arXiv subjects

L. Keller

Publications and source records attributed to L. Keller.

At least 19 recordsLinked to original sources

arXiv2026

Neutron scattering evidence for two-dimensionally coupled spin-dimerized antiferromagnetic lattice in {\alpha}-Cu2P2O7

The microscopic magnetic model of the low-dimensional quantum magnet alpha-Cu2P2O7 has remained controversial. We present a comprehensive study of its magnetic ground state and excitation spectrum using temperature-dependent inelastic neutron scattering, neutron diffraction, magnetization measurements, and comprehensive spin-wave modeling. Our results unambiguously establish alpha-Cu2P2O7 as a two-dimensionally coupled spin-dimerized antiferromagnetic (AF) lattice within the bc plane, with a dominant AF exchange J2 = 7.73 meV (hereafter referred to as "intradimer exchange") and weaker exchange couplings J1, J3, and J4 in the two-dimensional lattice (hereafter referred to as "interdimer exchange"), in agreement with LDA-based density functional theory and in contrast to previous GGA+U predictions. The dominant intradimer AF exchange is found between seventh-nearest-neighbor Cu-Cu ion pairs [d(Cu-Cu) = 5.125(3) A] rather than nearest-neighbor Cu-Cu ion pairs [d(Cu-Cu) = 3.014(1) A] of the structural dimers. Weak interlayer coupling (J5 = 0.03 meV) stabilizes long-range antiferromagnetic order below TN = 25 K. We further identify a weak single-ion anisotropy, associated with the distorted CuO5 polyhedra, that opens a gap in the spin-excitation spectrum and drives a field-induced metamagnetic transition. Systematic spin-wave calculations elucidate the distinct roles of interlayer coupling J5 and anisotropy term D in producing two distinct energy gaps at different antiferromagnetic zone centers. Complementary neutron diffraction and magnetization measurements as a function of applied magnetic field uncover a previously overlooked metamagnetic transition near 13 kOe and allow construction of the magnetic phase diagram in the H-T plane.

cond-mat.str-el↗

arXiv2025

Incommensurate magnetic order arising from frustrated interchain interactions in the spin-1/2 chain compound AgCuVO$_4$

Quantum spin chains with competing interactions offer a platform where low dimensionality and frustration--both acting to suppress magnetic order--intersect. We studied magnetic ordering in the spin-1/2 chain compound AgCuVO$_4$ using muon spin spectroscopy and neutron diffraction. Long-range magnetic order emerges at $T_N = 2.0(1)$ K, which is $\sim$1/200 of the dominant intrachain coupling $J$ and $\sim$1/15 of the interchain interactions. The collinear incommensurate amplitude-modulated magnetic structure features a reduced ordered moment of 0.13(3) $\mu_\mathrm{B}$, confined to the $ab$ plane and modulated along the $c$ axis--perpendicular to the spin chains--indicating frustrated interchain couplings. The low $T_N$, small moment, and incommensurate order highlight strong frustration, positioning AgCuVO$_4$ as a model system for exploring frustration in quantum spin chains.

cond-mat.str-el↗

arXiv2025

Pressure evolution of coplanar antiferromagnetism in heavy-fermion Ce$_{2}$CoAl$_{7}$Ge$_{4}$

Ce$_{2}$$M$Al$_{7}$Ge$_{4}$ ($M=$ Co, Ir, Ni or Pd) are heavy-fermion materials and host a variety of ground states ranging from magnetism to non-Fermi liquid behavior. The Co, Ir, and Ni members of the series undergo magnetic ordering with decreasing transition temperatures. In contrast, the Pd compound does not magnetically order down to 0.4 K and shows non-Fermi liquid behavior, suggesting proximity to a magnetic quantum critical point. Among the series, Ce$_{2}$CoAl$_{7}$Ge$_{4}$ orders antiferromagnetically below $T_N=1.9$ K along with heavy-fermion behavior below 15 K. We report the magnetic structure of the antiferromagnetic phase in Ce$_{2}$CoAl$_{7}$Ge$_{4}$ and the evolution of the magnetic transition under external pressure. Rietveld refinement of the neutron diffraction data suggests a coplanar antiferromagnetic structure with a wave vector $k = (1,1,1)$ and an ordered moment of $0.383 \pm 0.018 \> \mu_B$ in the antiferromagnetic phase. Electrical resistivity and AC calorimetry measurements under hydrostatic pressure reveal a suppression of the antiferromagnetic transition toward zero temperature around $p=1.1$ GPa. However, there is no evidence of non-Fermi liquid behavior associated with the suppression of magnetism by pressure, unlike the effect of transition-metal substitution.

