SrCu2(BO3)(2) under pressure: A first-principles study

Using density-functional theory (DFT) band-structure calculations, we study the crystal structure, lattice dynamics, and magnetic interactions in the Shastry-Sutherland magnet SrCu2(BO3)(2) under pressure, concentrating on experimentally relevant pressures up to 4 GPa. We first check that a ferromagnetic spin alignment shortens the nearest-neighbor Cu-Cu distance and reduces the Cu-O-Cu angle compared to the state with the antiferromagnetic spin alignment in the dimers, in qualitative agreement with the structural changes observed at ambient pressure as a function of temperature and applied field. Next, we determine the optimal crystal structures corresponding to the magnetic structures consistent with, respectively, the dimer phase realized at ambient pressure, the Neel ordered phase realized at high pressure, and two candidates for the intermediate phase with two types of dimers and different stackings. For each phase, we performed a systematic study as a function of pressure, and we determined the exchange interactions and the frequencies of several experimentally relevant phonon modes. In the dimer phase, the ratio of the inter- to intradimer couplings is found to increase with pressure, in qualitative agreement with various experiments. This increase is mostly due to the decrease of the intradimer coupling due to the reduction of the Cu-O-Cu angle under pressure. The phonon frequency of the pantograph mode is also found to increase with pressure, in qualitative agreement with recent Raman experiments. In the Neel phase, the frequency of the pantograph mode is larger than the extrapolated value from the dimer phase, again in agreement with the experimental results, and accordingly the intradimer coupling is smaller than the extrapolated value from the dimer phase. Finally, all tendencies inside the candidate intermediate phases are thoroughly worked out, including specific predictions for some Raman active phonon modes that could be used to pin down the nature of the intermediate phase.

Synthesis and Physical Properties of 3d, 4d and 5d Transition Metal Compounds

Lin Yang

In this thesis I study the synthesis and basic physical properties characterization on 3

d

, 4

d

and 5

d

transition metal compounds. Great success has been obtained in 3

d

transition metal compounds, in which the electric states are well localized due to the large on-site Coulomb repulsion

U

. Most stoichiometric 3

d

transition metal oxides are antiferromagnetic Mott insulators. Among them, low dimensional geometrically frustrated systems, such as

S

= 1/2 Kagome lattice antiferromagnets, are at the forefront of condensed matter research. Recently, high-quality single crystals of

\mathrm{Cu_2OSO_4}

, which are spin-1/2 antiferromagnets with low dimensional magnetism, have been successfully synthesized. The measurements of specific heat, susceptibility and magnetization were performed on this material. We found that the Kagome-like compound

\mathrm{Cu_2OSO_4}

shows typical signatures for a canted-AFM ground state with a weak ferromagnetic component. On the other hand,

4d

transition metal compounds were considered as weakly correlated systems because the electron correlation is expected to be weaker in

4d

transition metal compounds compared with the

3d

ones. The

4d

ones naturally bridge two different regimes of the strongly correlated

3d

compounds and the

5d

compounds. Most notably, for instance, it is intriguing that seemingly similar

\mathrm{Ca_2RuO_4}

and

\mathrm{Sr_2RuO_4}

display totally different behavior: the former is a Mott insulator while the latter is metallic and becomes superconducting at low temperature. Here we report the synthesis of large single crystals of

\mathrm{MoPO_5}

, and present their magnetic and thermodynamic properties. We found that the

4d^1

compound

\mathrm{MoPO_5}

is orbitally quenched and orders into an antiferromagnet with the moments along

c

axis. Spin-flop transition is observed which indicates magnetic anisotropy.

5d

orbitals are more extended and the Coulomb repulsion

U

values are expected to be further reduced compared with those of

3d

and

4d

transition metal compounds. Thus, insulating behaviors in

5d

transition metal compounds have been puzzling. A possible reason is the strong spin-orbit coupling. Here we show the ambient-pressure synthesis and physical properties of a new all-Ir

^{6+}

iridate

\mathrm{Ba_8Al_2IrO_{14}}

and a novel layered iridate

\mathrm{Ba_{21}Ir_9O_{43}}

. The synthesis, crystal structure, transport, and magnetic properties of them have been reported.

\mathrm{Ba_8Al_2IrO_{14}}

is a

p-

type band insulator and shows antiferromagnetic couplings but display no order down to 2 K.

\mathrm{Ba_{21}Ir_9O_{43}}

is an insulator with antiferromagnetic Curie-Weiss behavior, where a magnetic transition is suppressed down to low temperature of 9 K despite the large Curie-Weiss temperature of

-90

K. We also performed the pressure-dependent resistivity measurements of the

5d

compound

\mathrm{Ir_{0.95}Pt_{0.05}Te_{2}}

and found that the charge order with

q

=(1/5,0,1/5) dimer configuration is introduced and the superconductivity undergoes a dimensionality cross-over from 3 dimension to 2 dimension under pressure.

