arXiv subjects

Peter Klavins

Publications and source records attributed to Peter Klavins.

14 recordsLinked to original sources

arXiv2025

Knight shift measurements probing Fermi surface changes under pressure in CeRhIn$_5$

We report nuclear magnetic resonance (NMR) Knight shift measurements of the In(1) and In(2) sites in CeRhIn$_5$ as a function of pressure. In contrast to the $c$ axis, the in-plane components of the In(1) Knight shift tensor exhibit little to no pressure dependence. These results indicate that the dipolar component of the tensor is strongly suppressed at the In(1) site, while it remains constant with pressure at the In(2) site. We analyze the hyperfine coupling in terms of a tight binding model for the electronic structure, and determine that the pressure dependence of the In(1) shift cannot be explained in terms of changes to the crystal field parameters, but rather can be understood in terms of an increase in the 4f electron content at the Fermi surface. Our results indicate that the hyperfine coupling reflects changes in the electronic structure near a Kondo breakdown quantum critical point.

cond-mat.str-el↗

arXiv2025

Importance of anisotropic interactions for hard-axis/plane ordering of Ce-based ferromagnets

Ferromagnetic (FM) Kondo-lattice (KL) compounds often exhibit intriguing magnetic behavior driven by strong crystal electric field (CEF) anisotropy and exchange interactions. Recent studies suggest that many Ce-based and Yb-based KL ferromagnets order along the hard-axis or hard-plane determined by their CEF ground state. We performed a survey of Ce-based FM compounds, complemented with new single crystal synthesis, magnetization measurements, and analysis of the CEF scheme. Our results reveal that hard-axis/plane ordering is less common than previously reported, with most compounds ordering along their easy-axis/plane. We also find no clear correlation between the strength of the Kondo effect and whether a compound adopts hard-axis/plane or easy-axis/plane ordering. Instead, the key driver appears to be the competition between CEF anisotropy and exchange interaction anisotropy. A direct comparison of CeCuSi and CeAgSb$_2$ further indicates that antiferromagnetic (AFM) interactions along the CEF easy-axis can be crucial in stabilizing the hard-axis FM ordering observed in CeAgSb$_2$. Finally, our results suggest that an anisotropic Ruderman-Kittel-Kasuya-Yosida (RKKY) model could offer deeper insights into the complicated magnetic properties of Ce-based and other heavy-fermion systems.

cond-mat.str-el↗

arXiv2025

Incommensurate and commensurate antiferromagnetic orders in the kagome compound UV$_{6}$Sn$_{6}$

We report on the synthesis of single crystals of the kagome compound, UV$_6$Sn$_6$, and present the results of magnetization, electrical resistivity, heat capacity, x-ray, and neutron diffraction experiments to characterize the structure and magnetic properties. UV$_6$Sn$_6$ crystallizes in a large supercell of the HfFe$_6$Ge$_6$ parent structure with an hexagonal symmetry in which some of the U atoms are shifted by $c/2$ in an ordered fashion. Below $T_{N1}\approx$ 29 K, an incommensurate magnetic structure with a temperature-dependent wave vector $(0,0,k_z)$ is observed. Below $T_{N2}=$ 23.5 K, the wave vector locks in to $(0,0,0.5)$, forming an antiferromagnetic ground state. The U moments align along the $c$ axis retaining a large magnetic anisotropy. These findings highlight the role of the $5f$ orbitals from uranium in this structural family in driving both magnetic ordering and structural modulation and distinguish UV$_6$Sn$_6$ from its lanthanide-based analogs.

cond-mat.str-el↗

arXiv2025

Anomalous temperature-dependent magnetization in the nearly collinear antiferromagnet Y$_2$Co$_3$

Y$_2$Co$_3$ is a newly discovered antiferromagnetic (AFM) compound with distorted kagome layers. Previous investigations via bulk magnetization measurements suggested a complex noncollinear magnetic behavior, with magnetic moments primarily anti-aligned along the $b$ axis and some canting towards the $ac$ plane. In this study, we report the magnetic structure of Y$_2$Co$_3$ to be an A-type AFM structure with ferromagnetic (FM) interactions within the distorted kagome plane and an interplane antiferromagnetic interaction, as determined by single-crystal neutron diffraction. The magnetic moments align along the $b$ axis, with minimal canting towards the $c$ axis, at odds with the previous interpretation of bulk magnetization measurements. The magnetic moments on the two distinct Co sites are [0, -0.68(3), 0] $μ_B$ and [0, 1.25(4), 0.07(1)] $μ_B$. We attribute the previously reported "noncollinear" behavior to the considerable temperature dependence of itinerant AFM exchange interactions, induced by thermal contraction along the $b$ axis. Additionally, our examination of lattice constants through pressure studies reveals compensating effects on FM and AFM interactions, resulting in negligible pressure dependence of $T_\textrm{N}$.

