arXiv subjects

Alexander A. Tsirlin

Publications and source records attributed to Alexander A. Tsirlin.

At least 73 records · Page 4Linked to original sources

arXiv2020

Spin liquids in geometrically perfect triangular antiferromagnets

The cradle of quantum spin liquids, triangular antiferromagnets show strong proclivity to magnetic order and require deliberate tuning to stabilize a spin-liquid state. In this brief review, we juxtapose recent theoretical developments that trace the parameter regime of the spin-liquid phase, with experimental results for Co-based and Yb-based triangular antiferromagnets. Unconventional spin dynamics arising from both ordered and disordered ground states is discussed, and the notion of a geometrically perfect triangular system is scrutinized to demonstrate non-trivial imperfections that may assist magnetic frustration in stabilizing dynamic spin states with peculiar excitations.

cond-mat.str-el↗

arXiv2019

Large Easy Axis Anisotropy in the One-Dimensional Magnet BaMo(PO$_4$)$_2$

We present an extensive experimental and theoretical study on the low-temperature magnetic properties of the monoclinic anhydrous alum compound BaMo(PO$_4$)$_2$. The magnetic susceptibility reveals strong antiferromagnetic interactions $θ_{CW} = -167$~K and long-range magnetic order at $T_N=22$~K, in agreement with a recent report. Powder neutron diffraction furthermore shows that the order is collinear, with the moments near the $ac$ plane. Neutron spectroscopy reveals a large excitation gap $Δ= 15$~meV in the low-temperature ordered phase, suggesting a much larger easy-axis spin anisotropy than anticipated. However, the large anisotropy justifies the relatively high ordered moment, Néel temperature, and collinear order observed experimentally, and is furthermore reproduced in a first principles calculations using a new computational scheme. We therefore propose BaMo(PO$_4$)$_2$ to host $S=1$ antiferromagnetic chains with large easy-axis anisotropy, which has been theoretically predicted to realize novel excitation continua.

cond-mat.str-el↗

arXiv2019

Magnetic frustration in a metallic fcc lattice

Magnetic frustration in metals is scarce and hard to pinpoint, but exciting due to the possibility of the emergence of fascinating novel phases. The cubic intermetallic compound HoInCu$_4$ with all holmium atoms on an fcc lattice, exhibits partial magnetic frustration, yielding a ground state where half of the Ho moments remain without long-range order, as evidenced by our neutron scattering experiments. The substitution of In with Cd results in HoCdCu$_4$ in a full breakdown of magnetic frustration. Consequently we found a fully ordered magnetic structure in our neutron diffraction experiments. These findings are in agreement with the local energy scales and crystal electric field excitations, which we determined from specific heat and inelastic neutron scattering data. The electronic density of states for the itinerant bands acts as tuning parameter for the ratio between nearest-neighbor and next-nearest-neighbor interactions and thus for magnetic frustration.

cond-mat.str-el↗

arXiv2019

The origin of up-up-down-down magnetic order in Cu$_2$GeO$_4$

We use density-functional band-structure calculations to explore the origin of the up-up-down-down (UUDD) magnetic order in Cu$_2$GeO$_4$ with the frustrated $J_1-J_2$ spin chains coupled into layers within the spinel-like crystal structure. In contrast to earlier studies, we find that the nearest-neighbor coupling $J_1$ may be negligibly small, owing to a nearly perfect compensation of the ferromagnetic direct exchange and antiferromagnetic superexchange. Under this condition, weak symmetric anisotropy of the exchange couplings gives rise to the UUDD order observed experimentally and also elucidates the non-trivial ordering pattern between the layers, whereas a small Dzyaloshinsky-Moriya interaction causes a spin canting that may generate local electric polarization. We argue that the buckling of the copper chains plays a crucial role in the suppression of $J_1$ in Cu$_2$GeO$_4$ and sets this compound apart from other $J_1-J_2$ chain magnets.

