arXiv subjects

Y. Skourski

Publications and source records attributed to Y. Skourski.

At least 19 recordsLinked to original sources

arXiv2026

Ground-state properties of the $S=3/2$ anisotropic triangular lattice antiferromagnet Na$_3$Cr(PO$_4$)$_2$

We report the crystal structure and magnetic properties of a $S=3/2$ anisotropic triangular lattice compound Na$_3$Cr(PO$_4$)$_2$ employing single-crystal and powder x-ray diffraction, magnetization, heat capacity, and $^{31}$P nuclear magnetic resonance (NMR) experiments, supported by the band structure calculations. Magnetic susceptibility exhibits a broad maximum around 3.5 K, indicating the presence of a short-range antiferromagnetic order, typical of a low-dimensional spin system. Magnetization and heat capacity manifest an antiferromagnetic long-range ordering at around $T_{\rm N} \simeq 2.6$ K. This was further confirmed by the drastic NMR line broadening and a peak in the nuclear spin-lattice and spin-spin relaxation rates. The isothermal magnetization data exhibit a field-induced spin-flop transition at around $\mu_0H_{\rm SF} \simeq 1.7$ T reminiscent of an anisotropic two-dimensional magnet, before saturating above $\mu_0H_{\rm sat} \simeq 4.5$ T. The saturation field was further upheld by the field-dependent NMR relaxation measurements at low temperatures. The $^{31}$P NMR spectral shape confirms the commensurate antiferromagnetic nature of the ordering below $T_{\rm N}$. \textit{Ab initio} calculations reveal a significant deformation of the triangular spin lattice, resulting in triangles with two antiferromagnetic couplings of similar strength and a much weaker coupling along the third side of the triangle.

cond-mat.mtrl-sci↗

arXiv2026

Magnetic field-induced phases in a model S=1 Haldane chain system

An $S=1$ Haldane chain is a one-dimensional (1D) quantum magnet where strong fluctuations result in quantum disordered singlet ground state with a gapped excitation spectrum. The gap magnitude is primarily set by the dominant intrachain interaction ($J_\text{1D}$). An applied magnetic field closes the gap at $B_\text{c1}$ and drives the system into a gapless Tomonaga-Luttinger liquid (TLL) regime, followed by, at lower temperatures, a Bose-Einstein condensate (BEC) ground state, persisting up to $B_\text{c2} \propto 4 J_\text{1D}/gμ_B$. Almost all previously studied experimental realizations of such systems were based on transition-metal complexes which typically suffer from intrinsic anisotropies or large $J_\text{1D}$ values, limiting the access to the full theoretical phase diagram. We report a comprehensive study of TLL and BEC phases in the organic Haldane chain system 3,5-bis(N-tert-butylaminoxyl)-3'-nitrobiphenyl (BoNO). The absence of anisotropy and a moderate $J_\text{1D}$ enable exploration of the complete $B-T$ phase diagram. Through $^1$H nuclear magnetic resonance, combined with theoretical analysis, we characterize the TLL properties, map the BEC phase boundary $T_c (B)$, determine the associated critical exponent $ν\approx 0.66$ at $B_\text{c2}$, and demonstrate universal quasiparticle scaling in the quantum-critical regime. These results provide full experimental validation of theoretical predictions for field-induced phases in an $S=1$ Haldane chain, made over two decades ago.

cond-mat.str-el↗

arXiv2025

Large out-of-equilibrium magnetocaloric effect in rare-earth zirconate pyrochlores

