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

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.

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

Etude par microscopie électronique de particules nanostructurées d'oxalate de cobalt précipitées en milieu aqueux

The understanding of material self-assembling and self-organization mechanisms, at mesoscale, is crucial for nanotechnologies development. Such structures, hierarchically organized in superlattices or colloïdal crystals, are observed in inorganic or organo-metallic precipitates. The classical characterization, using electron microscopy and X-ray diffraction, of a synthesized powder is inadequate to describe the hierarchically organized polycrystalline structure of the final particles. Here, we have highlighted the successive steps that lead to the spontaneous formation of a cobalt oxalate dihydrate (COD) colloïdal crystals that precipitate in aqueous media. The characterization of the final particles, made by X-ray and electronic diffraction, has allowed us to determine the controversial crystalline phase of the precipitated COD. The β and γ COD phases has been discriminated by comparing experimental results with simulations of X-ray diffractogram and electron diffraction patterns for these both structures. The precipitated COD corresponds to the γ phase indicating that it comes from solid dehydration of cobalt oxalate tetrahydrate. The final powder characterization, performed by X-ray diffraction (XRD), atomic force microscopy (AFM), low voltage high resolution scanning electron microscopy (LVHRSEM) and transmission electron microscopy (TEM), gives some diverging results about monocrystalline or polycrystalline nature of the precipitates. XRD and AFM analysis show a polycrystalline structure of particles whereas LVHRSEM, TEM and electron diffraction observations indicate a monocrystalline structure. This apparent contradiction has led us to elaborate a new characterization method allowing to follow the growth and the structural evolution of the precipitate as a function of time. The cryo- preparation techniques are well adapted as they allow freezing and direct observation of solid state suspension by electron microscopy. The freeze/drying method has been modified and used for both SEM and TEM, and has allowed us to study the morphological and structural evolution of the COD precipitate from nanoscale to mesoscale. In addition, TEM analysis of samples prepared by classical techniques has been performed to complement cryo-electron microscopy observations. The combination of these two methods show that COD precipitation is a complex multi-step process leading to the formation of a core/shell heterostructure. The anisotropic core is porous and partially crystalline. It is formed by agglomeration of isolated primary particles and agregated ones (15-20 nm sized secondary particles). The crystalline shell corresponds to the final particles faces and is made up by the layer by layer self-assembling and alignment of 5-7 nm sized crystalline primary particles. These nanoparticles are aligned and perfectly ordered in strings of nanograins in the layer structure. The self-assembling occurs in the last step of growth where we have a lower ionic strength and supersaturation inducing slower kinetics of growth and aggregation. Moreover, the crystalline order in the primary and secondary particles increase with the time of reaction consistent with a continuous process of primary particle nucleation and growth. Our results show that self-assembling occurs layer by layer with terraces, kinks and steps that are present on the faces. This reminds the layer by layer crystalline growth models but in this case, the buiding blocks are colloïdal instead of atomic or molecular. Based on these different results, we propose an original model describing the COD precipitate growth. In addition, we have studied the poly(methacrylic acid-sodium salt) effect on the COD precipitation. This additive (PMMA) appeared to be a very good growth inhibitor. The PMMA effect on the final particles morphology is dramatic. Its concentration variation in solution slows down the nanoparticles aggregation rates along a [101] preferential direction. The crystal habit can then be controlled and we have synthesized platelets, cubes and rods with tailored length. The use of cryogenic electron microscopy has been shown to be a powerful tool for the understanding of time dependent aspects of particle growth. The use of such techniques for the study of other systems, where the final precipitate appearance may hide particle substructures, will help to elucidate growth mechanisms and allow finer control of nanostructured materials for many applications. The complex self-assembling and self-organization processes could then be highlighted and studied on a nanometer to micrometer scale.

