Intrachain antiferromagnetic exchange in a 1D branched-chain built of two different copper(II) centres interlinked by end-on azido and phenoxo bridges: electron density map, electrochemical and magnetic properties

A 1D, end-on azido- and phenoxo-bridged, coordination polymer, Cu-2(mu-NAPPR)(mu(1,1)-N-3)(2) (1), (NAPPR(2-) = 1,3-bis(naphthylideneimino) propane dianion), was synthesized and characterized. The 1D branched-chain is built of two types of slightly different, square-pyramidal, Cu(II) ions with different sets of donor atoms. Temperature-and field-dependent magnetic analyses reveal an antiferromagnetic interaction between copper centres in the main chain (Cu-2) mediated by EO-azide bridges along the chain. The interaction between the copper centres Cu1 (in the branches of the main chain) and Cu-2 is very weak and is included by means of an own Weiss constant theta'. The electron density map of 1 from synchrotron data shows that the d(x2-y2) orbitals of Cu-2 are not well described by the procrystal model, a difference which might be meaningful for the magnetic properties. Cyclic voltammetry studies show two different redox waves with a large difference between the redox potentials of the two types of copper centres (1.21 V). The resulting data are consistent with the crystal structure of this complex determined by single crystal X-ray diffraction.

Static and dynamic properties of non-collinear quantum antiferromagnets explored by neutron scattering

Luc Testa

This thesis is devoted to the investigation of static and dynamic properties of two different sets of quantum magnets with neutron scattering techniques and the help of linear spin wave theory. Both systems are copper-based with spin-1/2, which makes them ideal to study the interplay between purely quantum and semi-classical effects. I start with the analysis of the antiferromagnet SeCuO3, which has a canted spins structure. Through careful inelastic neutron scattering experiments on thermal and cold triple-axis spectrometers, I demonstrate that this compound exhibits three primary types of excitations that are intrinsically opposite : spin waves (magnons), singlet to triplet excitations (triplons), and fractional spins excitations (spinons). Such a strong coexistence and interdependence of these collective excitations has not been observed yet, thereby the quantification and description of the excitations in SeCuO3 leads the way to further theoretical work on multi-excitation spin systems, as well as the existence of quantum effects in high dimensional systems.\ My second project is on the extraction of the magnetic structure of three members of the A(BO)Cu4(PO4)4 chiral family, namely (A; B) = (Ba; Ti), (Sr; Ti) and (Pb; Ti), from spherical neutron polarimetry measurements. I prove that the first two compounds exhibit a highly non-collinear magnetic structure, with the Cu spins forming clusters of 'two-in--two-out' arrangements on each structural unit. This structure is stabilised by the presence of a strong Dzyaloshinskii-Moriya interaction, and explains the observation of magnetoelectric effects as emerging from quadrupole moments. The analysis of the latter compound did not lead to the confirmation of its magnetic structure due to strong nuclear-magnetic interference. I conclude this thesis by the investigation of the magnetic excitation spectrum of some members of the (A; B) family, probed by inelastic neutron scattering measurements. Indeed, its particular crystallographic structure makes it an ideal playground to study tetramerisation effects on the two dimensional square lattice. Additionally, the aforementioned Dzyaloshinskii-Moriya interaction ensures the presence of a structural gap, which competes with the quantum one emerging from tetramerisation effects. Using linear spin wave theory, I describe (Ba; Ti) as a chequerboard system with almost equal intra- and inter-plaquette couplings, with weak quantum effects. I also provide a qualitative description of (Pb; Ti), which exhibits similar physics, and conclude by presenting the first results on the highly symmetric compound (K; Nb), which shows hints of a strong quantum behaviour.

EPFL2021

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

Frustration and Randomness

Péter Szirmai

Spin systems with strong magnetic interactions might remain disordered avoiding conventional magnetic long-range ordering due to zero-point quantum fluctuations, and supporting a Quantum Spin Liquid (QSL) state. Long-range quantum entanglement in QSLs promotes the emergence of non-local excitations and topological properties, unveiling unexplored quantum phenomena. The immense fundamental interest in this exotic phase of matter evinces the need for new crystalline designs and novel combinations of experimental methods in the study of QSLs. The present PhD thesis focuses on two organic low-dimensional charge-transfer salt families that might host Quantum Spin Liquid states due to geometrical frustration and designedly introduced randomness. Using the combination of multi-frequency high-field Electron Spin Resonance (ESR) and Muon Spin Rotation (

