $ $\textit{R}$30$°$ CDW order with time-reversal symmetry breaking, provides a novel platform to explore the underlying mechanism behind the unconventional CDW. Here, we combine high-resolution angle-resolved photoemission spectroscopy, Raman scattering measurements and density functional theory to investigate the electronic structures and phonon modes of ScV$_{6}$Sn$_{6}$ and their evolution with temperature. We identify topologically nontrivial Dirac surface states and multiple van Hove singularities (VHSs) in the vicinity of the Fermi level, with one VHS near the K point exhibiting nesting wave vectors in proximity to the $\sqrt{3}$ x $\sqrt{3}$ $\textit{R}$30$°$ CDW wave vector. Additionally, Raman measurements indicate a strong intrinsic electron-phonon coupling in ScV$_{6}$Sn$_{6}$, as evidenced by the presence of a two-phonon mode and a large frequency amplitude mode. Our findings highlight the fundamental role of lattice degrees of freedom in promoting the CDW in ScV$_{6}$Sn$_{6}$ and provide important insights into the fascinating correlation phenomena observed in kagome metals.

cond-mat.str-el↗

arXiv2023

Competing spin-glass and spin-fluctuation states in NdxPr4-xNi3O8

Neodymium nickelates have attracted research interest due to their strongly correlated behaviour and remarkable magnetic properties. More importantly, superconductivity has recently been confirmed in thin-film samples of Sr-doped NdNiO2, bringing the layered rare earth nickel oxides into the research spotlight. In this report, we present results on a series of NdNiO2 analogues, NdxPr4-xNi3O8 (x = 0.1, 0.25, 1, 2, and 4) obtained by topotactic reduction, in which we observe systematic changes in the magnetic behaviour. As the Nd3+ content increases, the initially large spin-freezing region with magnetic frustration becomes smaller and gradually shifts to low temperatures, while the magnetic response gradually increases. The muon-spin spectroscopy measurements on Nd4Ni3O8 show that this phenomenon is likely due to the enhancement of spin fluctuations in NdxPr4-xNi3O8, which weakens the spin frustration behaviour for high Nd3+ contents and at low temperatures. These spin fluctuations can be caused by both Nd and Ni ions and could be one of the factors determining the occurrence of possible superconductivity.

cond-mat.supr-con↗

arXiv2023

Nodeless electron pairing in CsV$_3$Sb$_5$-derived kagome superconductors

The newly discovered kagome superconductors represent a promising platform for investigating the interplay between band topology, electronic order, and lattice geometry. Despite extensive research efforts on this system, the nature of the superconducting ground state remains elusive. In particular, consensus on the electron pairing symmetry has not been achieved so far, in part owing to the lack of a momentum-resolved measurement of the superconducting gap structure. Here we report the direct observation of a nodeless, nearly isotropic, and orbital-independent superconducting gap in the momentum space of two exemplary CsV$_3$Sb$_5$-derived kagome superconductors -- Cs(V$_{0.93}$Nb$_{0.07}$)$_3$Sb$_5$ and Cs(V$_{0.86}$Ta$_{0.14}$)$_3$Sb$_5$, using ultrahigh resolution and low-temperature angle-resolved photoemission spectroscopy (ARPES). Remarkably, such a gap structure is robust to the appearance or absence of charge order in the normal state, tuned by isovalent Nb/Ta substitutions of V. Moreover, we observe a signature of the time-reversal symmetry (TRS) breaking inside the superconducting state, which extends the previous observation of TRS-breaking CDW in the kagome lattice. Our comprehensive characterizations of the superconducting state provide indispensable information on the electron pairing of kagome superconductors, and advance our understanding of unconventional superconductivity and intertwined electronic orders.