cond-mat.mtrl-sci↗

arXiv2019

Cubic symmetry and magnetic frustration on the $fcc$ spin lattice in K$_2$IrCl$_6$

Cubic crystal structure and regular octahedral environment of Ir$^{4+}$ render antifluorite-type K$_2$IrCl$_6$ a model fcc antiferromagnet with a combination of Heisenberg and Kitaev exchange interactions. High-resolution synchrotron powder diffraction confirms cubic symmetry down to at least 20 K, with a low-energy rotary mode gradually suppressed upon cooling. Using thermodynamic and transport measurements, we estimate the activation energy of $Δ\simeq 0.7$ eV for charge transport, the antiferromagnetic Curie-Weiss temperature of $θ_{\rm CW}\simeq -43$ K, and the extrapolated saturation field of $H_s\simeq 87$ T. All these parameters are well reproduced \textit{ab initio} using $U_{\rm eff}=2.2$ eV as the effective Coulomb repulsion parameter. The antiferromagnetic Kitaev exchange term of $K\simeq 5$ K is about one half of the Heisenberg term $J\simeq 13$ K. While this combination removes a large part of the classical ground-state degeneracy, the selection of the unique magnetic ground state additionally requires a weak second-neighbor exchange coupling $J_2\simeq 0.2$ K. Our results suggest that K$_2$IrCl$_6$ may offer the best possible cubic conditions for Ir$^{4+}$ and demonstrates the interplay of geometrical and exchange frustration in a high-symmetry setting.

cond-mat.str-el↗

arXiv2019

Rearrangement of uncorrelated valence bonds evidenced by low-energy spin excitations in YbMgGaO4

DC-magnetization data measured down to 40 mK speak against conventional freezing and reinstate YbMgGaO$_4$ as a triangular spin-liquid candidate. Magnetic susceptibility measured parallel and perpendicular to the $c$-axis reaches constant values below 0.1 and 0.2 K, respectively, thus indicating the presence of gapless low-energy spin excitations. We elucidate their nature in the triple-axis inelastic neutron scattering experiment that pinpoints the low-energy ($E$ $\leq$ $J_0$ $\sim$ 0.2 meV) part of the excitation continuum present at low temperatures ($T$ $<$ $J_0$/$k_B$), but \emph{completely} disappearing upon warming the system above $T$ $\gg$ $J_0$/$k_B$. In contrast to the high-energy part at $E$ $>$ $J_0$ that is rooted in the breaking of nearest-neighbor valence bonds and persists to temperatures well above $J_0$/$k_B$, the low-energy one originates from the rearrangement of the valence bonds and thus from the propagation of unpaired spins. We further extend this picture to herbertsmithite, the spin-liquid candidate on the kagome lattice, and argue that such a hierarchy of magnetic excitations may be a universal feature of quantum spin liquids.

cond-mat.str-el↗

arXiv2019

Field-induced double dome and Bose-Einstein condensation in the crossing quantum spin chain system AgVOAsO4

We present inelastic neutron scattering data on the quantum paramagnet AgVOAsO4 that establish the system is a S=1/2 alternating spin chain compound and provide a direct measurement of the spin gap. We also present experimental evidence for two different types of field-induced magnetic order between mu_0H_c1 = 8.4T and mu_0H_c2 = 48.9T, which may be related to Bose-Einstein condensation (BEC) of triplons. Thermodynamic measurements in magnetic fields up to 60T and temperatures down to 0.1K reveal a H-T phase diagram consisting of a dome encapsulating two ordered phases with maximum ordering temperatures of 3.8K and 5.3K respectively. This complex phase diagram is not expected for a single-Q BEC system and therefore establishes AgVOAsO4 as a promising multi-Q BEC candidate capable of hosting exotic vortex phases.

cond-mat.str-el↗

arXiv2019

Gapless spin-liquid state in the structurally disorder-free triangular antiferromagnet NaYbO$_2$

