1Recent discovery of both gapped and gapless topological phases in weakly correlated electron systems has introduced various relativistic particles and a number of exotic phenomena in condensed matter physics [1][2][3][4][5] . The Weyl fermion 6-8 is a prominent example of three dimensional (3D), gapless topological excitation, which has been experimentally identified in inversion symmetry breaking semimetals 4,5 . However, their realization in spontaneously time reversal symmetry (TRS) breaking magnetically ordered states of correlated materials has so far remained hypothetical 7, 9, 10 . Here, we report a set of experimental evidence for elusive magnetic Weyl fermions in Mn 3 Sn, a non-collinear antiferromagnet that exhibits a large anomalous Hall effect even at room temperature 11 . Detailed comparison between our angle resolved photoemission spectroscopy (ARPES) measurements and density functional theory (DFT) calculations reveals significant bandwidth renormalization and damping effects due to the strong correlation among Mn 3d electrons. Moreover, our transport measurements have unveiled strong evidence for the chiral anomaly of Weyl fermions, namely, the emergence of positive magnetoconductance only in the presence of parallel electric and magnetic fields. The magnetic Weyl fermions of Mn 3 Sn have a significant technological potential, since a weak field (∼ 10 mT) is adequate for controlling the distribution of Weyl points and the large fictitious field (∼ a few 100 T) in the momentum space. Our discovery thus lays the foundation for a new field of science and technology involving the magnetic Weyl excitations of strongly correlated electron systems.Traditionally, topological properties have been considered for the systems supporting gapped bulk excitations 1 . However, over the past few years three dimensional gapless systems such asWeyl and Dirac semimetals have been discovered, which combine two seemingly disjoint notions 2 of gapless bulk excitations and band topology [2][3][4][5] . In 3D inversion or TRS breaking systems, two nondegenerate energy bands can linearly touch at pairs of isolated points in the momentum (k) space, giving rise to the Weyl quasiparticles. The touching points or Weyl nodes act as the unit strength (anti) monopoles of underlying Berry curvature [4][5][6][7] , leading to the protected zero energy surface states also known as the Fermi-arcs 4,5,7 , and many exotic bulk properties such as large anomalous Hall effect (AHE) 12 , optical gyrotropy 13 , and chiral anomaly 6,[14][15][16][17][18][19] . Interestingly, the Weyl fermions can describe low energy excitations of both weakly and strongly correlated electron systems. In weakly correlated, inversion symmetry breaking materials, where the symmetry breaking is entirely caused by the crystal structure rather than the collective properties of electrons, the ARPES has provided evidence for long-lived bulk Weyl fermions and the surface Fermi arcs 4, 5 .On the other hand, the magnetic Weyl fermions have been predicted for several...

In metallic ferromagnets, the Berry curvature of underlying quasiparticles can cause an electric voltage perpendicular to both magnetization and an applied temperature gradient, a phenomenon called the anomalous Nernst effect (ANE) [1,2]. Here, we report the observation of a giant ANE in the full-Heusler ferromagnet Co 2 MnGa, reaching S yx ∼ −6 µV/K at room T , one order of magnitude larger than the maximum value reported for a magnetic conductor [3]. With increasing temperature, the transverse thermoelectric conductivity or Peltier coefficient α yx shows a crossover between T -linear and −T log(T ) behaviors, indicating the violation of Mott formula at high temperatures. Our numerical and analytical calculations indicate that the proximity to a quantum Lifshitz transition between type-I and type-II magnetic Weyl fermions [4-6] is responsible for the observed crossover properties and an enhanced α yx . The

Frustrated magnetic materials, in which local conditions for energy minimization are incompatible because of the lattice structure, can remain disordered to the lowest temperatures. Such is the case for Ba(3)CuSb(2)O(9), which is magnetically anisotropic at the atomic scale but curiously isotropic on mesoscopic length and time scales. We find that the frustration of Wannier's Ising model on the triangular lattice is imprinted in a nanostructured honeycomb lattice of Cu(2+) ions that resists a coherent static Jahn-Teller distortion. The resulting two-dimensional random-bond spin-1/2 system on the honeycomb lattice has a broad spectrum of spin-dimer-like excitations and low-energy spin degrees of freedom that retain overall hexagonal symmetry.

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The magnetic properties of YVO 3 single crystals have been studied in the temperature range from 350 to 4.2 K and in magnetic fields up to 7 T. It is found that in an applied field less than 4 kOe remarkable magnetization reversals occur at two distinct temperatures: an abrupt switch at T s ϭ77 K associated with a first-order structure phase transition and a gradual reversal at T*Ϸ95 K without a structural anomaly. Most interestingly, the magnetization always switches to the opposite direction if the crystal is cooled or warmed through T s and T* in a field less than ϳ500 Oe. In higher magnetic fields the magnetization does not change sign but has a minimum at T* and a sudden change at T s. A possible mechanism for the observed peculiar magnetic behavior is discussed, related to the competition of the single-ion magnetic anisotropy and the antisymmetric Dzyaloshinsky-Moriya interaction accompanied by a change of orbital ordering.