cond-mat.str-el↗

arXiv2024

Multiple metamagnetic transitions in helical antiferromagnet CeVGe$_3$

We report on neutron diffraction, magnetoresistance, magnetization, and magnetic torque measurements under high magnetic field in the helical antiferromagnet CeVGe$_3$. This compound exhibits Kondo lattice coherence and helical antiferromagnetic (AFM) ordering at ambient pressure, similar to the well-studied CeRhIn$_5$. Our measurements reveal that CeVGe$_3$ undergoes a magnetic transition from an incommensurate (ICM) AFM state to an up-up-down-down commensurate (CM) AFM structure, followed by a transition to a novel phase at higher fields. A quantum phase transition occurs around 21.3 T. This rich magnetic field phase diagram closely resembles that of CeRhIn$_5$. Furthermore, angle-dependent magnetoresistance measurements reveal that all transitions in CeVGe$_3$ occur from the field component along the $ab$ plane. These findings highlight the intricate interplay among exchange interactions, crystal field effects, ground state properties, and crystalline symmetries.

cond-mat.str-el↗

arXiv2024

Easy-plane ferromagnetic ordering and crystal-field ground state in the Kondo lattice CeCuSi

We report the successful growth of CeCuSi single crystals using a metallic flux method and the physical properties using structural, magnetic, electrical transport, optical, and heat capacity measurements. CeCuSi crystallizes in a hexagonal-bar shape, and single crystal x-ray diffraction confirms the ZrBeSi-type structure (space group $P6_{3}/mmc$). CeCuSi orders ferromagnetically below $T_\textrm{C}=15.5$ K with easy magnetization direction within the basal plane. The Ce$^{3+}$ ions are situated within a triangular lattice with a point group of $D_{3d}$. We perform a detailed crystalline electric field (CEF) analysis of the anisotropic magnetic susceptibility, the Schottky anomaly in heat capacity, and the Raman-active excitations. The results indicate a ground state doublet with magnetic moment primarily in the basal plane, and a ferromagnetic interaction along both directions. The exponential behavior in resistivity and in heat capacity below $T_\textrm{C}$ can also be well explained by the ferromagnetic magnon model. We found that CeCuSi does not exhibit the CEF hard axis ordering observed in many ferromagnetic Kondo lattice (FM-KL) compounds. Our CEF analysis suggests that the exchange interactions along both axes are ferromagnetic, potentially explaining the absence of hard-axis ordering.

cond-mat.str-el↗

arXiv2024

Superconductivity Mediated by Nematic Fluctuations in Tetragonal $\textrm{Fe}\textrm{Se}_{1-x}\textrm{S}_{x}$

Nematic phases, where electrons in a solid spontaneously break rotational symmetry while preserving the translational symmetry, exist in several families of unconventional superconductors [1, 2]. Although superconductivity mediated by nematic fluctuations is well established theoretically [3-7], it has yet to be unambiguously identified experimentally [8, 9]. A major challenge is that nematicity is often intertwined with other degrees of freedom, such as magnetism and charge order. The FeSe$_{1-x}$S$_x$ family of iron based superconductors provides a unique opportunity to explore this concept, as it features an isolated nematic phase that can be suppressed by sulfur substitution at a quantum critical point (QCP) near $x_c = 0.17$, where nematic fluctuations are the largest [10-12]. Here, we performed scanning tunneling spectroscopy measurements to visualize Boguliubov quasiparticle interference patterns, from which we determined the momentum structure of the superconducting gap near the Brillouin zone $\Gamma$ point of FeSe$_{0.81}$S$_{0.19}$. The results reveal an anisotropic, near nodal gap with minima that are $45^\circ$ rotated with respect to the Fe-Fe direction, characteristic of a nematic pairing interaction, contrary to the usual isotropic gaps due to spin mediated pairing in other tetragonal Fe-based superconductors. The results are also in contrast with pristine FeSe, where the pairing is mediated by spin fluctuations and the gap minima are aligned with the Fe-Fe direction. Therefore, the measured gap structure demonstrates not only a fundamental change of the pairing mechanism across the phase diagram of FeSe$_{1-x}$S$_x$, but it also indicates the existence of superconductivity mediated by nematic fluctuations in FeSe$_{0.81}$S$_{0.19}$.