We present the structural characterization and low-temperature magnetism of the triangular-lattice delafossite NaYbO$_2$. Synchrotron x-ray diffraction and neutron scattering exclude both structural disorder and crystal-electric-field randomness, whereas heat-capacity measurements and muon spectroscopy reveal the absence of magnetic order and persistent spin dynamics down to at least 70\,mK. Continuous magnetic excitations with the low-energy spectral weight accumulating at the $K$-point of the Brillouin zone indicate the formation of a novel spin-liquid phase in a triangular antiferromagnet. This phase is gapless and shows a non-trivial evolution of the low-temperature specific heat. Our work demonstrates that NaYbO$_2$ practically gives the most direct experimental access to the spin-liquid physics of triangular antiferromagnets.

cond-mat.str-el↗

arXiv2018

Stripe order and magnetic anisotropy in the $S=1$ antiferromagnet BaMoP$_2$O$_8$

Magnetic behavior of BaMoP$_2$O$_8$ with the spatially anisotropic triangular arrangement of the $S=1$ Mo$^{4+}$ ions is explored using thermodynamic measurements, neutron diffraction, and density-functional band-structure calculations. A broad maximum in the magnetic susceptibility around 46\,K is followed by the stripe antiferromagnetic order with the propagation vector ${\mathbf k}=(\frac{1}{2},\frac{1}{2},\frac{1}{2})$ formed below $T_N\simeq 21$ K. This stripe phase is triggered by a pronounced one-dimensionality of the spin lattice, where one of the in-plane couplings, $J_2\simeq 4.6$ meV, is much stronger than its $J_1\simeq 0.4$ meV counterpart, and stabilized by the weak easy-axis anisotropy. The ordered moment of 1.42(9) $μ_B$ at 1.5 K is significantly lower than the spin-only moment of 2 $μ_B$ due to a combined effect of quantum fluctuations and spin-orbit coupling.

cond-mat.str-el↗

arXiv2018

Unravelling the complex magnetic structure of multiferroic pyroxene NaFeGe2O6: A combined experimental and theoretical study

Magnetic order and the underlying magnetic model of the multiferroic pyroxene NaFeGe2O6 are systematically investigated by neutron powder diffraction, thermodynamic measurements, density-functional bandstructure calculations, and Monte-Carlo simulations. Upon cooling, NaFeGe2O6 first reveals one-dimensional spin-spin correlations in the paramagnetic state below about 50 K, revealed by magnetic diffuse scattering. The sinusoidal spin-density wave with spins along the a-direction sets in at 13 K, followed by the cycloidal configuration with spins lying in the (ac) plane below 11.6 K. Microscopically, the strongest magnetic coupling runs along the structural chains, J1 ' 12 K, which is likely related to the one-dimensional spin-spin correlations. The interchain couplings J2 ' 3:8K and J3 ' 2:1K are energetically well balanced and compete, thus giving rise to the incommensurate order in sharp contrast to other transition-metal pyroxenes, where one type of the interchain couplings prevails. The magnetic model of NaFeGe2O6 is further completed by the weak single-ion anisotropy along the a-direction. Our results resolve the earlier controversies regarding the magnetic order in NaFeGe2O6 and establish relevant symmetries of the magnetic structures. These results, combined with symmetry analysis, enable us to identify the possible mechanisms of the magnetoelectric coupling in this compound. We also elucidate microscopic conditions for the formation of incommensurate magnetic order in pyroxenes.

cond-mat.str-el↗

arXiv2018

Gapped ground state in the zigzag pseudospin-1/2 quantum antiferromagnetic chain compound PrTiNbO6