We explore the magnetic properties of Nd$_2$Zr$_2$O$_7$ and Pr$_2$Zr$_2$O$_7$ single crystals subjected to pulsed magnetic fields up to 60 T using magnetization and magnetocaloric-effect (MCE) measurements, with initial temperatures ranging from 2 to 31K. The MCE data exhibit pronounced and unconventional hysteresis loops, in which the sample temperature increases during both the up-sweep and down-sweep of the field. In Nd$_2$Zr$_2$O$_7$, the MCE further displays a striking plateau as a function of time, followed by a rapid temperature rise that begins at the maximum applied field, across pulses with differing peak-field strengths. Our magnetization measurements reveal an inferred temperature of the magnetic subsystem that differs significantly from the directly measured sample temperature and exhibits opposite hysteresis: the temperature is higher on the up-sweep than the down-sweep, unlike the direct measurements. These observations indicate a breakdown of thermal equilibrium between magnetic and lattice degrees of freedom on the timescale of the pulse ($\sim 10^{-1}$s). We interpret the results using a phenomenological model involving two thermally coupled subsystems - the magnetic ions and phonons, and a thermal reservoir, which accounts well for the behavior of Pr$_2$Zr$_2$O$_7$. However, it fails to reproduce the plateau seen in Nd$_2$Zr$_2$O$_7$. Agreement with Nd$_2$Zr$_2$O$_7$ data is improved substantially if we allow the thermal coupling between the magnetic and the lattice subsystems to depend on the product $\frac{HdH}{dt}$. Our results reveal anomalously slow heat transfer between magnetic and lattice subsystems and point toward a novel mechanism for dynamically controlling the heat flow in Nd$_2$Zr$_2$O$_7$ via the rate of magnetic field variation.

cond-mat.str-el↗

arXiv2025

Inhomogeneous dynamic state in the double trillium lattice antiferromagnet KBaFe$_2$(PO$_4$)$_3$

The three-dimensional (3D) magnet KBaFe$_2$(PO$_4$)$_3$ hosts a double-trillium lattice of Fe$^{3+}$ (spin, $S=5/2$) ions offering a prototypical platform to study the frustration induced effects in 3D. Through magnetization, specific heat, $^{31}$P nuclear magnetic resonance (NMR), and muon spin relaxation ($μ$SR) experiments, supported by first principles calculations, we uncover an unconventional ground state. Despite strong antiferromagnetic interactions with a large Curie-Weiss temperature $θ_{\rm CW} = -70(2)$ K, no magnetic long-range order is observed down to 30 mK. Below $T^{\ast}\simeq 3.5$ K, the NMR linewidth becomes nearly field-independent and the spin-spin relaxation rate $1/T_2$ saturates, accompanied by an inhomogeneous distribution of transverse nuclear magnetization $M_{xy}$. The latter indicates the emergence of short-range dynamical correlations, which was further corroborated by a robust and field-insensitive broad maximum in specific heat. In $μ$SR, we detect neither a static internal field nor spin freezing; instead the relaxation remains dynamic and is best described by two coexisting dynamic relaxation channels: a dominant fast (sporadic) channel and a slower Markovian component. Their differing weights and fluctuation rates suggest microscopic inhomogeneity in spin dynamics. Altogether, KBaFe$_2$(PO$_4$)$_3$ exemplifies a rare high-spin stochiometric 3D antiferromagnet that evades ordering and instead fosters a mosaic of spin dynamics driven by strong geometric frustration intrinsic to the trillium lattice.

cond-mat.mtrl-sci↗

arXiv2025

Intricacies of Frustrated Magnetism in the Kondo Metal YbAgGe

The combination of localized magnetic moments, their frustration and interaction with itinerant electrons is a key challenge of condensed matter physics. Frustrated magnetic interactions promote degenerate ground states with enhanced fluctuations, a topic that is predominantly studied in magnetic insulators. The coupling between itinerant and localized electrons in metals add complexity to the problem, and presently formulated only for extreme cases in which the itinerant electrons mediate exchange between localized spins (RKKY interaction) or suppress the formation of magnetic moments (Kondo screening). Here, we report an in-depth experimental study of the distorted Kagome metal YbAgGe, unravelling the open questions of how frustration, localized magnetism and itinerant electrons are intertwined in frustrated Kondo metals. We find that coupled itinerant and localized electrons give rise to dynamic magnetic correlations below T* ~ 20 K. At lower temperature, frustrated magnetic interactions establish anisotropic magnetic short-range correlations that culminate into antiferromagnetic long-range order below TN = 0.68 K with a significantly reduced modulated magnetic moment. We show that local moment Hamiltonians can yield limited understanding of the microscopic behaviour in frustrated metals, and prompt the extension of more sophisticated model Hamiltonians incorporating itinerant effects.