cond-mat.supr-con↗

arXiv2023

$μ$SR measurements on Sr$_2$RuO$_4$ under $\langle 110 \rangle$ uniaxial stress

Muon spin rotation/relaxation ($μ$SR) and polar Kerr effect measurements provide evidence for a time-reversal symmetry breaking (TRSB) superconducting state in Sr$_2$RuO$_4$. However, the absence of a cusp in the superconducting transition temperature ($T_{\rm c}$) vs. stress and the absence of a resolvable specific heat anomaly at TRSB transition temperature ($T_{\rm TRSB}$) under uniaxial stress challenge a hypothesis of TRSB superconductivity. Recent $μ$SR studies under pressure and with disorder indicate that the splitting between $T_{\rm c}$ and $T_{\rm TRSB}$ occurs only when the structural tetragonal symmetry is broken. To further test such behavior, we measured $T_\text{c}$ through susceptibility measurements, and $T_\text{TRSB}$ through $μ$SR, under uniaxial stress applied along a $\langle 110 \rangle$ lattice direction. We have obtained preliminary evidence for suppression of $T_\text{TRSB}$ below $T_\text{c}$, at a rate much higher than the suppression rate of $T_\text{c}$.

cond-mat.supr-con↗

arXiv2022

Discovery of charge order and corresponding edge state in kagome magnet FeGe

Kagome materials often host exotic quantum phases, including spin liquids, Chern gap, charge order, and superconductivity. Existing scanning microscopy studies of the kagome charge order have been limited to non-kagome surface layers. Here we tunnel into the kagome lattice of FeGe to uncover features of the charge order. Our spectroscopic imaging identifes a 2x2 charge order in the magnetic kagome lattice, resembling that discovered in kagome superconductors. Spin-mapping across steps of unit-cell-height demonstrates that this charge order emerges from spin-polarized electrons with an antiferromagnetic stacking order. We further uncover the correlation between antiferromagnetism and charge order anisotropy, highlighting the unusual magnetic coupling of the charge order. Finally, we detect a pronounced edge state within the charge order energy gap, which is robust against the irregular shape of the kagome lattice edges. We discuss our results with the theoretically considered topological features of the kagome charge order including orbital magnetism and bulk-boundary correspondence.

cond-mat.str-el↗

arXiv2022

Time reversal invariant single gap superconductivity with upper critical field larger than Pauli limit in NbIr$_2$B$_2$

Recently, compounds with noncentrosymmetric crystal structure have attracted much attention for providing a rich playground in search for unconventional superconductivity. NbIr$_2$B$_2$ is a new member to this class of materials harboring superconductivity below $T_{\rm c} = 7.3(2)$~K and very high upper critical field that exceeds Pauli limit. Here we report on muon spin rotation ($μ$SR) experiments probing the temperature and field dependence of effective magnetic penetration depth in this compound. Our transverse-field -$μ$SR results suggest a fully gaped $s$-wave superconductvity. Further, the estimated high value of upper critical field is also supplemented by high field transport measurements. Remarkably, the ratio $T_{\rm c}$/$λ^{-2}(0)$ obtained for NbIr$_2$B$_2$ ($\sim$2) is comparable to those of unconventional superconductors. Zero-field $μ$SR data reveals no significant change in the muon spin relaxation rate above and below $T_{\rm c}$, evincing that time-reversal symmetry is preserved in the superconducting state. The presented results will stimulate theoretical investigations to obtain a microscopic understanding of the origin of superconductivity with preserved time reversal symmetry in this unique noncentrosymmetric system.

cond-mat.supr-con↗

arXiv2022

Microscopic evidence for anisotropic multigap superconductivity in the CsV$_3$Sb$_5$ kagome superconductor

The recently discovered kagome superconductor CsV$_3$Sb$_5$ ($T_c \simeq 2.5$ K) has been found to host charge order as well as a non-trivial band topology, encompassing multiple Dirac points and probable surface states. Such a complex and phenomenologically rich system is, therefore, an ideal playground for observing unusual electronic phases. Here, we report on microscopic studies of its anisotropic superconducting properties by means of transverse-field muon spin rotation ($μ$SR) experiments. The temperature dependences of the in-plane and out-of-plane components of the magnetic penetration depth $λ_{ab}^{-2}(T)$ and $λ_{c}^{-2}(T)$ indicate that the superconducting order parameter exhibits a two-gap ($s+s$)-wave symmetry, reflecting the multiple Fermi surfaces of CsV3Sb5. The multiband nature of its superconductivity is further validated by the different temperature dependences of the anisotropic magnetic penetration depth $γ_λ(T)$ and upper critical field $γ_{\rm B_{c2}}(T)$, both in close analogy with the well known two-gap superconductor MgB$_2$. Remarkably, the high value of the $T_c/λ^{-2}(0)$ ratio in both field orientations strongly suggests the unconventional nature of superconductivity. The relaxation rates obtained from zero field $μ$SR experiments do not show noticeable change across the superconducting transition, indicating that superconductivity does not break time reversal symmetry.