We report a single-crystal study on the magnetism of the rare-earth compound PrTiNbO$_6$ that experimentally realizes the zigzag pseudospin-$\frac{1}{2}$ quantum antiferromagnetic chain model. Random crystal electric field caused by the site mixing between non-magnetic Ti$^{4+}$ and Nb$^{5+}$, results in the non-Kramers ground state quasi-doublet of Pr$^{3+}$ with the effective pseudospin-$\frac{1}{2}$ Ising moment. Despite the antiferromagnetic intersite coupling of about 4 K, no magnetic freezing is detected down to 0.1 K, whilst the system approaches its ground state with almost zero residual spin entropy. At low temperatures, a sizable gap of about 1 K is observed in zero field. We ascribe this gap to off-diagonal anisotropy terms in the pseudospin Hamiltonian, and argue that rare-earth oxides open an interesting venue for studying magnetism of quantum spin chains.

cond-mat.str-el↗

arXiv2018

Absence of zero-point entropy in a triangular Ising antiferromagnet

Frustrated Ising magnets host exotic excitations, such as magnetic monopoles in spin ice. The ground state (GS) in this case is characterized by an extensive degeneracy and associated residual entropy going back to the pioneering work by G. Wannier who established large residual entropy of nearly 50%Rln2 per mole spins in a triangular Ising antiferromagnet (TIAF) already in 1950. Here, we endeavor to verify this result experimentally using TmMgGaO4, a novel rare-earth-based frustrated antiferromagnet with Ising spins arranged on a perfect triangular lattice. Contrary to theoretical expectations, we find almost no residual entropy and ascribe this result to the presence of a weak second-neighbor coupling J2zz ~ 0.09J1zz that lifts the GS degeneracy and gives rise to several ordered states, the stripe order, 1/3-plateau, and 1/2-plateau. TmMgGaO4 gives experimental access to these novel phases of Ising spins on the triangular lattice.

cond-mat.str-el↗

arXiv2017

Models and Materials for Generalized Kitaev Magnetism

The exactly solvable Kitaev model on the honeycomb lattice has recently received enormous attention linked to the hope of achieving novel spin-liquid states with fractionalized Majorana-like excitations. In this review, we analyze the mechanism proposed by G. Jackeli and G. Khaliullin to identify Kitaev materials based on spin-orbital dependent bond interactions and provide a comprehensive overview of its implications in real materials. We set the focus on experimental results and current theoretical understanding of planar honeycomb systems (Na$_2$IrO$_3$, $α$-Li$_2$IrO$_3$, and $α$-RuCl$_3$), three-dimensional Kitaev materials ($β$- and $γ$-Li$_2$IrO$_3$), and other potential candidates, completing the review with the list of open questions awaiting new insights.

cond-mat.str-el↗

arXiv2017

Magnetism of coupled spin tetrahedra in ilinskite-type KCu$_{5}$O$_2$(SeO$_3$)$_2$Cl$_3$

Synthesis, thermodynamic properties, and microscopic magnetic model of ilinskite-type KCu$_{5}$O$_2$(SeO$_3$)$_2$Cl$_3$ built by corner-sharing Cu$_4$ tetrahedra are reported, and relevant magnetostructural correlations are discussed. Quasi-one-dimensional magnetic behavior with the short-range order around 50\,K and the absence of long-range order down to at least 2\,K is observed experimentally and explained in terms of weakly coupled spin ladders (tubes) with a complex topology formed upon fragmentation of the tetrahedral network. This fragmentation is rooted in the non-trivial effect of the SeO$_3$ groups that render the Cu--O--Cu superexchange strongly ferromagnetic.

cond-mat.mtrl-sci↗

arXiv2017

Collinear order and chirality-reorientation transition in the Cairo pentagonal magnet Bi$_4$Fe$_5$O$_{13}$F