EPFL2004

Thermopower as a Unique Tool to Probe Quantum Phase Transitions

Stevan Arsenijevic

In my thesis, transport measurements such as resistivity and, more importantly, thermopower S, were used to explore the phase diagram of bad metals. Bad metals are electronically correlated systems whose ground state lies close to a quantum phase transition. By tuning the control parameters, such as temperature (T ), magnetic field (B), hydrostatic pressure (p) or chemical substitution (x), we can induce phase transitions between the various electronic, magnetic and structural phases. Here, the thermopower is presented as a unique tool for probing quantum phase transition because it is a measure of the entropy of conducting electrons. The main part of the thesis is dedicated to the study of Fe-based superconductors (FeSC) discovered in 2008. Their parent compound has an antiferromagnetic (AF) ground state, where the itinerant electrons form a spin-density wave (SDW), a periodic modulation of spin density. This coincides or is preceded by a structural, tetragonal-to-orthorhombic transition. The nesting between the electron and hole Fermi surface is believed to be the driving mechanism for the SDW state. By changing the structural or chemical properties the AF ground state of FeSC is suppressed, giving way to superconductivity (SC). The remaining antiferromagnetic fluctuations above the transition can provide a glue for SC pairing. Here, the analysis of the thermopower S/T of BaFe1−xCoxAs2 (BFCA) in the x-T phase diagram shows the signatures of the spin fluctuation which have a dome-like dependence and follow the trend of superconducting Tc . The logarithmic increase of S/T upon decreasing T is ascribed to the proximity of the spin-density-wave quantum critical point. It can be understood as an increase of entropy due to the incommensurate AF spin fluctuations. We can ascribe the high values of thermopower in BFCA at intermediate- and room-temperatures to the influence of low-T quantum criticality. To probe the response of the electronic system in FeSC to structural changes, we performed measurements under pressure of the parent compound BaFe2As2 (BFA), the SC electron-doped BFCA, and hole-doped Ba1−xKxFe2As2 (BKFA). In the parent compound pressure suppresses the structural/SDW transition, similar to the effect of doping. For doped systems, in order to describe the behavior of thermopower in the high-T range (above 100K) we used a semi-metallic two-band model which was fitted to the data in order to extract the pressure dependence of the band parameters. In both doping cases the effect of pressure was similar, an increase of the band overlap and of the effective number of charge carriers. With this model we can explain the high-T , x and p dependence of thermopower in both electron- and hole-doped BFA. In a structurally simpler Fe-chalcogenide Fe1+yTe1−xSex compound, the excess of Fe has a Kondo-like influence on the charge carriers which dramatically changes the physics of the normal state. To probe the normal state, pressure, doping, magnetic Fe-excess concentration (y) and temperature were used as control parameters. At low-T a characteristic upturn of resistivity (ρmag ) is observed, followed by an increase of thermopower (Smag ), which we identify as the magnetic contribution caused by the spin-flip scattering events. Increasing the y resulted in an increase of ρmag , and a decrease of Smag , which is in agreement with the behavior of canonical Kondo-systems. Pressure suppresses the magnetic contribution to transport, thus increasing the itinerancy of the system. MnSi is another system in which the sensitivity of thermopower to entropy brings new information related to the complex magnetic structure. Pressure was used to drive the system from a helically ordered, canonical Fermi-liquid (FL) phase with T 2-resistivity to the intrinsically disordered, non-Fermi-liquid (NFL) phase above pc with T3/2-dependence. Our contour plot of S/T demonstrated how powerful the thermopower technique is, by reproducing the whole previously established T -p phase diagram. At the phase transition from the magnetically-ordered FL phase to the disordered NFL, the thermopower is dramatically enhanced. We bring useful information about the mysterious partial order (PO) phase inside the NFL phase, previously detected only by neutron scattering. The fluctuating helices scenario can describe the observed increase of entropy/thermopower in the PO phase. At ambient pressure, close to the helical transition of MnSi, a moderate magnetic field can stabilize the skyrmion lattice - the lattice of topological magnetic whirls, vortices. We observe a signature of the skyrmion lattice as a minute drop in thermopower. It is located exactly in the same region of the T − B phase diagram where an increase in magnetoresistance and Hall effect was reported previously. This feature originates from the additional scattering of conducting electrons on magnetic vortices, while the change in S is dominated by the decrease of entropy as the stable skyrmion lattice is formed. Overall, resistivity was used to confirm the established phase diagram, while thermopower, as an interesting and not sufficiently understood technique, was used to probe the sensitive changes of the charge carriers at the Fermi surface. We explored various phases showing how useful thermopower is to probe the entropy of electronic system on the verge of quantum phase transition.

EPFL2012

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

Etude par microscopie électronique de particules nanostructurées d'oxalate de cobalt précipitées en milieu aqueux

EPFL2004

Thermopower as a Unique Tool to Probe Quantum Phase Transitions

Stevan Arsenijevic

EPFL2012