Hall effect of triplons in a dimerized quantum magnet

Identifying and Constructing Complex Magnon Band Topology

Magnetically ordered materials tend to support bands of coherent propagating spin wave, or magnon, excitations. Topologically protected surface states of magnons offer a new path toward coherent spin transport for spintronics applications. In this work we explore the variety of topological magnon band structures and provide insight into how to efficiently identify topological magnon bands in materials. We do this by adapting the topological quantum chemistry approach that has used constraints imposed by time reversal and crystalline symmetries to enumerate a large class of topological electronic bands. We show how to identify physically relevant models of gapped magnon band topology by using so-called decomposable elementary band representations, and in turn discuss how to use symmetry data to infer the presence of exotic symmetry enforced nodal topology.

Discovery of quantum phases in the Shastry-Sutherland compound SrCu2(BO3)2 under extreme conditions of field and pressure

The 2-dimensional layered oxide material SrCu₂(BO₃)₂, long studied as a realization of the Shastry-Sutherland spin topology, exhibits a range of intriguing physics as a function of both hydrostatic pressure and magnetic field, with a still debated intermediate plaquette phase appearing at approximately 20 kbar and a possible deconfined critical point at higher pressure. Here, we employ a tunnel diode oscillator (TDO) technique to probe the behavior in the combined extreme conditions of high pressure, high magnetic field, and low temperature. We reveal an extensive phase space consisting of multiple magnetic analogs of the elusive supersolid phase and a magnetization plateau. In particular, a 10 × 2 supersolid and a 1/5 plateau, identified by infinite Projected Entangled Pair States (iPEPS) calculations, are found to rely on the presence of both magnetic and non-magnetic particles in the sea of dimer singlets. These states are best understood as descendants of the full-plaquette phase, the leading candidate for the intermediate phase of SrCu₂(BO₃)₂.

SrCu2(BO3)2 is a 2D quantum antiferromagnet on a particular frustrated lattice showing multiple magnetization plateaus and quantum phase transitions under high pressure. Here the authors uncover novel magnetic phases in this material under combined effects of extreme magnetic field and pressure.

Topological Magnon Band Crossing in Y_{2}Ir_{2}O_{7}

Topological magnonic materials have attracted much interest because of the potential for dissipationless spintronic applications. Pyrochlore iridates are theoretically regarded as good candidates for designing topological magnon bands. However, experimental identification of topological magnon bands in pyrochlore iridates remains elusive. We explored this possibility in Y_{2}Ir_{2}O_{7} using Raman spectroscopy to measure both the single-magnon excitations and anomalous phonon shifts. From the single-magnon energies and tight-binding model calculations concerning the phonons, we determined the key parameters in the spin Hamiltonian. These confirm that Y_{2}Ir_{2}O_{7} hosts a nontrivial magnon band topology distinct from other pyrochlore iridate compounds. Our work demonstrates that pyrochlore iridates constitute a system in which the magnon band topology can be tailored and that Raman spectroscopy is a powerful technique to explore magnon band topology.

Dirac Surface States in Magnonic Analogs of Topological Crystalline Insulators

We propose magnonic analogs of topological crystalline insulators which possess Dirac surface states protected by the combined symmetry of time reversal and half translation. Constructing models of the topological magnon systems, we demonstrate that an energy current flows through the systems in response to an electric field, owing to the Dirac surface states with the spin-momentum locking. We also propose a realization of the magnonic analogs of topological crystalline insulators in the magnetic compound CrI_{3} with a monoclinic structure.

Temperature-Induced Plasmon Excitations for the α-T3 Lattice in Perpendicular Magnetic Field

We have investigated the α-T3 model in the presence of a mass term which opens a gap in the energy dispersive spectrum, as well as under a uniform perpendicular quantizing magnetic field. The gap opening mass term plays the role of Zeeman splitting at low magnetic fields for this pseudospin-1 system, and, as a consequence, we are able to compare physical properties of the the α-T3 model at low and high magnetic fields. Specifically, we explore the magnetoplasmon dispersion relation in these two extreme limits. Central to the calculation of these collective modes is the dielectric function which is determined by the polarizability of the system. This latter function is generated by transition energies between subband states, as well as the overlap of their wave functions.

