1Spin liquid is a state of electron spins where quantum fluctuation breaks magnetic ordering with keeping spin correlation [1]. It has been one of central topics of magnetism because of its relevance to fascinating phenomena such as high-T c superconductivity [2, 3] and topological states [4]. In spite of the profound physics, on the other hand, spin liquid itself has been quite difficult to utilize.Typical spin liquid states are realized in one-dimensional spin systems, called quantum spin chains [5, 6]. Here we show that a spin liquid in a spin-1/2 quantum chain generates and carries spin current via its long-range spin fluctuation. This is demonstrated by observing an anisotropic negative spin Seebeck effect [7][8][9][10][11][12] along the spin chains in Sr 2 CuO 3 . The result shows that spin current can flow even in an atomic channel owing to the spin liquid state, which can be used for atomic spin-current wiring.A flow of electrons spin angular momentum is called spin current [13]. In condensed matter science, transport properties of spin current have attracted considerable interest since the discovery of various spin-current phenomena [14, 15]. In spintronics [16], on the other hand, it is of critical importance to find materials which can carry spin angular momentum efficiently in integrated microscopic devices.Two types of spin current have experimentally been explored so far. The first one is conduction-electron spin current, which is mediated by an electron motion in metals and semiconductors. Its velocity and propagation length are thus limited by electron diffusion [17]. The other type is spin-wave spin current [18,19], where spin waves, wavelike propagation of spin motions in magnets, carry spin angular momentum. Its excitation gap is equal to a spin-wave gap, proportional to magnetic anisotropy. Importantly, spin-wave spin current can exist even in insulators in which spin relaxation via conduction electrons is absent, an advantage which may realize fast and long-range spin current transmission, opening a new field of insulator-based spintronics. However, spin-wave spin current in classical magnets may not be suitable for microscopic devices, since handling spin waves becomes difficult when devices are miniaturized toward atomic scale; in ferromagnets, spontaneous magnetization brings about significant stray fields, causing crosstalk. In an antiferromagnetic system, on the other hand, spin ordering patterns should be broken or interfered when a device is in atomic scale; in both cases, spin waves become vulnerable. Therefore, to realize spin-current transport in microscopic devices, spin ordering is expected to vanish with 2 keeping strong interaction among spins.Here, we would like to make a new type of spin current debut: spinon spin current, which may provide a channel for atomic spin transmission to satisfy the requirements. A spinon generally refers to magnetic elementary excitation in quantum spin liquid states [1]. When system size of a magnet is reduced to atomic scale, quantum spin fluct...
We have investigated the longitudinal thermal conductivity of α-RuCl3, the magnetic state of which is considered to be proximate to a Kitaev honeycomb model, along with the spin susceptibility and magnetic specific heat. We found that the temperature dependence of the thermal conductivity exhibits an additional peak around 100 K, which is well above the phonon peak temperature (∼ 50 K). The higher-temperature peak position is comparable to the temperature scale of the Kitaev couplings rather than the Néel temperatures below 15 K. The additional heat conduction was observed for all five samples used in this study, and was found to be rather immune to a structural phase transition of α-RuCl3, which suggests its different origin from phonons. Combined with experimental results of the magnetic specific heat, our transport measurement suggests strongly that the higher-temperature peak in the thermal conductivity is attributed to itinerant spin excitations associated with the Kitaev couplings of α-RuCl3. A kinetic approximation of the magnetic thermal conductivity yields a mean free path of ∼ 20 nm at 100 K, which is well longer than the nearest Ru-Ru distance (∼ 3Å), suggesting the long-distance coherent propagation of magnetic excitations driven by the Kitaev couplings. IntroductionQuantum spin liquid is a phase of magnetic insulator in which frustration or quantum fluctuation prohibits magnetic order while keeping spin correlation 1-3 . It induces rich physical phenomena depending on the types of spin liquids, which cannot be realized in standard ordered magnets. Several quantum-spin models have been proposed as possible realizations of quantum spin liquids along with candidate frustrated magnets such as κ- ( Among them, the Kitaev honeycomb model is unique in that the ground state is exactly calculated and known to be a two-dimensional (2D) quantum spin liquid 10 . In this model, spin-1/2 moments, S r , sit on a honeycomb lattice and interact via the bond-dependent Ising couplings; different spin components interact via Ising coupling for the three different bonds of the honeycomb lattice. These anisotropic couplings, called the Kitaev couplings J α S
We have studied the longitudinal spin Seebeck effect (LSSE) in the layered ferromagnetic insulators CrSiTe 3 and CrGeTe 3 covered by Pt films in the measurement configuration where spin current traverses the ferromagnetic Cr layers. The LSSE response is clearly observed in the ferromagnetic phase and, in contrast to a standard LSSE magnet Y 3 Fe 5 O 12 , persists above the critical temperatures in both CrSiTe 3 /Pt and CrGeTe 3 /Pt samples. With the help of a numerical calculation, we attribute the LSSE signals observed in the paramagnetic regime to exchange-dominated interlayer transport of in-plane paramagnetic moments reinforced by short-range ferromagnetic correlations and strong Zeeman effects.The longitudinal spin Seebeck effect (LSSE) generates spin currents in magnetic materials when a temperature gradient is applied [1]. By injecting this spin current into a paramagnetic metal it can be measured as a voltage through the inverse spin-Hall effect. Because of the simple bilayer structure needed to generate a thermoelectric voltage, LSSE devices have a potential use as thermoelectric conversion elements [2,3]. From the point of basic physics, the LSSE is sensitive to spin correlations [4,5], and thus can be exploited as a probe to study the dynamical spin susceptibility in magnetic materials [6,7]. The LSSE was originally found in ferro(ferri)magnets and later measured also in antiferromagnets [8][9][10][11] and paramagnets [12,13]. However, the LSSE has not been studied in magnetic materials with two-dimensional (2D) crystal structures, even though 2D materials, such as transitionmetal chalcogenides, have drawn extensive research attention due to their extraordinary magnetic properties [14]. The layered ferromagnetic insulators CrSiTe 3 and CrGeTe 3 have been studied recently due to their intriguing physical properties. First-principles calculations [15, 16] predicted that the ferromagnetism in CrSiTe 3 survives even down to a monolayer thickness and indeed ferromagnetism in bilayer flakes of CrGeTe 3 was experimentally confirmed [17]. The Cr layers possess a graphenelike honeycomb structure and exotic properties are expected to arise in heterostructures fabricated with related 2D materials by van der Waals epitaxy. Furthermore, it has been reported that CrGeTe 3 acts as an ideal ferromagnetic substrate for the growth of the popular topological insulator Bi 2 Te 3 [18].In this Rapid Communication, we studied the LSSE [1] in the ferromagnets CrSiTe 3 and CrGeTe 3 in contact with Pt films. The crystal structure of CrSiTe 3 and CrGeTe 3 is illustrated schematically in Fig. 1(a). The Cr 3+ (spin 3/2) ions form a honeycomb lattice in the ab plane with the Si or Ge atoms in the center of the hexagon and the Cr 3+ atoms are surrounded by octahedra of Te atoms [19][20][21]. The honeycomb layers stack along the c axis, held together by van der Waals interactions, forming a quasi-2D structure with a highly anisotropic magnetic environment. Interestingly, it was reported that CrSiTe 3 , with a Curie temperature of T C...
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