While it was speculated that 5d 4 systems would possess non-magnetic J = 0 ground state due to strong Spin-Orbit Coupling (SOC), all such systems have invariably shown presence of magnetic moments so far. A puzzling case is that of Ba 2 YIrO 6 , which in spite of having a perfectly cubic structure with largely separated Ir 5+ (d 4 ) ions, has consistently shown presence of weak magnetic moments. Moreover, we clearly show from Muon Spin Relaxation (µSR) measurements that a change in the magnetic environment of the implanted muons in Ba 2 YIrO 6 occurs as temperature is lowered below 10 K. This observation becomes counterintuitive, as the estimated value of SOC obtained by fitting the RIXS spectrum of Ba 2 YIrO 6 with an atomic j − j model is found to be as high as 0.39 eV, meaning that the system within this model is neither expected to possess moments nor exhibit temperature dependent magnetic response. Therefore we argue that the atomic j − j coupling description is not sufficient to explain the ground state of such systems, where despite having strong SOC, presence of hopping triggers delocalisation of holes, resulting in spontaneous generation of magnetic moments. Our theoretical calculations further indicate that these moments favour formation of spin-orbital singlets in the case of Ba 2 YIrO 6 , which is manifested in µSR experiments measured down to 60 mK.
The zero‐magnetic‐field nonlinear Hall effect (NLHE) refers to the second‐order transverse current induced by an applied alternating electric field; it indicates the topological properties of inversion‐symmetry‐breaking crystals. Despite several studies on the NLHE induced by the Berry‐curvature dipole in Weyl semimetals, the direct current conversion by rectification is limited to very low driving frequencies and cryogenic temperatures. The nonlinear photoresponse generated by the NLHE at room temperature can be useful for numerous applications in communication, sensing, and photodetection across a high bandwidth. In this study, observations of the second‐order NLHE in type‐II Dirac semimetal CoTe2 under time‐reversal symmetry are reported. This is determined by the disorder‐induced extrinsic contribution on the broken‐inversion‐symmetry surface and room‐temperature terahertz rectification without the need for semiconductor junctions or bias voltage. It is shown that remarkable photoresponsivity over 0.1 A W−1, a response time of approximately 710 ns, and a mean noise equivalent power of 1 pW Hz−1/2 can be achieved at room temperature. The results open a new pathway for low‐energy photon harvesting via nonlinear rectification induced by the NLHE in strongly spin–orbit‐coupled and inversion‐symmetry‐breaking systems, promising a considerable impact in the field of infrared/terahertz photonics.
We have investigated the thermodynamic and local magnetic properties of the Mott insulating system Ag3LiRu2O6 containing Ru 4+ (4d 4 ) for novel magnetism. The material crystallizes in a monoclinic C2/m structure with RuO6 octahedra forming an edge-shared two-dimensional honeycomb lattice with limited stacking order along the c-direction. The large negative Curie-Weiss temperature (θCW = −57 K) suggests antiferromagnetic interactions among Ru 4+ ions though magnetic susceptibility and heat capacity show no indication of magnetic long-range order down to 1.8 K and 0.4 K, respectively. 7 Li nuclear magnetic resonance (NMR) shift follows the bulk susceptibility between 120-300 K and levels off below 120 K. Together with a power-law behavior in the temperature dependent spin-lattice relaxation rate between 0.2 and 2 K, it suggest dynamic spin correlations with gapless excitations. Electronic structure calculations suggest an S = 1 description of the Ru-moments and the possible importance of further neighbour interactions as also bi-quadratic and ring-exchange terms in determining the magnetic properties. Analysis of our µSR data indicates spin freezing below 5 K but the spins remain on the borderline between static and dynamic magnetism even at 20 mK.
Here we present the structural and magnetic properties of a new honeycomb material Ag3LiMn2O6. The system Ag[Li 1/3 Mn 2/3 ]O2 belongs to a quaternary 3R-delafossite family and crystallizes in a monoclinic symmetry with space group C 2/m and the magnetic Mn 4+ (S = 3/2) ions form a honeycomb network in the ab-plane. An anomaly around 50 K and the presence of antiferromagnetic (AFM) coupling (Curie-Weiss temperature θCW ∼ −51 K) were inferred from our magnetic susceptibility data. The magnetic specific heat clearly manifests the onset of magnetic ordering in the vicinity of 48 K and the recovered magnetic entropy, above the ordering temperature, falls short of the expected value, implying the presence of short-range magnetic correlations. An asymmetric Bragg peak (characteristic of two dimensional order), seen in neutron diffraction, gains intensity even above the ordering temperature, thus showing the existence of short-range spin correlations. Our electron spin resonance ESR experiments corroborate the bulk magnetic data. Additionally, the (ESR) line broadening on approaching the ordering temperature T N could be described in terms of a Berezinski-Kosterlitz-Thouless (BKT) scenario with T KT = 40(1) K. 7 Li NMR line-shift probed as a function of temperature tracks the static susceptibility (Kiso) of magnetically coupled Mn 4+ ions. The 7 Li spin-lattice relaxation rate (1/T 1) exhibits a sharp decrease below about 50 K. A critical divergence is absent at the ordering temperature perhaps because of the filtering out of the antiferromagnetic fluctuations at the Li site, i.e., at the centers of the hexagons in the honeycomb network. Combining our bulk and local probe measurements, we establish the presence of an ordered ground state for the honeycomb system Ag3LiMn2O6. Our ab initio electronic structure calculations suggest that in the ab-plane, the nearest neighbor (NN) exchange interaction is strong and AFM, while the next NN and the third NN exchange interactions are FM and AFM respectively. The interplanar exchange interaction is found to be relatively small. In the absence of any frustration the system is expected to exhibit long-range, AFM order, in agreement with experiment. arXiv:1903.08366v2 [cond-mat.str-el]
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