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A. Zorko

Publications and source records attributed to A. Zorko.

At least 19 recordsLinked to original sources

Exotic magnetism and persistent spin dynamics in a frustrated Jeff = 1/2 triangular lattice antiferromagnet

The delicate interplay between competing degrees of freedom, anisotropy, and frustration-induced strong quantum fluctuations in pseudospin-$J_{\rm eff}=1/2$ rare-earth triangular-lattice antiferromagnets offers a promising platform for the experimental realization of exotic states with nontrivial low-energy excitations. Here, we present thermodynamic, inelastic neutron scattering (INS), and muon spin relaxation ($μ$SR) investigations of the frustrated magnet K$_3$NdTe$_2$O$9$, in which Nd$^{3+}$ ions constitute a structurally perfect triangular lattice with no detectable site disorder. The experiments reveal the realization of a Kramers doublet ground state with $J{\rm eff}=1/2$ moments, well separated from the first excited state, which interact antiferromagnetically with an exchange interaction of $\sim$0.6 K between the Nd$^{3+}$ moments in the triangular plane. The absence of oscillations and the so-called 1/3 plateau in the zero-field $μ$SR asymmetry down to 50 mK rules out long-range magnetic ordering and spin freezing on the $μ$SR time scale, respectively. The temperature dependence of the zero-field $μ$SR relaxation rate is well described by the Orbach relaxation mechanism, indicating the existence of fluctuating moments in the ground state of this frustrated magnet. Our results demonstrate exotic magnetism and persistent spin dynamics down to 50 mK. These observations establish this new family of frustrated rare-earth triangular-lattice antiferromagnets as a promising venue for the experimental realization of nontrivial quantum states with exotic low-energy excitations.

cond-mat.str-el

Quantum spin liquid on a 3D bipartite lattice of spin trimers stabilized by enhanced effective anisotropy

Quantum spin liquids (QSLs) represent highly entangled states of matter in which frustration-induced quantum fluctuations suppress any symmetry-breaking phase transition down to absolute zero, giving rise to fractionalized excitations and emergent gauge fields. Theoretically, bond anisotropy can stabilize QSLs even on bipartite lattices, as exemplified by the Kitaev honeycomb model; however, no material has so far been established to realize such a state as its true ground state. Here we identify the three-dimensional spin-trimer magnet KBa$_3$Ca$_4$Cu$_3$V$_7$O$_{28}$ as a promising candidate for a bipartite quantum spin liquid persisting to the lowest temperatures. Strongly coupled Cu$^{2+}$ trimers form effective pseudospin-1/2 degrees of freedom upon cooling, which in turn constitute a three-dimensional bipartite network. Bulk thermodynamic measurements, neutron scattering, $μ$SR, and NMR detect no spin freezing or symmetry-breaking phase transition down to 20 mK, but instead reveal a gapless dynamical ground state with algebraic spin autocorrelations. Complementary Monte Carlo and exact-diagonalization calculations show that this state is stabilized by a strong enhancement of effective anisotropy: a weak microscopic Cu-Cu exchange anisotropy of approximately 15 percent is generically amplified at the trimer level, producing effective pseudospin-pseudospin interaction anisotropies of 60 to 100 percent. Our results establish trimer-based networks as a promising platform for realizing anisotropy-stabilized quantum entangled states, even in three-dimensional bipartite systems with only weak microscopic anisotropy.

cond-mat.str-el

Melting upon cooling in a quantum magnet

Heating enhances thermal fluctuations and typically leads to melting of solids, but in exceptional cases, heating can also cause liquids to solidify. The paradigm of this counterintuitive phenomenon is solidification of liquid $^3$He upon increasing temperature, known as the Pomeranchuk effect. Here we show that such inverse melting also appears in quantum magnetism. We find that, on cooling, the Ising-like triangular-lattice antiferromagnet erbium heptatantalate first develops a three-sublattice long-range magnetic order -- analogous to a solid -- which then, unexpectedly, melts at even lower temperatures into a short-range correlated spin-stripe state -- analogous to a liquid. We propose that such an unprecedented ``spin Pomeranchuk effect" can generically arise from strong competition between spin-spin interactions in frustrated magnets, and provides a novel avenue to transformations between exotic magnetic phases.