Evidence has been found for a change in the ordered occupation of the vanadium d-orbitals at the 77K phase transition in YVO3, manifested by a change in the type of Jahn-Teller distortion. The orbital ordering above 77K is not destroyed at the magnetic ordering temperature of 116K, but is present as far as a second structural phase transition at 200K. The transition between orbital orderings is caused by an increase in octahedral tilting with decreasing temperature. [5,6].In this letter we report on the orbital ordering in YVO 3 . The orbital ordering of t 2g electron systems is reflected structurally to a lesser extent than that of e g systems but can still have a dramatic influence on the physical properties. We show that the orbital ordering in YVO 3 takes place at 200K which, contrary to previous assumptions, is far above the antiferromagnetic (AFM) ordering temperature T N = 116K. Moreover, there is a change in symmetry of the orbital ordering at T S = 77K. This is induced by an increased tilting of the octahedra with decreasing temperature, and it changes the easy axis of the V 3+ d 2 S = 1 spin, resulting in a magnetic structure transition and a reversal of the net ferromagnetic moment of the canted AFM state [7].We have previously described the unusual magnetic properties of YVO 3 [7,8]. YVO 3 adopts a distorted perovskite structure with the space group P bnm [9] at room temperature. At T S there is a first order structural phase transition involving a sudden change in the unit cell volume, below which a JT ordered state is present [8]. A tetragonal distortion of the octahedra, where the long and short V-O bond distances alternate along the [110] and [110] directions of the ab plane, causes a splitting of the V t 2g orbitals into a doublet of lower energy and a singlet of higher energy. The doublet contains the xy orbital, which is always occupied, and either the zx or yz orbital. This ordered, alternating occupation of the V d zx and d yz orbitals between adjacent cations is shown schematically in Figs. 1 and 2. We believe that the change in magnetic structure [10] is caused by a change from C-type orbital ordering (OO) below T S to G-type OO above T S (Fig. 2). This statement is supported by the Goodenough-Kanamori rules [11], band structure calculations [12] and model Hartree-Fock calculations [13]. Experimentally, the only suggestion of G-type OO thus far has been provided by resonant X-ray scattering studies at the vanadium main K edge [14], although there is some debate about whether orbital ordering can be directly observed at this energy [15,16]. In terms of the crystal structure, G-type OO is incompatible with P bnm symmetry since two crystallographically distinct JT-distorted ab planes are required, with "out of phase" bonding arrangements; in P bnm all ab planes are rendered equivalent by the mirror planes at z = 1/4 and 3/4, and the bonding arrangement is "in phase". We provide here the first crystallographic evidence for G-type OO above T S .Single crystals of YVO 3 were prepared as prev...

In recent years, large effort has been put into the development and characterization of new colossal-ε' materials. For example, the recent discovery (1,2) of "colossal" values of the dielectric constant, ε', up to about 10 5 in CaCu 3 Ti 4 O 12 (CCTO) has aroused tremendous interest and a huge number of publications deals with its investigation and optimization. Aside of the extensively investigated CCTO, there are also some reports of other colossal-ε' materials (e.g., refs. (3,4,5,6,7)), mainly transition metal oxides. While there is no clear definition, the term "colossal" typically denotes values of ε' > 10 4 . Such materials are very appealing for the further miniaturization of capacitive components in electronic devices and also in giant capacitors that may replace batteries for energy storage.Of course, colossal dielectric constants are also found in ferroelectrics where close to the phase transition very large values are reached. However, ferroelectrics are characterized by a strong temperature dependence of ε' around their critical temperature, which restricts their applicability. In contrast, CCTO and other materials stand out due to their colossal-ε' values being nearly constant over a broad temperature range around room temperature. But in all these materials a strong frequency dependence is observed, revealing the signature of relaxational contributions, namely a steplike decrease of ε' above a certain, temperaturedependent frequency, accompanied by a peak in the dielectric loss. Intrinsic relaxations are commonly observed, e.g., in materials containing dipolar molecules, which reorient in accord with the ac field at low frequencies, but cannot follow at high frequencies. However, the extensive investigations of CCTO, have quite clearly revealed that the observed relaxation features are due to a nonintrinsic effect, termed Maxwell-Wagner (MW) relaxation (8,9,10). It arises from heterogeneity of the sample, which is composed of a bulk region with relatively high conductivity and one or several relatively insulating thin layers. The equivalent circuit describing such a sample leads to a relaxation-like frequency and temperature dependence (10). The insulating layers can arise, for example, from surface effects (e.g., depletion regions of Schottky diodes at the electrodes) or internal barriers (e.g., grain boundaries). However, this is rather irrelevant from an application point of view (e.g., external surface layers are used to enhance the capacitance in ferroelectrics-based multi-layer ceramic capacitors). Thus, although in CCTO the exact mechanism is not yet finally clarified, the interest in this material is still high. This is, amongst others, demonstrated by the fact that since its discovery in 2000, twelve socalled "highly-cited" papers on this topic have appeared (source: ISI Web of Science, Nov. 2008). Unfortunately, at room temperature the relaxation in CCTO leads to a decrease of ε' in the MHz region and around GHz only values of the order of 100 are observed (8,11,12). In contrast, electron...

Inelastic light scattering spectra of several members of the RFe3(BO3)4 family reveal a cascade of phase transitions as a function of temperature, starting with a structural, weakly first order, phase transition followed by two magnetic phase transitions. Those consist of the ordering of the Fe-spin sublattice revealed by all the compound, and a subsequent spin-reorientational transition for GdFe3(BO3)4. The Raman data evidence a strong coupling between the lattice and magnetic degrees of freedom in these borates. The Fe-sublattice ordering leads to a strong suppression of the low energy magnetic scattering, and a multiple peaked two-magnon scattering continuum is observed. Evidence for short-range correlations is found in the 'paramagnetic' phase by the observation of a broad magnetic continuum in the Raman data, which persists up to surprisingly high temperatures.