Thermal-transport studies of kagomé antiferromagnets

Searching for the ground state of a kagomé Heisenberg antiferromagnet (KHA) has been one of the central issues of condensed-matter physics, because the KHA is expected to host spin-liquid phases with exotic elementary excitations. Here, we show our longitudinal ([Formula: see text]) and transverse ([Formula: see text]) thermal conductivities measurements of the two kagomé materials, volborthite and Ca kapellasite. Although magnetic orders appear at temperatures much lower than the antiferromagnetic energy scale in both materials, the nature of spin liquids can be captured above the transition temperatures. The temperature and field dependence of [Formula: see text] is analyzed by spin and phonon contributions, and large sample variations of the spin contribution are found in volborthite. Clear changes in [Formula: see text] are observed at the multiple magnetic transitions in volborthite, showing different magnetic thermal conduction in different magnetic structures. These magnetic contributions are not clearly observed in low-[Formula: see text] crystals of volborthite, and are almost absent in Ca kapellasite, showing the high sensitivity of the magnetic excitation in [Formula: see text] to the defects in crystals. On the other hand, a clear thermal Hall signal has been observed in the lowest-[Formula: see text] crystal of volborthite and in Ca kapellasite. Remarkably, both the temperature dependence and the magnitude of [Formula: see text] of volborthite are found to be very similar to those of Ca kapellasite, despite of about an order of magnitude difference in [Formula: see text] We find that [Formula: see text] of both compounds is well reproduced, both qualitatively and quantitatively, by spin excitations described by the Schwinger-boson mean-field theory applied to KHA with the Dzyaloshinskii-Moriya interaction. This excellent agreement demonstrates not only that the thermal Hall effect in these kagomé antiferromagnets is caused by spins in the spin liquid phase, but also that the elementary excitations of this spin liquid phase are well described by the bosonic spin excitations.

Synthesis, structure, and electronic properties of the Li11RbGd4Te6O30 single crystal

Materials with spin dimers have attracted much attention in the last several decades because they could provide a playground to embody simple quantum spin models. For example, the Bose–Einstein condensation of magnons has been observed in TlCuCl₃ with anti-ferromagnetic Cu₂Cl₆ dimers. In this work, we have synthesized a new kind of single-crystal Li₁₁RbGd₄Te₆O₃₀ with Gd₂O₁₅ dimers. This material belongs to the rhombohedral system with the lattice parameters: a = b = c = 16.0948 Å and α = β = γ = 33.74°. First-principles calculations indicate that Li₁₁RbGd₄Te₆O₃₀ is a wide-bandgap (about 4.5 eV) semiconductor. But unlike many other well studied quantum dimer magnets with an anti-ferromagnetic ground state, the Gd₂O₁₄ dimers in Li₁₁RbGd₄Te₆O₃₀ show ferromagnetic intra-dimer exchange interactions according to our calculations. Our work provides a new material which could possibly extend the studies of the spin dimers.

The prime novelty of this research is the synthesis and theory analyses of a new kind of single crystal compound Li₁₁RbGd₄Te₆O₃₀ with Gd₂O₁₅ dimers.

Topological phases of higher Chern numbers in Kitaev-Heisenberg ferromagnet with further-neighbor interactions

Emergence of multiple topological phases with a series of Chern numbers, [Formula: see text]1, [Formula: see text]1, [Formula: see text]2, [Formula: see text]2, [Formula: see text]3 and [Formula: see text]4, is found in a ferromagnetic Kitaev-Heisenberg-spin-anisotropic model on honeycomb lattice with next-next-nearest-neighbor interactions in the presence of an external magnetic field. Magnon Chern insulating dispersions of this two-band model are studied by using linear spin-wave theory formulated on the exact ferromagnetic ground state. Magnon edge states are obtained for the respective topological phases along with density of states. A topological phase diagram of this model is presented. Behavior of thermal Hall conductivity for those phases is studied. Sharp jumps of thermal hall conductivity is noted near the vicinity of phase transition points. Topological phases of the Kitaev spin-liquid compounds, [Formula: see text]-RuCl₃, X₂IrO₃, X  =  (Na, Li) and CrY₃, Y  =  (Cl, Br, I) are characterized based on this theoretical findings.

Triplon band splitting and topologically protected edge states in the dimerized antiferromagnet

Search for topological materials has been actively promoted in the field of condensed matter physics for their potential application in energy-efficient information transmission and processing. Recent studies have revealed that topologically invariant states, such as edge states in topological insulators, can emerge not only in a fermionic electron system but also in a bosonic system, enabling nondissipative propagation of quasiparticles. Here we report the topologically nontrivial triplon bands measured by inelastic neutron scattering on the spin-1/2 two-dimensional dimerized antiferromagnet Ba₂CuSi₂O₆Cl₂. The excitation spectrum exhibits two triplon bands that are clearly separated by a band gap due to a small alternation in interdimer exchange interaction, consistent with a refined crystal structure. By analytically modeling the triplon dispersion, we show that Ba₂CuSi₂O₆Cl₂ is the first bosonic realization of the coupled Su-Schrieffer-Heeger model, where the presence of topologically protected edge states is prompted by a bipartite nature of the lattice.