cond-mat.supr-con↗

arXiv2023

Magnetic structure and Kondo lattice behavior in CeVGe$_3$: an NMR and neutron scattering study

We present nuclear magnetic resonance (NMR), neutron diffraction, magnetization, and transport measurements on a single crystal and powder of CeVGe$_3$. This material exhibits heavy fermion behavior at low temperature, accompanied by antiferromagnetic (AFM) order below 5.8 K. We find that the magnetic structure is incommensurate with AFM helical structure, characterized by a magnetic modulated propagation vector of $(0, 0, 0.49)$ with in-plane moments rotating around the $c$-axis. The NMR Knight shift and spin-lattice relaxation rate reveal a coherence temperature $T^*\sim 15$ K, and the presence of significant antiferromagnetic fluctuations reminiscent of the archetypical heavy fermion compound CeRhIn$_5$. We further identify a metamagnetic transition above $H_m\sim 2.5$ T for magnetic fields perpendicular to $c$. We speculate that the magnetic structure in this field-induced phase consists of a superposition with both ferromagnetic and antiferromagnetic components, which is consistent with the NMR spectrum in this region of the phase diagram. Our results thus indicate that CeVGe$_3$ is a hexagonal structure analog to tetragonal CeRhIn$_5$.

cond-mat.str-el↗

arXiv2022

Anomalous depinning of magnetic domain walls within the ferromagnetic phase of the Weyl semimetal Co3Sn2S2

We report bulk magnetization measurements and spatially resolved measurements of magnetic domains in Co3Sn2S2 single crystals. The results indicate that a previously reported magnetic anomaly around 130 K is due to an anomalous domain wall depinning upon cooling. Our measurements also reveal a hysteresis between field-cooled-cooling (FCC) and field-cooled-warming (FCW) magnetization curves acquired under a constant magnetic field below 300 Oe. This observation rules out the possibility that the anomaly stems from a second-order phase transition. Our results further suggest that changes in the shape of hysteresis loops from 5 K to 170 K are caused by an unusual temperature-dependent domain nucleation field that changes sign around 130 K. The Kerr rotation images of the magnetic domains confirm that the domain walls depin between 120 K and 140 K.

cond-mat.mtrl-sci↗

arXiv2022

Suppression of ferromagnetism governed by a critical lattice parameter in CeTiGe$\_3$ with hydrostatic pressure or V substitution

We combine structural and magnetic measurements to compare the different magnetic phase diagrams between the pressure and substitution studies in CeTiGe3. We report on the structural, magnetic, and electrical transport properties of single crystals of CeTi1-xVxGe3 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.9, and 1), and of polycrystalline samples (x = 0.5, 0.6, 0.7, 0.8), as well as structural properties of CeTiGe3 under pressure up to 9 GPa. The ferromagnetic ordering in CeTiGe3 is suppressed with V doping in CeTi1-xVxGe3, and suggests a possible ferromagnetic quantum critical point near x = 0.45. We perform a detailed crystalline electric field (CEF) analysis, and the magnetic susceptibility data in pure CeTiGe3 and CeVGe3 can be well explained by the CEF model. The proposed CEF energy levels suggest that there is a gradual change of the ground state from 5/2 state in CeTiGe3 to 1/2 state in CeVGe3, and a suppression of CEF splitting energies near the quantum critical region. When hydrostatic pressure is used instead of chemical substitution, the quantum critical point is avoided by the appearance of magnetic phases above around 4.1 GPa. In the substitution study, the ferromagnetic and antiferromagnetic regions are well separated, whereas they touch in the pressure study. We observe a different trend in the temperature dependence of the resistivity maximum in both studies, suggesting that the CEF splitting energy is suppressed by V substitution but enhanced by pressure. We also observe different responses in lattice constants between the two studies, highlighting the fact that substitution effects cannot be reduced to chemical pressure effects only. Nevertheless, when the magnetic phase diagrams of both hydrostatic pressure and substitution are compared, we find a common critical lattice constant c = 5.78 A where the ferromagnetic ordering is suppressed in both studies.

cond-mat.str-el↗

arXiv2022

Copper migration and surface oxidation of $\text{Cu}_{x}\text{Bi}_2\text{Se}_3$ in ambient pressure environments

