Using transmission electron microscopy, the anomalies in resistivity and magnetic susceptibility at ~262 K in IrTe2 are found to accompany the superlattice peaks with q[over q=(1/5,0,-1/5). The wave vector is consistent with our theoretical calculation for the Fermi surface nesting vector, indicating that the ~262 K transition is of the charge-orbital density wave (DW) type. We also discovered that both Pd intercalation and substitution induce bulk superconductivity with T(c) up to ~3 K, which competes with DW in a quantum critical pointlike manner.

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We have theoretically designed the half-metallic (HM) antiferromagnets (AFMs) in thiospinel systems, Mn(CrV)S4 and Fe0.5Cu0.5(V0.5Ti1.5)S4, based on the electronic structure studies in the local-spin-density approximation (LSDA). We have also explored electronic and magnetic properties of parent spinel compounds of the above systems; CuV2S4 and CuTi2S4 are found to be HM ferromagnets in their cubic spinel structures, while MnCr2S4 is a ferrimagnetic insulator. We have discussed the feasibility of material synthesis of HM-AFM thiospinel systems.PACS numbers: 71.20. Be, 75.25.+Z, 75.50.Ee Since the first theoretical report of Heusler halfmetallic (HM) ferromagnet NiMnSb by de Groot et al.[1], much effort has been devoted to developing the HM magnetic materials, in which the conduction electrons at the Fermi level E F are 100% spin-polarized [2]. Especially, the HM antiferromagnet (AFM) attracts great attention because it is a non-magnetic metal but its conduction electrons are 100% spin-polarized. It can be used as a probe of the spin-polarized scanning tunneling microscope without perturbing the spin-character of samples. Further, the HM-AFM is expected to play a vital role in the future spintronic devices that utilize the spin polarization of the carriers.The first HM-AFM, V 7 MnFe 8 Sb 7 In, which is a derivative of the Heusler compound, was proposed by van Leuken and de Groot [3]. Anther possibility was suggested by Pickett [4] in the double perovskite system such as La 2 VMnO 6 . In this case, V and Mn have antiferromagnetically aligned magnetic moments that exactly cancel each other. To date, there has been no successful experimental realization of the HM-AFM.The thiospinel FeCr 2 S 4 in its metallic phase has the HM ferrimagnetic state with nominal valence configurations of Fe 2+ (d 6 ) and Cr 3+ (d 3 ) [5][6][7]. The magnetic moments of Fe and Cr are 4µ B and −3µ B , respectively, which produce the integer total magnetic moment of −2µ B per formula unit. From this, one can expect that FeV 2 S 4 becomes a HM-AFM, since V has one less electron than Cr and so the magnetic moment of two V 3+ (d 2 : S=1) ions would cancel that of Fe 2+ (d 6 : S=2), still possessing the HM property. Indeed the local-spindensity approximation (LSDA) band calculation yields the HM-AFM electronic structure of FeV 2 S 4 [8]. Unfortunately, FeV 2 S 4 does not exist in the cubic spinel structure but in the hexagonal NiAs structure (Cr 3 S 4 -type) with a complicated magnetic configuration [9]. So the above expectation does not work for FeV 2 S 4 in nature.Motivated by the above expectation, we attempt to search for the HM-AFM in other thiospinel compounds. Most of the thiospinel compounds of AB 2 S 4 -type (A, B: transition metals) with cubic structure have a ferrimagnetic ground state. Usually, the magnetic moment of the A ion in the tetrahedral site is antiferromagnetically polarized with that of the B ion in the octahedral site. Under this circumstance, there are some pairs of A and B which give rise to the exactly cancelled ma...

Based on the theoretical exploration of electronic structures, we propose that the ordered double perovskites LaAVRuO6 and LaVO3/ARuO3 (001) superlattice (A = Ca, Sr and Ba) are strong candidates for half-metallic (HM) antiferromagnets (AFMs). We have shown that the HM-AFM nature in LaAVRuO6 is very robust regardless of (i) divalent ion replacement at A-sites, (ii) oxygen site relaxation, (iii) the inclusion of the Coulomb correlation, and (iv) cation disorder. A type of the double exchange interaction is expected to be responsible for the half-metallicity and the antiferromagnetism in these systems.Since the observation of the room temperature colossal magnetoresistance (CMR) phenomenon in Sr 2 FeMoO 6 [1], intensive research efforts have been devoted to understanding electronic and magnetic properties of double perovskites with A 2 BB ′ O 6 formula unit (F.U.). The high transition temperature T C and the low field MR in double perovskites suggest the high spin-polarization of conduction electrons and the half-metallic (HM) ground state [1,2]. In fact, the HM property is considered to be closely related to the CMR phenomena observed in various materials [3,4]. It has also been proposed that the double perovskites can be a suitable candidate for the half-metallic (HM) antiferromagnet (AFM) [5]. The HM-AFM is a nonmagnetic metal, but its conduction electrons are perfectly spin-polarized. The HM-AFM is expected to play a vital role in the advanced spintronic devices that utilize the spin polarization of the conduction carriers. Furthermore, the success in synthesizing the ordered La 2 CrFeO 6 as an artificial superlattice of (111) layers of LaFeO 3 /LaCrO 3 stimulates research on developing new double perovskites with exotic properties [6]. The purpose of present work is to search for candidate materials in double perovskites having the HM-AFM characteristics.The first HM-AFM was proposed by van Leuken and de Groot [7] on the basis of the Heusler compound. Pickett [5] has also suggested that the double perovskite La 2 VMnO 6 can be a promising candidate for the HM-AFM. The local spin-density approximation (LSDA) band calculation in La 2 VMnO 6 indicates that only the minority t 2g bands of both V and Mn contribute to the density of states (DOS) 4+ ion has 2µ B in low spin state, and so, if they are antiferromagnetically coupled, the total magnetic moment will be zero. Another valence configuration of V 4+ (3d 1 ) and Ru 3+ (4d 5 ), albeit not so plausible, would also produce zero total magnetic moment.We have investigated electronic structures of mixedcation double perovskites LAVRO (A=Ca, Sr, Ba) using both the LSDA and the LSDA + U (U : Coulomb correlation interaction) scheme on the basis of the linearized muffin-tin orbitals (LMTO) band method [10]. We have considered LAVRO as a combined form of LaVO 3 and ARuO 3 perovskites. LaAVRO with the antiferromagnetic coupling of V and Ru spins corresponds to a superlattice having the layered structure of stacking along the [111] direction, as in La 2 CrFeO 6 . In t...

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