Topological edge states can emerge not only in a fermionic electron system but also in a bosonic system. Here, Nawa et al. report topologically nontrivial triplon bands in a two-dimensional dimerized quantum antiferromagnet Ba₂CuSi₂O₆Cl₂, suggesting a bosonic realization of the coupled Su-Schrieffer-Heeger model.

Nontrivial Triplon Topology and Triplon Liquid in Kitaev-Heisenberg-type Excitonic Magnets

The combination of strong spin-orbit coupling and correlations, e.g., in ruthenates and iridates, has been proposed as a means to realize quantum materials with nontrivial topological properties. We discuss here Mott insulators where on-site spin-orbit coupling favors a local J_{tot}=0 singlet ground state. We investigate excitations into a low-lying triplet, triplons, and find them to acquire nontrivial band topology in a magnetic field. We also comment on magnetic states resulting from triplon condensation, where we find, in addition to the same ordered phases known from the J_{tot}=1/2 Kitaev-Heisenberg model, a triplon liquid taking the parameter space of Kitaev's spin liquid.

Degeneracy of the 1/8 Plateau and Antiferromagnetic Phases in the Shastry-Sutherland Magnet TmB_{4}

The 1/8 fractional plateau phase (1/8 FPP) in Shastry-Sutherland lattice (SSL) spin systems has been viewed an exemplar of emergence on an Archimedean lattice. Here we explore this phase in the Ising magnet TmB_{4} using high-resolution specific heat (C) and magnetization (M) in the field-temperature plane. We show that the 1/8 FPP is smoothly connected to the antiferromagnetic phase on ramping the field from H=0. Thus, the 1/8 FPP is not a distinct thermodynamic ground state of TmB_{4}. The implication of these results for Heisenberg spins on the SSL is discussed.

Thermal Critical Points and Quantum Critical End Point in the Frustrated Bilayer Heisenberg Antiferromagnet

We consider the finite-temperature phase diagram of the S=1/2 frustrated Heisenberg bilayer. Although this two-dimensional system may show magnetic order only at zero temperature, we demonstrate the presence of a line of finite-temperature critical points related to the line of first-order transitions between the dimer-singlet and -triplet regimes. We show by high-precision quantum Monte Carlo simulations, which are sign-free in the fully frustrated limit, that this critical point is in the Ising universality class. At zero temperature, the continuous transition between the ordered bilayer and the dimer-singlet state terminates on the first-order line, giving a quantum critical end point, and we use tensor-network calculations to follow the first-order discontinuities in its vicinity.

Discovery of coexisting Dirac and triply degenerate magnons in a three-dimensional antiferromagnet

Topological magnons are emergent quantum spin excitations featured by magnon bands crossing linearly at the points dubbed nodes, analogous to fermions in topological electronic systems. Experimental realisation of topological magnons in three dimensions has not been reported so far. Here, by measuring spin excitations (magnons) of a three-dimensional antiferromagnet Cu₃TeO₆ with inelastic neutron scattering, we provide direct spectroscopic evidence for the coexistence of symmetry-protected Dirac and triply degenerate nodes, the latter involving three-component magnons beyond the Dirac–Weyl framework. Our theoretical calculations show that the observed topological magnon band structure can be well described by the linear-spin-wave theory based on a Hamiltonian dominated by the nearest-neighbour exchange interaction J₁. As such, we showcase Cu₃TeO₆ as an example system where Dirac and triply degenerate magnonic nodal excitations coexist, demonstrate an exotic topological state of matter, and provide a fresh ground to explore the topological properties in quantum materials.

Topological magnonic materials display exotic properties which may enable high-efficiency and low-cost spintronic devices. Here the authors demonstrate a three-dimensional antiferromagnet Cu₃TeO₆ that hosts symmetry-protected Dirac and triply degenerate magnons.

Dynamics and Instabilities of the Shastry-Sutherland Model

We study the excitation spectrum in the dimer phase of the Shastry-Sutherland model by using an unbiased variational method that works in the thermodynamic limit. The method outputs dynamical correlation functions in all possible channels. This output is exploited to identify the order parameters with the highest susceptibility (single or multitriplon condensation in a specific channel) upon approaching a quantum phase transition in the magnetic field versus the J^{'}/J phase diagram. We find four different instabilities: antiferro spin nematic, plaquette spin nematic, stripe magnetic order, and plaquette order, two of which have been reported in previous studies.

