Through first-principles calculations, we study the electronic structure of double-perovskite iridates with Ir in the d 4 valence state. Contrary to the expected strong spin-orbit driven J = 0 nonmagnetic state, we find finite moment at the Ir site, exhibiting breakdown of the J = 0 state. We further find the band structure effect rather than the crystal field effect to be responsible for this breakdown. The antiferromagnetic superexchange interaction between Ir moments, in general, makes these compounds insulating.

We show using detailed magnetic and thermodynamic studies and theoretical calculations that the ground state of Ba 3 ZnIr 2 O 9 is a realization of a novel spin-orbital liquid state. Our results reveal that Ba 3 ZnIr 2 O 9 with Ir 5þ (5d 4 ) ions and strong spin-orbit coupling (SOC) arrives very close to the elusive J ¼ 0 state but each Ir ion still possesses a weak moment. Ab initio density functional calculations indicate that this moment is developed due to superexchange, mediated by a strong intradimer hopping mechanism. While the Ir spins within the structural Ir 2 O 9 dimer are expected to form a spin-orbit singlet state (SOS) with no resultant moment, substantial frustration arising from interdimer exchange interactions induce quantum fluctuations in these possible SOS states favoring a spin-orbital liquid phase down to at least 100 mK. DOI: 10.1103/PhysRevLett.116.097205 5d transition metal compounds often exhibit unusual electronic and magnetic properties due to the presence of strong spin-orbit coupling (SOC), comparable to their onsite Coulomb (U) and crystal field (Δ CFE ) interactions [1,2]. In the strong spin-orbit coupling regime, M J ( P m j ) becomes the only valid quantum number instead of m l (orbital) and m s (spin), and the multiplets and their degeneracies are solely determined by the total angular momentum J. The electronic and magnetic responses of a system in such limits are not yet well understood and have generated significant curiosity in recent times. For example, the curious insulating state of the layered tetravalent iridates (Ir 4þ ; 5d 5 ) has recently been explained within single particle theories assuming splitting of t 2g bands into a set of fully filled quartet bands separated from another set of half-filled narrow doublet bands due to finite SOC. The half-filled doublet bands further split into fully occupied lower and empty upper Hubbard bands in the presence of relatively small Hubbard U [3-5].The pentavalent Iridates (Ir 5þ ; 5d 4 ) are more intriguing, where in the strong SOC limit all the spin-orbit entangled electrons will be confined to singlet J ¼ 0 (M J ¼ 0) ground state, with no net moment. The evolution of ground and excited states of a low spin 5d t 4 2g Ir 5þ ion as a function of SOC parameter λ 0 is illustrated in Fig. 1(a) and a J ¼ 0 ground state is realized in the strong SOC limit [6]. A possibility of excitonic magnetism has been predicted for these systems where the energy scale of the singlet-triplet splitting determined by SOC is comparable to superexchange interaction promoted by hopping [10]. The breakdown of the J ¼ 0 nonmagnetic state in d 4 systems can also be realized within a single electron picture primarily driven by band-structure effect that allows the hybridization between the quartet and the doublet redistributed orbitals (eigenstates of the spin-orbit coupled Hamiltonian). Overall, d 4 Ir compounds in the strong SOC limit may host weak magnetic moment unless the λ 0 becomes so large that any excitonic or hopping-assisted magnetism become...

We report magnetic susceptibility (χ) and heat capacity (C p ) measurements along with ab-initio electronic structure calculations on PbCuTe 2 O 6 , a compound made up of a three dimensional 3D network of corner-shared triangular units. The presence of antiferromagnetic interactions is inferred from a Curie-Weiss temperature (θ CW ) of about −22 K from the χ(T ) data. The magnetic heat capacity C m data show a broad maximum at T max ≃ 1.15 K (i.e.T max /θ CW ≃ 0.05), which is analogous to the the observed broad maximum in the C m /T data of a hyper-Kagome system, Na 4 Ir 3 O 8 . In addition, C m data exhibit a weak kink at T * ≃ 0.87 K. While the T max is nearly unchanged, the T * is systematically suppressed in an increasing magnetic field (H) up to 80 kOe. For H ≥ 80 kOe, the C m data at low temperatures exhibit a characteristic power-law (T α ) behavior with an exponent α slightly less than 2. Hopping integrals obtained from the electronic structure calculations show the presence of strongly frustrated 3D spin interactions along with non-negligible unfrustrated couplings. Our results suggest that PbCuTe 2 O 6 is a candidate material for realizing a 3D quantum spin liquid state at high magnetic fields.

We report α-Cu2V2O7 to be an improper multiferroic with the simultaneous development of electric polarization and magnetization below TC = 35 K. The observed spontaneous polarization of magnitude 0.55 µCcm −2 is highest among the copper based improper multiferroic materials. Our study demonstrates sizable amount of magneto-electric coupling below TC even with a low magnetic field. The theoretical calculations based on density functional theory (DFT) indicate magnetism in α-Cu2V2O7 is a consequence of ferro-orbital ordering driven by polar lattice distortion due to the unique pyramidal (CuO5) environment of Cu. The spin orbit coupling (SOC) further stabilize orbital ordering and is crucial for magnetism. The calculations indicate that the origin of the giant ferroelectric polarization is primarily due to the symmetric exchange-striction mechanism and is corroborated by temperature dependent X-ray studies.PACS numbers: 75.85.+t, 71.20.-b Recently multiferroic materials with mutually coupled ferroelectric (FE) and magnetic orders have attracted considerable interest for their versatile technological as well as fundamental importance. [1-4] A strong magneto-electric (ME) coupling is expected in improper magnetic mutiferroics where ferroelectricity is induced by a specific magnetic order. In the last one decade, several magnetic multiferroics have been discovered [5][6][7][8][9][10] where FE polarization is either associated with spiral magnetic structure induced by spin-orbit coupling (SOC) [11,12] or by symmetric exchange striction (SES) mechanism in case of collinear magnets. [7,13] Due to the secondary nature of the electric order, the value of the FE polarization in such magnetic multiferroics is much smaller (generally ∼ 0.01 µC.cm −2 ) compared to the 'proper' FE.[4] A recent breakthrough in this direction is the discovery of giant ferroelectricity (∼ 0.3 µC.cm −2 ) and large ME coupling in mixed valent manganate CaMn 7 O 12 below about 90 K [14] mediated by both Dzyaloshinski-Moriya (DM) interaction as well as exchange striction mechanism. [15] In this respect cuprates may be an attractive option as the orbital degrees of freedom and strong Coulomb correlations present in cuprates may not only lead to lattice distortion and magnetism but also possibly induce a coupling between them which are essential ingredients for multiferroicity.In view of the above, we investigated the Cu-based oxide Cu 2 V 2 O 7 in its orthorhombic α phase. Cu 2 V 2 O 7 crystallizes in at least three different polymorphs, namely α, β and γ-phases where only the α phase is noncentrosymmetric [16][17][18] and is important in the present context. It consists of magnetic Cu 2+ (3d 9 , S = 1 2 ) and nonmagnetic V 5+ (3d 0 , S = 0) metal ions making it a system having both partially filled and empty d shells similar to BiFeO 3 , BiMnO 3 , Pb(Fe 2/3 W 1/3 )O 3 etc. [19,20]. All Cu2+ ions are equivalent with fivefold coordination to oxygen atoms forming a distorted [CuO 5 ] polyhedron. Each Cu-polyhedron is linked with another two via edge s...

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