cond-mat.str-el↗

arXiv2025

Proximate spin-liquid behavior in the double trillium lattice antiferromagnet K$_2$Co$_2$(SO$_4$)$_3$

We report proximate quantum spin liquid behavior in K$_2$Co$_2$(SO$_4$)$_3$ with the magnetic Co$^{2+}$ ions embedded on a highly frustrated three-dimensional double trillium lattice. Single-crystal and high-resolution synchrotron powder x-ray diffraction experiments reveal a structural phase transition at $T_{\rm t} \simeq 125$ K from high-temperature cubic to low-temperature monoclinic phase with the three-fold superstructure. Magnetization and heat capacity consistently show the formation of the $J_{\rm eff} =1/2$ state of Co$^{2+}$ below 50 K. In zero field, K$_2$Co$_2$(SO$_4$)$_3$ shows signatures of static magnetic order formed below $T^* \simeq 0.6$ K, but muon spin relaxation experiments reveal a large fluctuating component that persists down to at least 50 mK, reminiscent of quantum spin liquid (QSL). Static order is completely suppressed in the small magnetic field of $\sim 1$ T, and low-temperature heat capacity demonstrates the $T^2$ behavior above this field, another fingerprint of QSL. Ab initio calculations show a competition of several antiferromagnetic couplings that render K$_2$Co$_2$(SO$_4$)$_3$ a promising pseudospin-$\frac12$ material for studying quantum magnetism in the double trillium lattice geometry.

cond-mat.mtrl-sci↗

arXiv2025

Magnetic phases and zone-folded phonons in a frustrated van der Waals magnet

2D magnetic materials have attracted extensive research interest due to their potential application in nanospintronics, optospintronics, and in magnonics. Ferromagnetic as well as antiferromagnetic layered materials have been demonstrated and successfully inserted into van der Waals heterostructures. However, the effects of magnetic frustration in van der Waals materials and the possibilities offered by spin configurations characterized by nonlinear spin arrangements have not been fully considered yet. Herein, we establish the magnetic phase diagram of bulk CrOCl, a frustrated van der Waals magnet, using magnetization and magneto-optical spectroscopy techniques. In particular, we use the magnetic superstructures relative to the crystallographic unit cell and the associated rich zone-folded phonon series to describe the magnetic field induced phases. Theoretical calculations taking into account the competing nearest neighbors magnetic exchange interactions provide a unique insight into the lattice vibrations of this class of magnetic system. This study expands the scope of 2D magnetic materials and provides a methodology to characterize frustrated van der Waals magnets.

cond-mat.mes-hall↗

arXiv2025

Strange metallicity encompasses high magnetic field-induced superconductivity in UTe2

A strange metallic state - characterized by an electrical resistivity that rises linearly in temperature $T$ at a rate governed by Planckian energetic dissipation - is present in a number of unconventional superconductors. Despite widespread investigative efforts, the microscopic properties of strange metallicity continue to defy elementary description. Here we investigate heavy fermion UTe$_2$, which hosts a suite of exotic spin-triplet superconducting phases, the most extreme of which resides in a narrow angular window of intense magnetic fields $>$ 40 T. Through angle-dependent magnetotransport measurements in pulsed magnetic fields, we find that this superconductive phase emerges from a strange metallic state with Planckian $T$-linear resistivity confined to a small region of phase space where the field-induced superconductivity is strongest, suggesting a shared underlying mechanism. These findings reveal a novel setting for strange metallicity - in contrast to the general case of singlet superconductivity emerging on the border of antiferromagnetism, here strange metallicity accompanies field-induced triplet superconductivity within a spin-polarized magnetic state. This result demonstrates the remarkable ubiquity of strange metallicity across diverse materials settings, and highlights UTe$_2$ as a unique platform for exploring the interplay between unconventional superconductivity and quantum criticality.

cond-mat.str-el↗

arXiv2025

A quantum critical line bounds the high field metamagnetic transition surface in UTe$_2$