We show that interlayer spins play a dual role in the Cairo pentagonal magnet Bi$_4$Fe$_5$O$_{13}$F, on one hand mediating the three-dimensional (3D) magnetic order and on the other driving spin-reorientation transitions both within and between the planes. The corresponding sequence of magnetic orders unraveled by neutron diffraction and Mössbauer spectroscopy features two orthogonal magnetic structures described by opposite local vector chiralities, and an intermediate, partly disordered phase with nearly collinear spins. A similar collinear phase has been predicted theoretically to be stabilized by quantum fluctuations, but Bi$_4$Fe$_5$O$_{13}$F is very far from the relevant parameter regime. While the observed in-plane reorientation cannot be explained by any standard frustration mechanism, our ab initio band-structure calculations reveal strong single-ion anisotropy of the interlayer Fe$^{3+}$ spins that turns out to be instrumental in controlling the local vector chirality and the associated interlayer order.

cond-mat.str-el↗

arXiv2017

Two-gap superconductivity in Mo$_{8}$Ga$_{41}$ and its evolution upon the V substitution

Zero-field and transverse-field muon spin rotation/relaxation ($μ$SR) experiments were undertaken in order to elucidate microscopic properties of a strongly-coupled superconductor Mo$_{8}$Ga$_{41}$ with $T_{\text{c}}=9.8$ K. The upper critical field extracted from the transverse-field $μ$SR data exhibits significant reduction with respect to the data from thermodynamic measurements indicating the coexistence of two independent length scales in the superconducting state. Accordingly, the temperature-dependent magnetic penetration depth of Mo$_{8}$Ga$_{41}$ is described using the model, in which two s-wave superconducting gaps are assumed. The V for Mo substitution in the parent compound leads to the complete suppression of one superconducting gap, and Mo$_{7}$VGa$_{41}$ is well described within the single s-wave gap scenario. The reduction in the superfluid density and the evolution of the low-temperature resistivity upon the V substitution indicate the emergence of a competing state in Mo$_{7}$VGa$_{41}$ that may be responsible for the closure of one of the superconducting gaps.

cond-mat.supr-con↗

arXiv2017

Nearest-neighbor resonating valence bonds in YbMgGaO4

Since its proposal by Anderson, resonating valence bonds (RVB) formed by a superposition of fluctuating singlet pairs have been a paradigmatic concept in understanding quantum spin liquids (QSL). Here, we show that excitations related to singlet breaking on nearest-neighbor bonds describe the high-energy part of the excitation spectrum in YbMgGaO4, the effective spin-1/2 frustrated antiferromagnet on the triangular lattice, as originally considered by Anderson. By a thorough single-crystal inelastic neutron scattering (INS) study, we demonstrate that nearest-neighbor RVB excitations account for the bulk of the spectral weight above 0.5 meV. This renders YbMgGaO4 the first experimental system where putative RVB correlations restricted to nearest neighbors are observed, and poses a fundamental question of how complex interactions on the triangular lattice conspire to form this unique many-body state.

cond-mat.str-el↗

arXiv2017

Frustrated magnetic planes with intricate interaction pathways in the mineral langite Cu$_4$(OH)$_6$SO$_4\cdot 2$H$_2$O

Magnetic and crystallographic properties of the mineral langite Cu$_4$(OH)$_6$SO$_4\cdot 2$H$_2$O are reported. Its layered crystal structure features a peculiar spatial arrangement of spin-$\frac12$ Cu$^{2+}$ ions that arises from a combination of corner- and edge-sharing chains. Experimentally, langite orders antiferromagnetically at $T_N\simeq 5.7$ K as revealed by magnetization and specific heat measurements. Despite this very low energy scale of the magnetic transition, langite features significantly stronger couplings on the order of 50-70 K. Half of the Cu$^{2+}$ spins are weakly coupled and saturate around 12T, where the magnetization reaches 0.5$μ_B$/Cu. These findings are rationalized by density-functional band-structure calculations suggesting a complex interplay of frustrated exchange couplings in the magnetic planes. A simplified model of coupled magnetic sublattices explains the experimental features qualitatively. To start from reliable structural data, the crystal structure of langite in the 100-280 K temperature range has been determined by single-crystal x-ray diffraction, and the hydrogen positions were refined computationally.

cond-mat.str-el↗