cond-mat.str-el

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) $μ_\mathrm{B}$, 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

Comment on "Neutron diffraction evidence of the 3-dimensional structure of Ba2MnTeO6 and misidentification of the triangular layers within the face-centred cubic lattice"

Frustrated magnetism continues to attract significant attention due to its potential to host novel quantum many-body phenomena and associated exotic excitations that transcend existing paradigms. Herein, we present our reply to the comment on our recent thermodynamic and muon spin relaxation studies on a frustrated double perovskite, Ba2MnTeO6 (henceforth BMTO). Previous studies by four independent groups, including our group, suggested a trigonal space group based on single-crystal and polycrystalline samples of BMTO, while the recent comment reports a cubic space group based on polycrystalline samples. We believe that the structure is fairly intricate because of the slight variations between the two space groups, refining the crystal structure of BMTO remains an unresolved problem that needs additional high-resolution XRD and neutron diffraction studies on high-quality single crystals. It is thought, however, that structural assignments will not greatly influence any of the primary findings related to the magnetism and spin dynamics of BMTO. These consist of a magnetic phase transition at around 21 K, the observation of antiferromagnetic magnon excitations exhibiting a gap of 1.4 K beneath the phase transition, the presence of short-range spin correlations well above the antiferromagnetic phase transition, and the persistence of spin dynamics even within the magnetically ordered phase. It is important to note that the magnetization, specific heat, and muon spin relaxation findings that constitute the core of our earlier study are independent; the interpretation of these findings did not rely on any specific space group. Concerning the final allocation of the symmetry of BMTO, a definitive differentiation in certain physical characteristics resulting from the symmetry is still necessary.

cond-mat.str-el

Magnetic properties and field-induced phenomena in the Jeff = 1/2 distorted kagome antiferromagnet

The intertwining between competing degrees of freedom, anisotropy, and frustration-induced quantum fluctuations offers an ideal ground to realize exotic quantum phenomena in the rare-earth-based kagome lattice. The magnetic susceptibility reveals the presence of two energy scales in agreement with the INS results. The higher energy state is dominated by CEF excitations, where the lowest Kramers ground-state doublet is well separated from the excited state suggesting that the compound realizes a low-energy state at low temperatures. The second energy scale is witnessed via thermodynamic results that reveal an anomaly at 0.3 K typical of a phase transition, which is attributed to the presence of complex magnetic ordering phenomena. The broad maximum in the specific heat well above 0.3 K indicates the presence of short-range spin correlations that is corroborated by muon spin relaxation rate results. The isothermal magnetization reveals a field-induced 1/3 magnetization plateau at low temperatures. muSR relaxation rate experiments, on the other hand, neither show the signature of a phase transition nor spin-freezing down to 34 mK. The ZF muSR relaxation is governed by the Orbach process and reveals the presence of a fluctuating state owing to the depopulation of crystal field levels reflected as a constant value of relaxation rate in the temperature range 0.4-10 K. NMR results indicate the presence of fluctuating Nd3+ moments down to 1.8 K consistent with muSR experiments. Our comprehensive results reveal that a field-induced quantum critical phenomenon is at play in this frustrated kagome magnet and enable us to construct a phase diagram exemplifying the proximity effect of competing magnetic states. This sets the stage to investigate the broad RE3BWO9 family of rare-earth kagome magnets promising to host exotic quantum states driven by spin-orbit coupling and geometrical frustration.

cond-mat.str-el

Experimental signatures of quantum and topological states in frustrated magnetism

