Crystal Structure, Transport, and Magnetic Properties of an Ir6+ Compound Ba8Al2IrO14

The novel iridate Ba8Al2IrO14 was prepared as single crystals by self-flux method, thereby providing a rare example of an all-Ir(VI) compound that can be synthesized under ambient pressure conditions. The preparation of all-Ir6+ iridate without using traditional high-pressure techniques has to our knowledge previously only been reported in Nd2K2IrO7 and Sm2K2IrO7. The monoclinic crystal structure (space group C2/m, No.12) is stable down to 90 K and contains layers of IrO6 octahedra separated by Ba and AlO4 tetrahedra. The material exhibits insulating behavior with a narrow band gap of ∼0.6 eV. The positive Seebeck coefficient indicates hole-like dominant charge carriers. Susceptibility measurement shows antiferromagnetic coupling with no order down to 2 K.

Disorder-induced electronic, magnetic, and optoelectronic properties of two-dimensional materials

Cheol Yeon Cheon

Technological advancement has been in cadence with material development by improving the purity of single crystals and, at the same time, controlling their imperfections. These capabilities have been especially vital for developing new technolo-gies based on two-dimensional (2D) van der Waals (vdW) materials for future electronic and optoelectronic applications. This is because the inherent properties of 2D vdW materials is highly susceptible to the presence of intrinsic structural defects and ex-trinsic disorders due to large surface-area-to-volume ratio. The successful reduction of these disorders has significantly im-proved material properties and led to the discovery of novel physical phenomena in vdW materials. On the other hand, structural defects â for instance, 0-dimensional point defects â can induce completely new properties that are otherwise absent in the pre-fect lattice. To harness the full potential of vdW materials, it is thus essential to produce high-quality crystals and understand how the disorder affects their material properties, which is the central idea of this dissertation.In this dissertation, we first present the work of high-quality epitaxial growth of NbS2. Based on atmospheric-pressure chemical vapor deposition, we have successfully synthesized the two polymorphs (2H and 3R) of NbS2 with the largest lateral size grown to date. Their distinct superconducting and metallic properties were examined under low-temperature charge transport, respectively. Our finding demonstrates the practical synthesis method for phase-controllable growth of 2D transition metal dichalcogenides and can benefit future studies in mesoscopic devices and large-area applications of 2D superconductors. Secondly, we present the work of discovering defect-induced novel properties in ultrathin layers of PtSe2. Although bulk PtSe2 is non-magnetic, we observe the appearance of magnetism in monolayer and bilayer PtSe2. We were able to measure the magnetoresistance (MR) of mono- and bilayer PtSe2 under perpendicular magnetic fields using proximitized graphene, and found antiferromagnetic and ferromagnetic MR responses for mono- and bilayer, respectively. The appearance of such different magnetic states is theoretically explained by the first-principle density functional theory calculation, suggesting the origin of induced-magnetic moments from intrinsic Pt vacancies for both layers. Moreover, we also found that structural disorder in PtSe2 can induce bulk photovoltaic effect (BPVE). The second-order optical nonlinear effects, such as BPVE, require broken structural inversion symmetry and crystal symmetry can be reduced by the presence of structural defects. The broken local inversion symmetry from structural disorder in centrosymmetric PtSe2 is manifested by the generation of zero-biased photo-current under homogenous illumination. We observe linear and circular polarization-dependent photocurrents in defective PtSe2, which is largely absent in the pristine crystal. Our findings in PtSe2 emphasize the importance of the structural disorder for generating completely new properties and stress the need for defect-engineering for realizing the practical use of PtSe2 in spintronic and photovoltaic applications.