\mu

SR) techniques supplemented by

^1

H Nuclear Magnetic Resonance and transport studies, I propose that organic frustrated magnetic systems are severely affected by the anion layers and by the weak antisymmetric exchange, the so-called Dzyaloshinskii-Moriya (DM) interaction. Amongst crystalline hybrid conductor-rotor structures comprising Brownian rotator anion layers, I survey experimentally four different materials illustrating a variety of magnetic ground states depending on their dimensionalities and interactions with the rotor components. In quasi-one-dimensional weakly coupled spin chain systems, the presence of the controlled disorder originating from the slowing-down of rotators creates a spin-gapped or long-range antiferromagnetically ordered ground state depending on the spatial relation of the anion and the spin-bearing cation layers. Remarkably, inhomogeneous distribution of the disorder of the rotor stopping positions induces a broad distribution of the N'{e}el temperatures at the microscopic length scales. In contrast, two quasi-one-dimensional distorted triangular lattice systems are identified as QSL candidates. In particular, low-temperature zero-field

\mu

SR and high-field ESR measurements in (EDT-TTF-CONH

_2

)

_2^{+}

[BABCO

^{-}

] (EDT-BCO) confirmed the absence of magnetic ordering down to 20 mK despite large,

J/k_{\text{B}}=365

K nearest-neighbor antiferromagnetic exchange interactions and a moderate anisotropy of

t'/t=1.6

of the interdimer transfer integrals. The linear field dependence of the ESR linewidth, i.e., the spectral density of the fluctuations, manifests fast spin excitations, reminiscent of motional narrowing. Despite a sizable DM interaction of 0.8 T suggested theoretically, the ESR lineshape remains unchanged down to 1.5 K, indicating the suppression of the effect of DM interaction. Longitudinal-field

\mu

SR measurements reveal one-dimensional diffusive spin transport in EDT-BCO, and predominant fluctuations at the staggered component corresponding to the wave vector of

q=\pi/a

. For comparison, I investigate the antisymmetric exchange in a nearly isotropic triangular lattice organic QSL candidate,

\kappa

-(BEDT-TTF)

_2

_2

(CN)

_3

(

\kappa

-Ag,

J/k_{\text{B}}=175

t'/t=0.97

). The recently discovered sister compound of the first two-dimensional QSL (

\kappa

-(BEDT-TTF)

_2

_2

(CN)

_3

) displays a spin liquid-superconductor phase transition above a critical pressure of

p_{\text{c}}>0.9

GPa (

T_{\text{c}}=5.2

K). In

\kappa

-Ag, multi-frequency and angle-dependent ambient-pressure ESR studies found large antiferromagnetic fluctuations, and a staggered moment of

\mu_{\text{s}}=6\cdot 10^{-3}

\mu_{\text{B}}

at 2 K and at 15 T. Via globally fitting the field- and temperature-dependence of the ESR linewidth using the one-dimensional field theory of Oshikawa and Affleck, I assigned

\mu_{\text{s}}

to a DM interaction of 1.2 T, weakly suppressed compared to its theoretical value of 5.5 T. Nevertheless, high-pressure multi-frequency ESR measurements reveal that the effect of the DM interaction is completely quenched with a moderate pressure of

p_{\text{DM}}=

0.3 GPa, as a result of moving away from the spiral-ordered phase. Above

p_{\text{DM}}

\mu

SR) techniques supplemented by

^1

H Nuclear Magnetic Resonance and transport studies, I propose that organic frustrated magnetic systems are severely affected by the anion layers and by the weak antisymmetric exchange, the so-called Dzyaloshinskii-Moriya (DM) interaction. Amongst crystalline hybrid conductor-rotor structures comprising Brownian rotator anion layers, I survey experimentally four different materials illustrating a variety of magnetic ground states depending on their dimensionalities and interactions with the rotor components. In quasi-one-dimensional weakly coupled spin chain systems, the presence of the controlled disorder originating from the slowing-down of rotators creates a spin-gapped or long-range antiferromagnetically ordered ground state depending on the spatial relation of the anion and the spin-bearing cation layers. Remarkably, inhomogeneous distribution of the disorder of the rotor stopping positions induces a broad distribution of the Néel temperatures at the microscopic length scales. In contrast, two quasi-one-dimensional distorted triangular lattice systems are identified as QSL candidates. In particular, low-temperature zero-field

\mu

SR and high-field ESR measurements in (EDT-TTF-CONH

_2

)

_2^{+}

[BABCO

^{-}

] (EDT-BCO) confirmed the absence of magnetic ordering down to 20 mK despite large,