Frustration in magnetic materials arising from competing exchange interactions can prevent the system from adopting long-range magnetic order and can instead lead to a diverse range of novel quantum and topological states with exotic quasiparticle excitations. Here, we review prominent examples of such emergent phenomena, including magnetically-disordered and extensively degenerate spin ices, which feature emergent magnetic monopole excitations, highly-entangled quantum spin liquids with fractional spinon excitations, topological order and emergent gauge fields, as well as complex particle-like topological spin textures known as skyrmions. We provide an overview of recent advances in the search for magnetically-disordered candidate materials on the three-dimensional pyrochlore lattice and two-dimensional triangular, kagome and honeycomb lattices, the latter with bond-dependent Kitaev interactions, and on lattices supporting topological magnetism. We highlight experimental signatures of these often elusive phenomena and single out the most suitable experimental techniques that can be used to detect them. Our review also aims at providing a comprehensive guide for designing and investigating novel frustrated magnetic materials, with the potential of addressing some important open questions in contemporary condensed matter physics.

cond-mat.str-el

Magnetic properties and spin dynamics in a spin-orbit driven Jeff= 1/2 triangular lattice antiferromagnet

Frustration-induced strong quantum fluctuations accompanied by spin-orbit coupling and crystal electric field can give rise to rich and diverse magnetic phenomena associated with unconventional low-energy excitations in rare-earth based quantum magnets. Herein, we present crystal structure, magnetic susceptibility, specific heat, muon spin relaxation(muSR), and electron spin resonance (ESR) studies on the polycrystalline samples of Ba6Yb2Ti4O17 in which Yb3+ ions constitute a perfect triangular lattice in ab-plane without detectable anti-site disorder between atomic sites. The Curie-Weiss fit of low-temperature magnetic susceptibility data suggest the spin-orbit entangled Jeff = 1/2 degrees of freedom of Yb3+ spin with weak antiferromagnetic exchange interactions in the Kramers doublet ground state. The zero-field specific heat data reveal the presence of long-range magnetic order at TN = 77 mK which is suppressed in a magnetic field 1 T. The broad maximum in specific heat is attributed to the Schottky anomaly implying the Zeeman splitting of the Kramers doublet ground state. The ESR measurements suggest the presence of anisotropic exchange interaction between the moments of Yb3+ spins and the well separated Kramers doublet state. muSR experiments reveal a fluctuating state of Yb3+ spins in the temperature range 0.1 K-100 K owing to depopulation of crystal electric field levels, which suggests that the Kramers doublets are well separated consistent with thermodynamic and ESR results. In addition to the intraplane nearest-neighbor superexchange interaction, the interplane exchange interaction and anisotropy are expected to stabilize the long-range ordered state in this triangular lattice antiferromagnet.

cond-mat.str-el

Magnetic properties of a spin-orbit entangled Jeff = 1/2 honeycomb lattice

The interplay between spin-orbit coupling, anisotropic magnetic interaction, frustration-induced quantum fluctuations and spin correlations can lead to novel quantum states with exotic excitations in rare-earth-based quantum magnets. Herein, we present the crystal structure, magnetization, electron spin resonance (ESR), specific heat, and nuclear magnetic resonance (NMR) experiments on the polycrystalline samples of Ba9Yb2Si6O24, in which Yb3+ ions form a perfect honeycomb lattice without detectable anti-site disorder. The magnetization data reveal antiferromagnetically coupled spin-orbit entangled Jeff = 1/2 degrees of freedom of Yb3+ ions in the Kramers doublet state. The ESR measurements reveal that the first excited Kramers doublet is 32.3(7) meV above the ground state. The specific heat results suggest the absence of any long-range magnetic order in the measured temperature range. Furthermore, the 29Si NMR results do not indicate any signature of magnetic ordering down to 1.6 K, and the spin-lattice relaxation rate reveals the presence of a field-induced gap that is attributed to the Zeeman splitting of Kramers doublet state in this quantum material. Our experiments detect neither spin freezing nor long-range magnetic ordering down to 1.6 K. The current results suggest the presence of short-range spin correlations in this spin-orbit entangled Jeff =1/2 rare-earth magnet on a honeycomb lattice.

cond-mat.str-el

Competing magnetic phases in the frustrated spin-1/2 chain compound $β$-TeVO$_4$ probed by NMR