cond-mat.str-el↗

arXiv2024

Structure and Magnetic Properties of a Maple Leaf antiferromagnet Ho$_3$ScO$_6$

Ho$_3$ScO$_6$ harbours a frustrated Maple Leaf Lattice (MLL). It crystalizes in the Mg$_3$TeO$_6$-type structure, and has a centrosymmetric trigonal space group (R$\bar{3}$). This system contains stacked layers of magnetic rings along the c-axis consisting of six magnetic Ho$^{3+}$ ions forming Ho hexagons, which are connected into a 2-dimensional network by equilateral and isosceles triangles to form a rare example of a MLL. Long range magnetic order is reached below $T_N=4.1$ K with a 120$^\circ$ spin arrangement on the equilateral triangles resulting in a positive vector chirality ground state configuration.

cond-mat.str-el↗

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↗

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

Revisiting the antiferromagnetic structure of $\rm Tb_{14}Ag_{51}$. The importance of distinguishing alternative symmetries for a multidimensional order parameter

We revisit the antiferromagnetic structure of $\rm Tb_{14}Ag_{51}$ [P. Fischer., $et\, al$. (2005). PRB, 72 134413] with the propagation vector $[{1\over3},{1\over3},{0}] $ and parent space group $P6/m$ using both magnetic symmetry and irreducible representation arguments. We have found a new magnetic structure under the hexagonal Shubnikov magnetic space group $P\bar{6}'$, which fits much better the experimental data. This new solution was obtained by constraining the spin arrangement to one of the three possible magnetic space groups of maximal symmetry that can be realised by a magnetic ordering transforming according to the 4-dimensional physically irreducible representation that is known to be relevant in this magnetic phase. The refined model, parameterised under $P\bar{6}'$, implicitly includes the presence of a third harmonic with the propagation vector at the gamma point $[0,0,0]$, which has an important weight in the final result. The structure consists of 13 symmetry-independent Tb magnetic moments with the same size of $8.48(2)μ_B$, propagating cycloidally in the $ab$-plane. The modulation has a substantial deviation from being purely sinusoidal due to the contribution of the mentioned third harmonic.

cond-mat.str-el↗

arXiv2022

Magnetic order and exchange couplings in the frustrated diamond lattice antiferromagnet MnSc$_2$Se$_4$

We report the magnetic properties of $A$-site spinel compound MnSc$_2$Se$_4$. The macroscopic magnetic measurements uncovers successive magnetic transitions at $T_{\rm{N1}}$= 2.04 K, followed by two further transitions at $T_{\rm{N2}}$=1.8 K and $T_{\rm{N3}}$=1.6 K. Neutron powder diffraction reveals that both, $T_{\rm{N2}} < T < T_{\rm{N1}}$ and $T <T_{\rm{N3}}$, orders are associated with the propagation vector $k$=(3/4 3/4 0), while the magnetic structures are collinear amplitude modulated and helical, respectively. Using neutron powder spectroscopy we demonstrated the effect of substitution of S by Se on the magnetic exchange. The energy range of the spin-wave excitations is supressed due to the chemical pressure of the $X$- ion in MnSc$_2X_4$ ($X$=S, Se) spinels.

cond-mat.str-el↗

arXiv2022

Magnetism of the 2D honeycomb layered Na2Ni2TeO6 compound driven by intermediate Na-layer crystal-structure