Quantum critical phenomena are widely studied across various materials families, from high temperature superconductors to magnetic insulators. They occur when a thermodynamic phase transition is suppressed to zero temperature as a function of some tuning parameter such as pressure or magnetic field. This generally yields a point of instability - a so-called quantum critical point - at which the phase transition is driven exclusively by quantum fluctuations. Here we show that the heavy fermion metamagnet UTe$_2$ possesses a quantum phase transition at extreme magnetic field strengths of over 70 T. Rather than terminating at one singular point, we find that the phase boundary is sensitive to magnetic field components in each of the three Cartesian axes of magnetic field space. This results in the three-dimensional transition surface being bounded by a continuous ring of quantum critical points, the locus of which forms an extended line of quantum criticality - a novel form of quantum critical phase boundary. Within this quantum critical line sits a magnetic field-induced superconducting state in a toroidal shape, which persists to fields over 70~T. We model our data by a phenomenological free energy expansion, and show how a three-dimensional quantum critical phase boundary - rather than a more conventional singular point of instability - anchors the remarkable high magnetic field phase landscape of UTe$_2$.

cond-mat.supr-con↗

arXiv2025

Ground state properties of a spin-$\frac{5}{2}$ frustrated triangular lattice antiferromagnet NH$_{4}$Fe(PO$_{3}$F)$_2$

Structural and magnetic properties of a two-dimensional spin-$\frac{5}{2}$ frustrated triangular lattice antiferromagnet NH$_{4}$Fe(PO$_{3}$F)$_2$ are explored via x-ray diffraction, magnetic susceptibility, high-field magnetization, heat capacity, and $^{31}$P nuclear magnetic resonance experiments on a polycrystalline sample. The compound portrays distorted triangular units of the Fe$^{3+}$ ions with anisotropic bond lengths. The magnetic susceptibility shows a broad maxima around $T^{\rm{max}}_χ\simeq 12$ K, mimicking the short-range antiferromagnetic order of a low-dimensional spin system. The magnetic susceptibility and NMR shift could be modeled assuming the spin-$5/2$ isotropic triangular lattice model and the average value of the exchange coupling is estimated to be $J/k_{\rm B} \simeq 1.7$ K. This value of the exchange coupling is reproduced well from the saturation field of the pulse field data. It shows the onset of a magnetic ordering at $T_{\rm N} \simeq 5.7$ K, setting the frustration ratio of $f = \frac{|θ_{\rm CW}|}{T_{\rm N}} \simeq 5.7$. Such a value of $f$ reflects moderate magnetic frustration in the compound. The d$M$/d$H$ vs $H$ plots of the low temperature magnetic isotherms exhibit a sharp peak at $H_{\rm SF} \simeq 1.45$ T, suggesting a field-induced spin-flop transition and magnetic anisotropy. The rectangular shape of the $^{31}$P NMR spectra below $T_{\rm N}$ unfolds that the ordering is commensurate antiferromagnet type. Three distinct phase regimes are clearly discerned in the $H - T$ phase diagram, redolent of a frustrated magnet with in-plane (XY-type) anisotropy.

cond-mat.mtrl-sci↗

arXiv2024

Magnon-phonon interactions in the spinel compound MnSc$_2$Se$_4$

We investigated the magnetic and magnetoelastic properties of MnSc$_2$Se$_4$ single crystals at low temperature under a magnetic field directed along the crystallographic [111] axis. The magnetization data at low temperature show a linear increase with magnetic field, until saturation is reached above 15 T. In ultrasound, a longitudinal acoustic mode shows a softening in field, which is absent for a transverse acoustic mode. We discuss these results using a microscopic model based on the framework of linear spin-wave theory. The magnetic and magnetoelastic data are qualitatively reproduced by considering magnon-phonon interactions arising from exchange-striction coupling between the crystal lattice and spin-wave fluctuations in the zero-temperature limit.

cond-mat.str-el↗

arXiv2024

Anomalous quasielastic scattering contribution in the centrosymmetric multi-$\mathbf{q}$ helimagnet SrFeO$_3$