In frustrated spin-1/2 chains the competition between the nearest- and next-nearest-neighbor exchange interactions leads to a rich phase diagram that becomes even richer in the presence of perturbations in their material realizations. These effects are still largely unexplored, so that new insight into static and dynamic magnetism, in particular by sensitive local probes, is highly desired. Here we present a comprehensive $^{17}$O nuclear magnetic resonance study of $β$-TeVO$_4$, where the anisotropy of the main exchange interactions and additional weak interchain exchange interactions complement the theoretical phase diagram. Our results confirm the dynamical nature of the intriguing spin-stripe phase that has been reported in previous studies. In addition, we find that the magnetic order in the high-field phase, which develops just below the magnetization saturation, is consistent with an unusual type of spin-density-wave (SDW) order with different alignments of the magnetic moments on the neighboring chains. This is reminiscent of the ordering in the SDW phase, realized in the absence of the magnetic field, and is thus most likely stabilized by magnetic anisotropy.

cond-mat.str-el

Spin liquid state in a rare-earth hyperkagome lattice

Quantum fluctuations enhanced by frustration and subtle interplay between competing degrees of freedom offer an ideal ground to realize novel states with fractional quantum numbers in quantum materials that defy standard theoretical paradigms. Quantum spin liquid (QSL) is a highly entangled state wherein frustration induced strong quantum fluctuations preclude symmetry breaking phase transitions down to zero temperature without any order parameter. Experimental realizations of QSL in quantum materials with spin dimensionality greater than one is very rare. Here, we present our thermodynamic, nuclear magnetic resonance, muon spin relaxation and inelastic neutron scattering studies of a new rare-earth hyperkagome compound Li3Yb3Te2O12 in which Yb3+ ions constitute a three dimensional spin-lattice without any detectable disorder. Our comprehensive experiments evince neither signature of magnetic ordering nor spin freezing down to 38 mK that suggest the realization of dynamic liquid-like ground state in this antiferromagnet. The ground state of this material is interpreted by a low energy Jeff = 1/2 degrees of freedom with short range spin correlations. The present results demonstrate a viable basis to explore spin-orbit driven enigmatic correlated quantum states in a new class of rare-earth based three dimensional frustrated magnets that may open new avenues in theoretical and experimental search for spin liquids.

cond-mat.str-el

Magnetic properties of triangular lattice antiferromagnets Ba3RB9O18 (R = Yb, Er)

Frustration, spin correlations and interplay between competing degrees of freedom are some of the key ingredients that underlie exotic states with fractional excitations in quantum materials. Rare-earth based two dimensional magnetic lattice wherein crystal electric field, spin-orbit coupling, anisotropy and electron correlation between rare-earth moments offer a new paradigm in this context. Herein, we present crystal structure, magnetic susceptibility and specific heat accompanied by crystal electric field calculations on the polycrystalline sample of Ba3RB9O18 (R = Yb, Er) in which R3+ ions form a perfect triangular lattice without anti-site disorder. The localized R3+ spins show neither long-range order nor spin-glass state down to 1.9 K in Ba3RB9O18. Magnetization data reveal a pseudospin Jeff = 1/2 ( Yb3+) in the Kramers doublet state and a weak antiferromagnetic interaction between Jeff = 1/2 moments in the Yb variant. On the other hand, the effective moment μeff = 8.8 μB was obtained from the Curie-Weiss fit of the low-temperature susceptibility data of Er variant suggests the admixture of higher crystal electric field states with the ground state. The Curie-Weiss fit of low-temperature susceptibility data for Er system unveils the presence of a relatively strong antiferromagnetic interaction between Er3+ moments compared to its Yb3+ analog. Ba3ErB9O18 does not show long-range magnetic ordering down to 500 mK. Furthermore, our crystal electric field calculations based on magnetization data of Ba3ErB9O18 suggest the presence of a small gap between the ground and first excited Kramers doublets. The broad maximum around 4 K in magnetic specific heat in zero-field is attributed to the thermal population of the first CEF excited state in Ba3ErB9O18, which is consistent with our CEF calculations.