Photoinduced Topological Phase Transitions in Topological Magnon Insulators

Topological magnon insulators are the bosonic analogs of electronic topological insulators. They are manifested in magnetic materials with topologically nontrivial magnon bands as realized experimentally in a quasi-two-dimensional (quasi-2D) kagomé ferromagnet Cu(1-3, bdc), and they also possess protected magnon edge modes. These topological magnetic materials can transport heat as well as spin currents, hence they can be useful for spintronic applications. Moreover, as magnons are charge-neutral spin-1 bosonic quasiparticles with a magnetic dipole moment, topological magnon materials can also interact with electromagnetic fields through the Aharonov-Casher effect. In this report, we study photoinduced topological phase transitions in intrinsic topological magnon insulators in the kagomé ferromagnets. Using magnonic Floquet-Bloch theory, we show that by varying the light intensity, periodically driven intrinsic topological magnetic materials can be manipulated into different topological phases with different sign of the Berry curvatures and the thermal Hall conductivity. We further show that, under certain conditions, periodically driven gapped topological magnon insulators can also be tuned to synthetic gapless topological magnon semimetals with Dirac-Weyl magnon cones. We envision that this work will pave the way for interesting new potential practical applications in topological magnetic materials.

Topological magnon bands and unconventional thermal Hall effect on the frustrated honeycomb and bilayer triangular lattice

In the conventional ferromagnetic systems, topological magnon bands and thermal Hall effect are due to the Dzyaloshinskii-Moriya interaction (DMI). In principle, however, the DMI is either negligible or it is not allowed by symmetry in some quantum magnets. Therefore, we expect that topological magnon features will not be present in those systems. In addition, quantum magnets on the triangular-lattice are not expected to possess topological features as the DMI or spin-chirality cancels out due to equal and opposite contributions from adjacent triangles. Here, however, we predict that the isomorphic frustrated honeycomb-lattice and bilayer triangular-lattice antiferromagnetic system will exhibit topological magnon bands and topological thermal Hall effect in the absence of an intrinsic DMI. These unconventional topological magnon features are present as a result of magnetic-field-induced non-coplanar spin configurations with nonzero scalar spin chirality. The relevance of the results to realistic bilayer triangular antiferromagnetic materials are discussed.

Thermal Hall Effect in a Phonon-Glass Ba_{3}CuSb_{2}O_{9}

A distinct thermal Hall signal is observed in a quantum spin liquid candidate Ba_{3}CuSb_{2}O_{9}. The transverse thermal conductivity shows a power-law temperature dependence below 50 K, where a spin gap opens. We suggest that because of the very low longitudinal thermal conductivity and the thermal Hall signals, a phonon Hall effect is induced by strong phonon scattering of orphan Cu^{2+} spins formed in the random domains of the Cu^{2+}-Sb^{5+} dumbbells in Ba_{3}CuSb_{2}O_{9}.

Emergence of nontrivial magnetic excitations in a spin-liquid state of kagomé volborthite

When quantum fluctuations destroy underlying long-range ordered states, novel quantum states emerge. Spin-liquid (SL) states of frustrated quantum antiferromagnets, in which highly correlated spins fluctuate down to very low temperatures, are prominent examples of such quantum states. SL states often exhibit exotic physical properties, but the precise nature of the elementary excitations behind such phenomena remains entirely elusive. Here, we use thermal Hall measurements that can capture the unexplored property of the elementary excitations in SL states, and report the observation of anomalous excitations that may unveil the unique features of the SL state. Our principal finding is a negative thermal Hall conductivity [Formula: see text] which the charge-neutral spin excitations in a gapless SL state of the 2D kagomé insulator volborthite Cu3V2O7(OH)2[Formula: see text]2H2O exhibit, in much the same way in which charged electrons show the conventional electric Hall effect. We find that [Formula: see text] is absent in the high-temperature paramagnetic state and develops upon entering the SL state in accordance with the growth of the short-range spin correlations, demonstrating that [Formula: see text] is a key signature of the elementary excitation formed in the SL state. These results suggest the emergence of nontrivial elementary excitations in the gapless SL state which feel the presence of fictitious magnetic flux, whose effective Lorentz force is found to be less than 1/100 of the force experienced by free electrons.