Centrosymmetric compounds which host three-dimensional topological spin structures comprise a distinct subclass of materials in which multiple-$\mathbf{q}$ magnetic order is stabilized by anisotropy and bond frustration in contrast to the more common path of antisymmetric exchange interactions. Here we investigate static and dynamic magnetic properties of the cubic perovskite SrFeO$_3$ $\unicode{x2013}$ a rare example of a centrosymmetric material hosting two types of topological spin textures: skyrmion- and hedgehog-lattice phases. Our detailed magnetization and dilatometry measurements describe the domain selection processes and phase transitions in SrFeO$_3$. Spin excitations are investigated using inelastic neutron scattering for all three zero-field phases. In the higher-temperature ordered phases, high-energy magnons increasingly lose coherence, so that spin fluctuations are dominated by a distinct quasielastic component at low energies. We anticipate that this could be generic to symmetric helimagnets in which the chiral symmetry is spontaneously broken by the magnetic order.

cond-mat.str-el↗

arXiv2024

Enhanced triplet superconductivity in next generation ultraclean UTe2

The unconventional superconductor UTe$_2$ exhibits numerous signatures of spin-triplet superconductivity -- a rare state of matter which could enable quantum computation protected against decoherence. UTe$_2$ possesses a complex phase landscape comprising two magnetic field-induced superconducting phases, a metamagnetic transition to a field-polarised state, along with pair- and charge-density wave orders. However, contradictory reports between studies performed on UTe$_2$ specimens of varying quality have severely impeded theoretical efforts to understand the microscopic origins of the exotic superconductivity. Here, we report a comprehensive suite of high magnetic field measurements on a new generation of pristine quality UTe$_2$ crystals. Our experiments reveal a significantly revised high magnetic field superconducting phase diagram in the ultraclean limit, showing a pronounced sensitivity of field-induced superconductivity to the presence of crystalline disorder. We employ a Ginzburg-Landau model that excellently captures this acute dependence on sample quality. Our results suggest that in close proximity to a field--induced metamagnetic transition the enhanced role of magnetic fluctuations -- that are strongly suppressed by disorder -- is likely responsible for tuning UTe$_2$ between two distinct spin-triplet superconducting phases.

cond-mat.supr-con↗

arXiv2024

Continuum excitations in a spin-supersolid on a triangular lattice

Magnetic, thermodynamic, neutron diffraction and inelastic neutron scattering are used to study spin correlations in the easy-axis XXZ triangular lattice magnet K2Co(SeO3)2. Despite the presence of quasi-2D "supersolid" magnetic order, the low-energy excitation spectrum contains no sharp modes and is instead a broad and structured multi-particle continuum. Applying a weak magnetic field drives the system into an m = 1/3 fractional magnetization plateau phase and restores sharp spin wave modes. To some extent, the behavior at zero field can be understood in terms of spin wave decay. However, the presence of clear excitation minima at the M-points of the Brillouin zone suggest that the spinon language may provide a more adequate description, and signals a possible proximity to a Dirac spin liquid state.

cond-mat.str-el↗

arXiv2024

Quantum interference between quasi-2D Fermi surface sheets in UTe2

UTe$_2$ is a spin-triplet superconductor candidate for which high quality samples with long mean free paths have recently become available, enabling quantum oscillation measurements to probe its Fermi surface and effective carrier masses. It has recently been reported that UTe$_2$ possesses a 3D Fermi surface component [Phys. Rev. Lett. 131, 036501 (2023)]. The distinction between 2D and 3D Fermi surface sections in triplet superconductors can have important implications regarding the topological properties of the superconductivity. Here we report the observation of oscillatory components in the magnetoconductance of UTe$_2$ at high magnetic fields. We find that these oscillations are well described by quantum interference between quasiparticles traversing semiclassical trajectories spanning magnetic breakdown networks. Our observations are consistent with a quasi-2D model of this material's Fermi surface based on prior dHvA-effect measurements. Our results strongly indicate that UTe$_2$ -- which exhibits a multitude of complex physical phenomena -- possesses a remarkably simple Fermi surface consisting exclusively of two quasi-2D cylindrical sections.