EPFL2017

Crystal Structure and Magnetic Properties of Two New Antiferromagnetic Spin Dimer Compounds; FeTe3O7X (X = Cl, Br)

Helmuth Berger, Jia Liu, Dong Zhang

Two new isostructural layered oxohalides FeTe3O7X (X = Cl, Br) were synthesized by chemical vapor transport reactions, and their crystal structures and magnetic properties were characterized by single-crystal X-ray diffraction, Raman spectroscopy, magnetic susceptibility and magnetization measurements, and also by density functional theory (DFT) calculations of the electronic structure and the spin exchange parameters. FeTe3O7X crystallizes in the monoclinic space group P2(1)/c with the unit cell parameters a = 10.7938(5), b = 7.3586(4), c = 10.8714(6) angstrom, beta = 111.041(5)degrees, Z = 4 for FeTe3O7CI, and a = 11.0339(10), b = 7.3643(10), c = 10.8892(10) angstrom, beta = 109.598(10)degrees, Z = 4 for FeTe3O7Br. Each compound has one unique Fe3+ ion coordinating a distorted [FeO5] trigonal bipyramid. Two such groups share edges to form [Fe2O8] dimers that are isolated from each other by Te4+ ions. The high-temperature magnetic properties of the compounds as well as spectroscopic investigations are consistent with an isolated antiferromagnetic spin dimer model with almost similar spin gaps of similar to 35 K for X = Cl and Br, respectively. However, deviations at low temperatures in the magnetic susceptibility and the magnetization data indicate that the dimers couple via an interdimer coupling. This interpretation is also supported by DFT calculations which indicate an interdimer exchange which amounts to 25% and 10% of the intradimer exchange for X = Cl and Br, respectively. The magnetic properties support the counterion character and a weak integration of halide ions into the covalent network similar to that in many other oxohalides.

2011

Two-dimensional square-lattice S=1/2 antiferromagnet Cu(pz)(2)(ClO4)(2)

Henrik Moodysson Rønnow, Nikolay Tsyrulin

We present an experimental study of the two-dimensional S=1/2 square-lattice antiferromagnet Cu(pz)(2)(ClO4)(2) (pz denotes pyrazine-C4H4N2) using specific-heat measurements, neutron diffraction, and cold-neutron spectroscopy. The magnetic field dependence of the magnetic ordering temperature was determined from specific-heat measurements for fields perpendicular and parallel to the square-lattice planes, showing identical field-temperature phase diagrams. This suggest that spin anisotropies in Cu(pz)(2)(ClO4)(2) are small. The ordered antiferromagnetic structure is a collinear arrangement with the magnetic moments along either the crystallographic b or c axis. The estimated ordered magnetic moment at zero field is m(0)=0.47 (5)mu(B) and thus much smaller than the available single-ion magnetic moment. This is evidence for strong quantum fluctuations in the ordered magnetic phase of Cu(pz)(2)(ClO4)(2). Magnetic fields applied perpendicular to the square-lattice planes lead to an increase in the antiferromagnetically ordered moment to m(0)=0.93 (5)mu(B) at mu H-0= 13.5 T evidence that magnetic fields quench quantum fluctuations. Neutron spectroscopy reveals the presence of a gapped spin excitations at the antiferromagnetic zone center and it can be explained with a slightly anisotropic nearest-neighbor exchange coupling described by J(xy)(1)= 1.563 (13) meV and J(z)(1) = 0.9979(2)J(1)(xy).

2010

Synthesis and Physical Properties of 3d, 4d and 5d Transition Metal Compounds

Lin Yang

In this thesis I study the synthesis and basic physical properties characterization on 3

d

, 4

d

and 5

d

transition metal compounds. Great success has been obtained in 3

d

transition metal compounds, in which the electric states are well localized due to the large on-site Coulomb repulsion

U

. Most stoichiometric 3

d

transition metal oxides are antiferromagnetic Mott insulators. Among them, low dimensional geometrically frustrated systems, such as

S

= 1/2 Kagome lattice antiferromagnets, are at the forefront of condensed matter research. Recently, high-quality single crystals of

\mathrm{Cu_2OSO_4}

\mathrm{Cu_2OSO_4}

shows typical signatures for a canted-AFM ground state with a weak ferromagnetic component. On the other hand,

4d

transition metal compounds were considered as weakly correlated systems because the electron correlation is expected to be weaker in

4d

transition metal compounds compared with the

3d

ones. The

4d

ones naturally bridge two different regimes of the strongly correlated

3d

compounds and the

5d

compounds. Most notably, for instance, it is intriguing that seemingly similar

\mathrm{Ca_2RuO_4}

and

\mathrm{Sr_2RuO_4}

\mathrm{MoPO_5}

, and present their magnetic and thermodynamic properties. We found that the

4d^1

compound

\mathrm{MoPO_5}

is orbitally quenched and orders into an antiferromagnet with the moments along

c

axis. Spin-flop transition is observed which indicates magnetic anisotropy.

5d

orbitals are more extended and the Coulomb repulsion

U

values are expected to be further reduced compared with those of

3d

and

4d

transition metal compounds. Thus, insulating behaviors in

5d

transition metal compounds have been puzzling. A possible reason is the strong spin-orbit coupling. Here we show the ambient-pressure synthesis and physical properties of a new all-Ir

^{6+}

iridate

\mathrm{Ba_8Al_2IrO_{14}}

and a novel layered iridate

\mathrm{Ba_{21}Ir_9O_{43}}

. The synthesis, crystal structure, transport, and magnetic properties of them have been reported.

\mathrm{Ba_8Al_2IrO_{14}}

is a

p-

type band insulator and shows antiferromagnetic couplings but display no order down to 2 K.

\mathrm{Ba_{21}Ir_9O_{43}}

is an insulator with antiferromagnetic Curie-Weiss behavior, where a magnetic transition is suppressed down to low temperature of 9 K despite the large Curie-Weiss temperature of

-90

K. We also performed the pressure-dependent resistivity measurements of the

5d

compound

\mathrm{Ir_{0.95}Pt_{0.05}Te_{2}}

and found that the charge order with

q

=(1/5,0,1/5) dimer configuration is introduced and the superconductivity undergoes a dimensionality cross-over from 3 dimension to 2 dimension under pressure.

EPFL2017