cond-mat.supr-con↗

arXiv2022

Charge order breaks time-reversal symmetry in CsV$_3$Sb$_5$

The recently discovered vanadium-based kagome metals $A$V$_{3}$Sb$_{5}$ ($A$~=~K,~Rb,~Cs) exhibit superconductivity at low-temperatures and charge density wave (CDW) order at high-temperatures. A prominent feature of the charge ordered state in this family is that it breaks time-reversal symmetry (TRSB), which is connected to the underlying topological nature of the band structure. In this work, a powerful combination of zero-field and high-field muon-spin rotation/relaxation is used to study the signatures of TRSB of the charge order in CsV$_3$Sb$_5$, as well as its anisotropic character. By tracking the temperature evolution of the in-plane and out-of-plane components of the muon-spin polarization, an enhancement of the internal field width sensed by the muon-spin ensemble was observed below $T_{\rm TRSB}=T_{\rm CDW}\simeq95$~K. Additional increase of the internal field width, accompanied by a change of the local field direction at the muon site from the $ab$-plane to the $c$-axis, was detected below $T^\ast\simeq30$~K. Remarkably, this two-step feature becomes well pronounced when a magnetic field of 8~T is applied along the crystallographic $c-$axis, thus indicating a field-induced enhancement of the electronic response at the CDW transition. These results point to a TRSB in CsV$_3$Sb$_5$ by charge order with an onset of ${\simeq}~95$~K, followed by an enhanced electronic response below ${\simeq}~30$~K. The observed two-step transition is discussed within the framework of different charge-order instabilities, which, in accordance with density functional theory calculations, are nearly degenerate in energy.

cond-mat.str-el↗

arXiv2022

Dynamic Magnetic Crossover at the Origin of the Hidden-Order in van der Waals Antiferromagnet CrSBr

The van der Waals material CrSBr stands out as a promising two-dimensional magnet. Especially, its high magnetic ordering temperature and versatile magneto-transport properties make CrSBr an important candidate for new devices in the emergent field of two-dimensional magnetic materials. To date, the magnetic and structural properties of CrSBr have not been fully elucidated. Here, we report on the detailed temperature-dependent magnetic and structural properties of this material, by comprehensively combining neutron scattering, muon spin relaxation spectroscopy, synchrotron X-ray diffraction, and magnetization measurements. We evidence that this material undergoes a transition to an A-type antiferromagnetic state below $T_{\rm N} \approx$ 140 K, with a pronounced two-dimensional character as deduced from the determined critical exponent of $β\approx $ 0.18. In our analysis of the field-induced metamagnetic transition, we find that the ferromagnetic correlations within the monolayers persist clearly above the Néel temperature in this material. Furthermore, we unravel the low-temperature (i.e. $T < T_{\rm N}$) magnetic hidden order within the long-range magnetically ordered state. We find that it is associated to a slowing down of the magnetic fluctuations, accompanied by a continuous reorientation of the internal magnetic field. These take place upon cooling below $T_s$ $\approx$ 100 K, until a spin freezing process occurs at $T$* $\approx$ 40 K. We argue this complex dynamic behavior to reflect a magnetic crossover driven by the in-plane uniaxial anisotropy, which is ultimately caused by the mixed-anion character of the material. Our findings indicate that the magnetic and structural properties of CrSBr widen its potential application as a component for spin-based electronic devices.

cond-mat.mtrl-sci↗