cond-mat.str-el

Signature of a randomness-driven spin-liquid state in a frustrated magnet

Collective behaviour of electrons, frustration induced quantum fluctuations and entanglement in quantum materials underlie some of the emergent quantum phenomena with exotic quasi-particle excitations that are highly relevant for technological applications. Herein, we present our thermodynamic and muon spin relaxation measurements, complemented by ab initio density functional theory and exact diagonalization results, on the recently synthesized frustrated antiferromagnet Li4CuTeO6, in which Cu2+ ions (S = 1/2) constitute disordered spin chains and ladders along the crystallographic [101] direction with weak random inter-chain couplings. Our thermodynamic experiments detect neither long-range magnetic ordering nor spin freezing down to 45 mK despite the presence of strong antiferromagnetic interaction between Cu2+ moments leading to a large effective Curie-Weiss temperature of -154 K. Muon spin relaxation results are consistent with thermodynamic results. The temperature and magnetic field scaling of magnetization and specific heat reveal a data collapse pointing towards the presence of random-singlets within a disorder-driven correlated and dynamic ground-state in this frustrated antiferromagnet.

cond-mat.str-el

Quantum spin liquid in the Ising triangular-lattice antiferromagnet neodymium heptatantalate

Disordered magnetic states known as spin liquids are of paramount importance in both fundamental and applied science. A classical state of this kind was predicted for the Ising antiferromagnetic triangular model more than half a century ago, while additional non-commuting exchange terms were proposed to induce its quantum version -- a quantum spin liquid. These predictions have not been yet confirmed experimentally. Here we report a discovery of such a state in the structurally perfect triangular-lattice antiferromagnet NdTa$_7$O$_{19}$. Its magnetic ground state is characterized by spin-1/2 degrees of freedom with Ising-like interactions and gives rise to low-frequency spin excitations persisting down to the lowest temperatures. Our study demonstrates the key role of strong spin-orbit coupling in stabilizing spin liquids that result from magnetic anisotropy and highlights the large family of rare-earth (RE) heptatantalates RETa$_7$O$_{19}$ as a novel framework for realization of these states, which represent a promising platform for quantum applications.

cond-mat.str-el

Universal fluctuating regime in triangular chromate antiferromagnets

We report x-ray diffraction, magnetic susceptibility, heat capacity, $^{1}$H nuclear magnetic resonance (NMR), and muon spin relaxation ($μ$SR) measurements, as well as density-functional band-structure calculations for the frustrated $S=3/2$ triangular lattice Heisenberg antiferromagnet (TLHAF) $α$-HCrO$_{2}$ (trigonal, space group: $R\bar{3}m$). This compound undergoes a clear magnetic transition at $T_{\rm N} \simeq 22.5$~K, as seen from the drop in the muon paramagnetic fraction and concurrent anomalies in the magnetic susceptibility and specific heat. Local probes (NMR and $μ$SR) reveal a broad regime with slow fluctuations down to $0.7\,T_{\rm N}$, this temperature corresponding to the maximum in the $μ$SR relaxation rate and in the NMR wipe-out. From the comparison with NaCrO$_{2}$ and $α$-KCrO$_{2}$, the fluctuating regime and slow dynamics below $T_{\rm N}$ appear to be hallmarks of the TLHAF with $ABC$ stacking that leads to a frustration of interlayer couplings between the triangular planes. This interlayer frustration is a powerful lever to generate spin states with persistent dynamics and may bear implications to spin-liquid candidates with the triangular geometry.

cond-mat.mtrl-sci

Magnetic ordering of the distorted kagome antiferromagnet Y$_3$Cu$_9$(OH)$_{18}$[Cl$_8$(OH)] prepared via optimal synthesis