The microscopic spin-spin correlations in the 2D layered spin-1 honeycomb lattice compound Na2Ni2TeO6 have been investigated by neutron diffraction and inelastic neutron scattering. The honeycomb lattice of spin-1 Ni2+ ions, within the crystallographic ab planes, are well separated along the c axis by an intermediate Na layer whose crystal structure contains chiral nuclear density distributions of Na ions. The chirality of the alternating Na layers is opposite. Such alternating chirality of the Na layer dictates the magnetic periodicity along the c axis where an up-up-down-down spin arrangement of the in-plane zigzag AFM structure is found. Besides, the above described commensurate (CM) zigzag AFM order state is found to coexist with an incommensurate (ICM) AFM state below the TN ~ 27.5 K. The ICM state is found to appear at much higher temperature ~ 50 K and persists down to lowest measured temperature of 1.7 K. Our reverse Monte Carlo (RMC) analysis divulges a two dimensional (2D) magnetic correlations (within the ab plane) of the ICM AFM state over the entire temperature range 1.7-50 K. Further, the spin-Hamiltonian has been determined by carrying out inelastic neutron scattering experiments and subsequent linear spin-wave theory analysis which reveals the presence of competing inplane exchange interactions up to 3 rd nearest neighbours consistent with the zigzag AFM ground state, and weak interplanar interaction as well as a weak single-ion-anisotropy. The values of the exchange constants yield that Na2Ni2TeO6 is situated well inside the zigzag AFM phase (spans over a wide ranges of J2/J1 and J3/J1 values) in the theoretical phase diagram. The present study, thus, provides a detailed microscopic understanding of the magnetic correlations and divulges the intertwining magneto-structural correlations.

cond-mat.str-el↗

arXiv2021

Magnetic correlations in triangular antiferromagnet FeGa$_2$S$_4$

The crystal structure and magnetic correlations in triangular antiferromagnet FeGa$_2$S$_4$ are studied by x-ray diffraction, magnetic susceptibility, neutron diffraction and neutron inelastic scattering. We report significant mixing at the cation sites and disentangle magnetic properties dominated by major and minor magnetic sites. The magnetic short-range correlations at 0.77 Å$^{-1}$ correspond to the major sites and being static at base temperature they evolve into dynamic correlations around 30 - 50 K. The minor sites contribute to the magnetic peak at 0.6 Å$^{-1}$, which vanishes at 5.5 K. Our analytical studies of triangular lattice models with bilinear and biquadratic terms provide the ratios between exchanges for the proposed ordering vectors. The modelling of the inelastic neutron spectrum within linear spin wave theory results in the set of exchange couplings $J_1=1.7$\,meV, $J_2=0.9$\,meV, $J_3=0.8$\,meV for the bilinear Heisenberg Hamiltonian. However, not all features of the excitation spectrum are explained with this model.

cond-mat.str-el↗

arXiv2021

Magnetic order in the quasi-one-dimensional Ising system RbCoCl$_3$

The dynamical properties of free and bound domain-wall excitations in Ising-chain materials have recently become the focus of intense research interest. New materials and spectrometers have made it possible to control the environment of coupled Ising chains by both effective internal and applied external fields, which can be both longitudinal and transverse, and thus to demonstrate how the resulting magnetic phase transitions and the nature of the associated excited states obey fundamental symmetry properties. In RbCoCl$_3$, the weakly coupled Ising chains form a triangular lattice whose frustrated geometry and magnetic ordering transitions at low temperature open new possibilities for the Ising-chain environment. We have investigated the structure and magnetism in RbCoCl$_3$ by high-resolution x-ray diffraction and neutron scattering measurements on powder and single crystal samples between 1.5 K and 300 K. Upon cooling, the Co$^{2+}$ spins develop one-dimensional antiferromagnetic correlations along the chain axis ($c$-axis) below 90 K. Below the first Néel temperature, $T_{N1}$ = 28 K, a partial 3D magnetic order sets in, with propagation vector ${\vec k}_1$ = (1/3,1/3,1), the moments aligned along the $c$-axis and every third chain uncorrelated from its neighbours. Only below a second magnetic phase transition at $T_{N2}$ = 13 K does the system achieve a fully ordered state, with two additional propagation vectors: ${\vec k}_2$ = (0,0,1) establishes a "honeycomb" $c$-axis order, in which 1/3 of the chains are subject to a strong effective mean field due to their neighbours whereas 2/3 experience no net field, while ${\vec k}_3$ = (1/2,0,1) governs a small, staggered in-plane ordered moment. We conclude that RbCoCl$_3$ is an excellent material to study the physics of Ising chains in a wide variety of temperature-controlled environments.