Chemical modifications such as intercalation can be used to modify surface properties or to further functionalize the surface states of topological insulators. Using ambient pressure X-ray photoelectron spectroscopy, we report copper migration in $\text{Cu}_{x}\text{Bi}_2\text{Se}_3$, which occurs on a timescale of hours to days after initial surface cleaving. The increase in near-surface copper proceeds along with the oxidation of the sample surface and large changes in the selenium content. These complex changes are further modelled with core-level spectroscopy simulations, which suggest a composition gradient near the surface which develops with oxygen exposure. Our results shed light on a new phenomenon that must be considered for intercalated topological insulators$\unicode{x2014}$and intercalated materials in general$\unicode{x2014}$that surface chemical composition can change when specimens are exposed to ambient conditions.

cond-mat.mes-hall↗

arXiv2022

Robust antiferromagnetism in Y$_2$Co$_3$

We report on a solution-growth based method to synthesise single crystals of Y$_2$Co$_3$ and on its structural and magnetic properties. We find that Y$_2$Co$_3$ crystallizes in the La2Ni3-type orthorhombic structure with space group Cmce (No. 64), with Co forming distorted kagome lattices. Y$_2$Co$_3$ orders antiferromagnetically below $T_N$ = 252 K. Magnetization measurements reveal that the moments are primarily aligned along the b axis with evidence for some canting. Band-structure calculations indicate that ferromagnetic and antiferromagnetic orders are nearly degenerate, at odds with experimental results. Magnetization measurements under pressure up to 1 GPa reveal that the N/'eel temperature decreases with the slope of -1.69 K/GPa. We observe a field-induced spin-flop transition in the magnetization measurements at 1.5 K and 21 T with magnetic field along the b direction. The magnetization is not saturated up to 35 T, indicating that the antiferromagnetic ordering in Y$_2$Co$_3$ is quite robust, which is surprising for such a Co-rich intermetallic.

cond-mat.str-el↗

arXiv2022

Stabilization of CeGe$_3$ with Ti and O featuring tetravalent Ce ions: (Ce$_{0.85}$Ti$_{0.15}$)Ge$_3$O$_{0.5}$

Sub-oxides with the anti-perovskite structure constitute a large part of the interstitial stabilized compounds with novel chemical and physical properties, especially in the systems containing rare earth elements and/or early transition metal elements. A new sub-oxide compound, (Ce$_{0.85}$Ti$_{0.15}$)Ge$_3$O$_{0.5}$, is synthesized by flux growth. The compound crystallizes in a cubic structure which can be considered as an ordered vacancy variant of anti-perovskite-structure or an ordered interstitial variant of the Cu$_3$Au type, with a space group of Fm-3m (225). The structure is refined from X-ray single crystal methods. The compound may be considered as a compound stabilized by chemical pressure generated by the substitution and the interstitials atoms. The stability of the compounds in RTr$_3$ systems (R = rare earth elements except Pm and Sc, Tr = tetrel elements such as Si, Ge, Sn, Pb) seem to be highly correlated to the ratio of the rare earth elements' radii to the main group elements' atomic radii. The magnetic property of the compound confirms the 4$^+$ valence of the Ce atoms. The resistivity measurement of the compound shows that it is metallic.

cond-mat.mtrl-sci↗

arXiv2021

Dirac lines and loop at the Fermi level in the Time-Reversal Symmetry Breaking Superconductor LaNiGa$_2$

Unconventional superconductors have Cooper pairs with lower symmetries than in conventional superconductors. In most unconventional superconductors, the additional symmetry breaking occurs in relation to typical ingredients such as strongly correlated Fermi liquid phases, magnetic fluctuations, or strong spin-orbit coupling in noncentrosymmetric structures. In this article, we show that the time-reversal symmetry breaking in the superconductor LaNiGa$_2$ is enabled by its previously unknown topological electronic band structure. Our single crystal diffraction experiments indicate a nonsymmorphic crystal structure, in contrast to the previously reported symmorphic structure. The nonsymmorphic symmetries transform the $k_z=π/c$ plane of the Brillouin zone boundary into a node-surface. Band-structure calculations reveal that distinct Fermi surfaces become degenerate on the node-surface and form Dirac lines and a Dirac loop at the Fermi level. Two symmetry related Dirac points remain degenerate under spin-orbit coupling. ARPES measurements confirm the calculations and provide evidence for the Fermi surface degeneracies on the node-surface. These unique topological features enable an unconventional superconducting gap in which time-reversal symmetry can be broken in the absence of other typical ingredients. LaNiGa$_2$ is therefore a topological crystalline superconductor that breaks time-reversal symmetry without any overlapping magnetic ordering or fluctuations. Our findings will enable future discoveries of additional topological superconductors.

cond-mat.supr-con↗