cond-mat.supr-con↗

arXiv2023

Repulsive Tomonaga-Luttinger Liquid in Quasi-one-dimensional Alternating Spin-$1/2$ Antiferromagnet NaVOPO$_4$

We probe the magnetic field-induced Tomonaga-Luttinger liquid (TLL) state in the bond-alternating spin-$1/2$ antiferromagnetic (AFM) chain compound NaVOPO$_4$ using thermodynamic as well as local $μ$SR and $^{31}$P NMR probes down to milli-K temperatures in magnetic fields up to 14~T. The $μ$SR and NMR relaxation rates in the gapless TLL regime decay slowly following characteristic power-law behaviour, enabling us to directly determine the interaction parameter $K$ as a function of the magnetic field. These estimates are cross-checked using magnetization and specific heat data. The field-dependent $K$ lies in the range of $0.4 < K < 1$ and indicates repulsive nature of interactions between the spinless fermions, in line with the theoretical predictions. This renders NaVOPO$_4$ the first experimental realization of TLL with repulsive fermionic interactions in hitherto studied $S=1/2$ bond-alternating AFM-AFM chain compounds.

cond-mat.str-el↗

arXiv2023

Magnetic structure and phase diagram of the Heisenberg-Ising spin chain antiferromagnetic PbCo$_{2}$V$_{2}$O$_{8}$

The effective spin-1/2 antiferromagnetic Heisenberg-Ising chain materials, ACo$_2$V$_2$O$_8$, A = Sr, Ba, are a rich source of exotic fundamental phenomena and have been investigated for their model magnetic properties both in zero and non-zero magnetic fields. Here we investigate a new member of the family, namely PbCo$_2$V$_2$O$_8$. We synthesize powder and single crystal samples of PbCo$_2$V$_2$O$_8$ and determine its magnetic structure using neutron diffraction. Furthermore, the magnetic field/temperature phase diagrams for magnetic field applied along the c, a, and [110] crystallographic directions in the tetragonal unit cell are determined via magnetization and heat capacity measurements. A complex series of phases and quantum phase transitions are discovered that depend strongly on both the magnitude and direction of the field. Our results show that \pcvo is an effective spin-1/2 antiferromagnetic Heisenberg-Ising chain with properties that are in general comparable to those of SrCo$_2$V$_2$O$_8$ and BaCo$_2$V$_2$O$_8$. One interesting departure from the results of these related compounds, is however, the discovery of a new field-induced phase for the field direction $H\|$[110] which has not been previously observed.

cond-mat.str-el↗

arXiv2023

Double magnetic transitions and exotic field induced phase in the triangular lattice antiferromagnets Sr$_3$Co(Nb,Ta)$_2$O$_9$

Two triangular lattice antiferromagnets Sr$_3$Co(Nb,Ta)$_2$O$_9$ with an effective $j_{\rm eff}=1/2$ of Co$^{2+}$ are synthesized and their magnetic properties are investigated via magnetization and heat capacity measurements. The leading in-plane antiferromagnetic exchange coupling is estimated to be $J/k_{\rm B} \simeq 4.7$ K and 5.8 K, respectively. Both the compounds feature two-step magnetic transitions at low temperatures [($T_{\rm N1} \simeq 1.47$ K and $T_{\rm N2} \simeq 1.22$ K) and ($T_{\rm N1} \simeq 0.88$ K and $T_{\rm N2} \simeq 0.67$ K), respectively], driven by weak easy-axis anisotropy. Under magnetic field Sr$_3$CoNb$_2$O$_9$ evinces a plateau at $1/3$ magnetization. Interestingly, the high field magnetization of Sr$_3$CoTa$_2$O$_9$ reveals an exotic regime (between $H_{\rm S1}$ and $H_{\rm S2}$), below the fully polarized state in which the heat capacity at low temperatures is governed by a power law ($C_{\rm p} \propto T^α$) with a reduced exponent $α\simeq 2$. These results demonstrate an unusual field induced state with gapless excitations in the strongly frustrated magnet Sr$_3$CoTa$_2$O$_9$. The complete $T-H$ phase diagram is discussed for both the compounds.

cond-mat.str-el↗