cond-mat.str-el↗

arXiv2020

Magnetism and anomalous transport in the Weyl semimetal PrAlGe: Possible route to axial gauge fields

In magnetic Weyl semimetals, where magnetism breaks time-reversal symmetry, large magnetically sensitive anomalous transport responses are anticipated that could be useful for topological spintronics. The identification of new magnetic Weyl semimetals is therefore in high demand, particularly since in these systems Weyl node configurations may be easily modified using magnetic fields. Here we explore experimentally the magnetic semimetal PrAlGe, and unveil a direct correspondence between easy-axis Pr ferromagnetism and anomalous Hall and Nernst effects. With sizes of both the anomalous Hall conductivity and Nernst effect in good quantitative agreement with first principles calculations, we identify PrAlGe as a system where magnetic fields can connect directly to Weyl nodes via the Pr magnetization. Furthermore, we find the predominantly easy-axis ferromagnetic ground state co-exists with a low density of nanoscale textured magnetic domain walls. We describe how such nanoscale magnetic textures could serve as a local platform for tunable axial gauge fields of Weyl fermions.

cond-mat.str-el↗

arXiv2020

Neutron powder diffraction study of NaMn$_2$O$_4$ and Li$_{0.92}$Mn$_2$O$_4$: New insights on spin-charge-orbital ordering

The high-pressure synthesized quasi-one-dimensional compounds NaMn$_2$O$_4$ and Li$_{0.92}$Mn$_2$O$_4$ are both antiferromagnetic insulators, and here their atomic and magnetic structures were investigated using neutron powder diffraction. The present crystal structural analyses of NaMn2O4 reveal that Mn3+/Mn4+ charge-ordering state exist even at low temperature (down to 1.5 K). It is evident from one of the Mn sites shows a strongly distorted Mn3+ octahedra due to the Jahn-Teller effect. Above TN = 39 K, a two-dimensional short-range correlation is observed, as indicated by an asymmetric diffuse scattering. Below TN, two antiferromagnetic transitions are observed (i) a commensurate long-range Mn3+ spin ordering below 39 K, and (ii) an incommensurate Mn4+ spin ordering below 10 K. The commensurate magnetic structure (kC = 0.5, -0.5, 0.5) follows the magnetic anisotropy of the local easy axes of Mn3+, while the incommensurate one shows a spin-density-wave order with kIC = (0,0,0.216). For Li$_{0.92}$Mn$_2$O$_4$, on the other hand, absence of a long-range spin ordered state down to 1.5 K is confirmed.

cond-mat.mtrl-sci↗

arXiv2020

Observation of plaquette fluctuations in the spin-1/2 honeycomb lattice

Quantum spin liquids are materials that feature quantum entangled spin correlations and avoid magnetic long-range order at T = 0 K. Particularly interesting are two-dimensional honeycomb spin lattices where a plethora of exotic quantum spin liquids have been predicted. Here, we experimentally study an effective S=1/2 Heisenberg honeycomb lattice with competing nearest and next-nearest neighbor interactions. We demonstrate that YbBr$_3$ avoids order down to at least T=100 mK and features a dynamic spin-spin correlation function with broad continuum scattering typical of quantum spin liquids near a quantum critical point. The continuum in the spin spectrum is consistent with plaquette type fluctuations predicted by theory. Our study is the experimental demonstration that strong quantum fluctuations can exist on the honeycomb lattice even in the absence of Kitaev-type interactions, and opens a new perspective on quantum spin liquids.

cond-mat.str-el↗

arXiv2020

Topological Magnetic Phase in the Candidate Weyl Semimetal CeAlGe

We report the discovery of topological magnetism in the candidate magnetic Weyl semimetal CeAlGe. Using neutron scattering we find this system to host several incommensurate, square-coordinated multi-$\vec{k}$ magnetic phases below $T_{\rm{N}}$. The topological properties of a phase stable at intermediate magnetic fields parallel to the $c$-axis are suggested by observation of a topological Hall effect. Our findings highlight CeAlGe as an exceptional system for exploiting the interplay between the nontrivial topologies of the magnetization in real space and Weyl nodes in momentum space.