EPFL2022

Magnetic properties of the magnetoelectric compound Cu2OSeO3: Magnetization and Se-77 NMR study

Jean-Philippe Ansermet, Helmuth Berger, Simon Granville

We present magnetization and Se-77 Nuclear Magnetic Resonance (NMR) measurements in single crystals of the magnetoelectric compound Cu2OSeO3. The temperature and field dependence of the magnetization suggest a ferrimagnetic ordering at T-c similar or equal to 60 K in a 3up-1down configuration. The easy axis of the magnetization is along the [100] crystallographic direction. The Se-77 NMR line shape data collected at 14.09 T are consistent with the symmetries imposed by the cubic P2(1)3 space group in the temperature range 20-290 K and confirm that the magnetic transition is not accompanied by any lowering of the crystal symmetry as has recently been proposed by Bos et al. [Phys. Rev. B 78 094416 (2008)].

Iop Publishing Ltd, Dirac House, Temple Back, Bristol Bs1 6Be, England2011

Synthesis, crystal growth and physical properties of the frustrated spin ladder material BiCu2PO6

Shuang Wang

In recent years quantum antiferromagnets with an intrinsically disordered (“spin liquid”) ground state and an energy gap in the spin excitation spectrum have received a great deal of attention. In search for new experimental realizations of spin-ladder and related models, we have embarked upon a new interesting family of low-dimensional materials with the general formula BiCu2AO6 (A = P, V and As). Among them, BiCu2PO6 shows a structure where Cu spins appear to form a two-leg zigzag ladder, which is believed to be a valuable model of a quantum magnet. However, for detailed studies of quantum magnetism, single crystals are indispensible. Therefore, in this thesis, synthesis and characterization of the newly-discovered spin ladder material BiCu2PO6 (BCPO) and its derivatives in the form of powder and single crystal are presented. Polycrystalline samples of Bi(Cu1-xZnx)2PO6 (x= 0.01, 0.02, 0.03, 0.04, 0.05 and 1), Bi1-y(Ca, Sr, La )yCu2PO6 (y = 0.15, 0.5 and 1) and BiCu2(P1-zVz)O6 (z = 1) were synthesized in this work. The solid state synthesis reactions, phase purity and thermal properties of the samples, substitution solubility, and preliminary magnetic susceptibility have been studied by thermal analysis (TG-DTA), X-ray powder diffraction (XRD), and Scanning Electron Microscope (SEM) as well as DC magnetization measurements, respectively. The BiCu2PO6 structure shows a remarkable chemical flexibility and interesting substitution effects. For detailed studies of the magnetic properties, well characterized large single crystals are indispensable. Thus in this thesis, a number of single crystal growth experiments have been performed using the Travelling Solvent Floating Zone method (TSFZ). Under the elaborated crystal growth conditions (growth rate of ~1 mm/h, gas mixture of 20% O2 in Argon, total 3 pressure (6~6.5 bar), high quality cm -size single crystals of Bi(Cu1-xZnx)2PO6 (x = 0, 0.01, 0.03, 0.05) have been successfully grown for the first time. For these crystals phase purity, crystal morphology/orientation, defect density, chemical composition distribution, crystal structure (room temperature and 1.5 K) and single crystal quality were thoroughly studied based on X-ray and neutron powder diffraction, optical and scanning electron microscopy, X-ray/neutron Laue diffraction, ICP-OES/X-ray fluorescence and two-axis neutron diffraction. Moreover, the specific heat Cp (H,T), AC susceptibility χ(H,T) and DC magnetization M(H,T) measurements on well-characterized single crystal samples of the undoped BCPO and 5% Zn-doped BCPO with the field applied along main crystallographic axes were performed in order to study the unique low temperature magnetic behaviour exhibited in this system, such as anisotropy of the magnetic properties, impurity-induced magnetism by the substitution of non-2+ magnetic cations (e.g. Zn ) and frustration. The magnetic phase diagram (H-T) under low field has been constructed. Further results obtained by close collaborators, based on the measurements made on single crystal samples grown in this thesis, including field-induced quantum phase transitions, NMR, Raman studies are attached directly at the end of this thesis.