Experimental studies of high-purity kagome-lattice antiferromagnets (KAFM) are of great importance in attempting to better understand the predicted enigmatic quantum spin-liquid ground state of the KAFM model. However, realizations of this model can rarely evade magnetic ordering at low temperatures due to various perturbations to its dominant isotropic exchange interactions. Such a situation is for example encountered due to sizable Dzyaloshinskii-Moriya magnetic anisotropy in YCu$_3$(OH)$_6$Cl$_3$, which stands out from other KAFM materials by its perfect crystal structure. We find evidence of magnetic ordering also in the distorted sibling compound Y$_3$Cu$_9$(OH)$_{18}$[Cl$_8$(OH)], which has recently been proposed to feature a spin-liquid ground state arising from a spatially anisotropic kagome lattice. Our findings are based on a combination of bulk susceptibility, specific heat, and magnetic torque measurements that disclose a Néel transition temperature of $T_N=11$~K in this material, which might feature a coexistence of magnetic order and persistent spin dynamics as previously found in YCu$_3$(OH)$_6$Cl$_3$. Contrary to previous studies of single crystals and powders containing impurity inclusions, we use high-purity single crystals of Y$_3$Cu$_9$(OH)$_{18}$[Cl$_8$(OH)] grown via an optimized hydrothermal synthesis route that minimizes such inclusions. This study thus demonstrates that the lack of magnetic ordering in less pure samples of the investigated compound does not originate from the reduced symmetry of spin lattice but is instead of extrinsic origin.

cond-mat.str-el

Dynamical spin correlations of the kagome antiferromagnet

Temperature-dependent dynamical spin correlations, which can be readily accessed via a variety of experimental techniques, hold the potential of offering a unique fingerprint of quantum spin liquids and other intriguing dynamical states. In this work we present an in-depth study of the temperature-dependent dynamical spin structure factor $S({\bf q}, ω)$ of the antiferromagnetic (AFM) Heisenberg spin-1/2 model on the kagome lattice with additional Dzyaloshinskii--Moriya (DM) interactions. Using the finite-temperature Lanczos method on lattices with up to $N = 30$ sites we find that even without DM interactions, chiral low-energy spin fluctuations of the $120^\circ$ AFM order parameter dominate the dynamical response. This leads to a nontrivial frequency dependence of $S({\bf q}, ω)$ and the appearance of a pronounced low-frequency mode at the M point of the extended Brillouin zone. Adding an out-of-plane DM interactions $D^z$ gives rise to an anisotropic dynamical response, a softening of in-plane spin fluctuations, and, ultimately, the onset of a coplanar AFM ground-state order at $D^z > 0.1 J$. Our results are in very good agreement with existing inelastic neutron scattering and temperature-dependent NMR spin-lattice relaxation rate ($1/T_1$) data on the paradigmatic kagome AFM herbertsmithite, where the effect of its small $D^z$ on the dynamical spin correlations is shown to be rather small, as well as with $1/T_1$ data on the novel kagome AFM YCu$_3$(OH)$_6$Cl$_3$, where its substantial $D^z \approx 0.25 J$ interaction is found to strongly affect the spin dynamics.

cond-mat.str-el

Development of short and long-range magnetic order in the double perovskite based frustrated triangular lattice antiferromagnet Ba2MnTeO6

Oxide double perovskites wherein octahedra formed by both 3d elements and sp-based heavy elements give rise to unconventional magnetic ordering and correlated quantum phenomena crucial for futuristic applications. Here, by carrying out experimental and first principles investigations, we present the electronic structure and magnetic phases of Ba2MnTeO6, where Mn^2+ ions with S = 5/2 spins constitute a perfect triangular lattice. The magnetic susceptibility reveals a large Curie- Weiss temperature -152 K suggesting the presence of strong antiferromagnetic interactions between Mn^2+ moments in the spin lattice. A phase transition at 20 K is revealed by magnetic susceptibility and specific heat which is attributed to the presence of a sizeable inter-plane interactions. Below the transition temperature, the specific heat data show antiferromagnetic magnon excitations with a gap of 1.4 K. Furthermore, muon spin-relaxation reveals the presence of static internal fields in the ordered state and provides strong evidence of short-range spin correlations for T > TN. The DFT+U calculations and spin-dimer analysis infer that Heisenberg interactions govern the inter and intra-layer spin-frustrations in this perovskite. The inter and intra-layer exchange interactions are of comparable strengths (J1 = 4.6 K, J2 = 0.92 J1). However, a weak third nearest-neighbor ferromagnetic inter-layer interaction exists (J3=-0.04 J1) due to double-exchange interaction via the linear path Mn-O-Te-O-Mn. The combined effect of J2 and J3 interactions stabilizes a three dimensional long-range magnetic ordering in this frustrated magnet.

cond-mat.str-el