cond-mat.str-el↗

arXiv2019

Distortion mode anomalies in bulk PrNiO$_3$ illustrating the potential of symmetry-adapted distortion mode analysis for the study of phase transitions

The origin of the metal-to-insulator transition in RNiO3 perovskites with trivalent 4f ion has challenged the condensed matter research community for almost three decades. A drawback for progress in this direction has been the lack of studies combining physical properties and accurate structural data covering the full nickelate phase diagram. Here we focus on a small region close to the itinerant limit (R = Pr, 1.5K < T < 300K), where we investigate the gap opening and the simultaneous emergence of charge order in PrNiO3. We combine electric resistance, magnetization, and heat capacity measurements with high resolution neutron and synchrotron x-ray powder diffraction data that, in contrast to previous studies, we analyze in terms of symmetry-adapted distortion modes. Such analysis allows to identify the contribution of the different modes to the global distortion in a broad temperature range. Moreover, it shows that the structural changes at the MIT, traditionally described in terms of the evolution of the interatomic distances and angles, appear as abrupt increases of all nonzero mode amplitudes at TMIT = TN ~130K accompanied by the appearance of new modes below this temperature. A further interesting observation is the existence of a nearly perfect linear correlation between the amplitude of the breathing mode associated to the charge order and the staggered magnetization below the MIT. Our data also uncover a previously unnoticed anomaly at T* ~60K (~0.4 x TMIT), clearly visible in the electrical resistance, lattice parameters and some mode amplitudes. Since phase coexistence is only observed in a small temperature region around TMIT (~10K), these observations suggest the existence of a hidden symmetry in the insulating phase. We discuss some possible origins, among them the theoretically predicted existence of polar distortions induced by the non-centrosymmetric magnetic order.

cond-mat.str-el↗

arXiv2019

Positron driven muon source for a muon collider

The design of a future multi-TeV muon collider needs new ideas to overcome the technological challenges related to muon production, cooling, accumulation and acceleration. In this paper a layout of a positron driven muon source known as the Low EMittance Muon Accelerator (LEMMA) concept is presented. The positron beam, stored in a ring with high energy acceptance and low emittance, is extracted and driven to a multi-target system, to produce muon pairs at threshold. This solution alleviates the issues related to the power deposited and the integrated Peak Energy Density Deposition (PEDD) on the targets. Muons produced in the multi-target system will then be accumulated before acceleration and injection in the collider. A multi-target line lattice has been designed to cope with the focusing of both the positron and muon beams. Studies on the number, material and thickness of the targets have been carried out. A general layout of the overall scheme and a description is presented, as well as plans for future R&D.

physics.acc-ph↗

arXiv2019

Tunable Berry Curvature Through Magnetic Phase Competition in a Topological Kagome Magnet

Magnetic topological phases of quantum matter are an emerging frontier in physics and material science. Along these lines, several kagome magnets have appeared as the most promising platforms. However, the magnetic nature of these materials in the presence of topological state remains an unsolved issue. Here, we explore magnetic correlations in the kagome magnet Co_3Sn_2S_2. Using muon spin-rotation, we present evidence for competing magnetic orders in the kagome lattice of this compound. Our results show that while the sample exhibits an out-of-plane ferromagnetic ground state, an in-plane antiferromagnetic state appears at temperatures above 90 K, eventually attaining a volume fraction of 80% around 170 K, before reaching a non-magnetic state. Strikingly, the reduction of the anomalous Hall conductivity above 90 K linearly follows the disappearance of the volume fraction of the ferromagnetic state. We further show that the competition of these magnetic phases is tunable through applying either an external magnetic field or hydrostatic pressure. Our results taken together suggest the thermal and quantum tuning of Berry curvature field via external tuning of magnetic order. Our study shows that Co_3Sn_2S_2 is a rare example where the magnetic competition drives the thermodynamic evolution of the Berry curvature field, thus tuning its topological state.

cond-mat.mtrl-sci↗