J/k_{\text{B}}=365

K nearest-neighbor antiferromagnetic exchange interactions and a moderate anisotropy of

t'/t=1.6

\mu

SR measurements reveal one-dimensional diffusive spin transport in EDT-BCO, and predominant fluctuations at the staggered component corresponding to the wave vector of

q=\pi/a

. For comparison, I investigate the antisymmetric exchange in a nearly isotropic triangular lattice organic QSL candidate,

\kappa

-(BEDT-TTF)

_2

_2

(CN)

_3

(

\kappa

-Ag,

J/k_{\text{B}}=175

t'/t=0.97

). The recently discovered sister compound of the first two-dimensional QSL (

\kappa

-(BEDT-TTF)

_2

_2

(CN)

_3

) displays a spin liquid-superconductor phase transition above a critical pressure of

p_{\text{c}}>0.9

GPa (

T_{\text{c}}=5.2

K). In

\kappa

-Ag, multi-frequency and angle-dependent ambient-pressure ESR studies found large antiferromagnetic fluctuations, and a staggered moment of

\mu_{\text{s}}=6\cdot 10^{-3}

\mu_{\text{B}}

at 2 K and at 15 T. Via globally fitting the field- and temperature-dependence of the ESR linewidth using the one-dimensional field theory of Oshikawa and Affleck, I assigned

\mu_{\text{s}}

p_{\text{DM}}=

0.3 GPa, as a result of moving away from the spiral-ordered phase. Above

p_{\text{DM}}

, a linear field dependence and fast spin fluctuations are found, similarly to the ambient-pressure EDT-BCO. Furthermore, detailed analysis of the high-pressure ESR linewidth gives evidence of the effect of high pressure upon inherent charge fluctuations.

EPFL2018

Frustration and Randomness

Péter Szirmai

\mu

SR) techniques supplemented by

^1

\mu

SR and high-field ESR measurements in (EDT-TTF-CONH

_2

)

_2^{+}

[BABCO

^{-}

] (EDT-BCO) confirmed the absence of magnetic ordering down to 20 mK despite large,

J/k_{\text{B}}=365

K nearest-neighbor antiferromagnetic exchange interactions and a moderate anisotropy of

t'/t=1.6

\mu

SR measurements reveal one-dimensional diffusive spin transport in EDT-BCO, and predominant fluctuations at the staggered component corresponding to the wave vector of

q=\pi/a

. For comparison, I investigate the antisymmetric exchange in a nearly isotropic triangular lattice organic QSL candidate,

\kappa

-(BEDT-TTF)

_2

_2

(CN)

_3

(

\kappa

-Ag,

J/k_{\text{B}}=175

t'/t=0.97

). The recently discovered sister compound of the first two-dimensional QSL (

\kappa

-(BEDT-TTF)

_2

_2

(CN)

_3

) displays a spin liquid-superconductor phase transition above a critical pressure of

p_{\text{c}}>0.9

GPa (

T_{\text{c}}=5.2

K). In

\kappa

-Ag, multi-frequency and angle-dependent ambient-pressure ESR studies found large antiferromagnetic fluctuations, and a staggered moment of

\mu_{\text{s}}=6\cdot 10^{-3}

\mu_{\text{B}}

at 2 K and at 15 T. Via globally fitting the field- and temperature-dependence of the ESR linewidth using the one-dimensional field theory of Oshikawa and Affleck, I assigned

\mu_{\text{s}}

p_{\text{DM}}=

0.3 GPa, as a result of moving away from the spiral-ordered phase. Above

p_{\text{DM}}

\mu

SR) techniques supplemented by

^1

H Nuclear Magnetic Resonance and transport studies, I propose that organic frustrated magnetic systems are severely affected by the anion layers and by the weak antisymmetric exchange, the so-called Dzyaloshinskii-Moriya (DM) interaction. Amongst crystalline hybrid conductor-rotor structures comprising Brownian rotator anion layers, I survey experimentally four different materials illustrating a variety of magnetic ground states depending on their dimensionalities and interactions with the rotor components. In quasi-one-dimensional weakly coupled spin chain systems, the presence of the controlled disorder originating from the slowing-down of rotators creates a spin-gapped or long-range antiferromagnetically ordered ground state depending on the spatial relation of the anion and the spin-bearing cation layers. Remarkably, inhomogeneous distribution of the disorder of the rotor stopping positions induces a broad distribution of the Néel temperatures at the microscopic length scales. In contrast, two quasi-one-dimensional distorted triangular lattice systems are identified as QSL candidates. In particular, low-temperature zero-field

\mu

SR and high-field ESR measurements in (EDT-TTF-CONH

_2

)

_2^{+}

[BABCO

^{-}

] (EDT-BCO) confirmed the absence of magnetic ordering down to 20 mK despite large,

J/k_{\text{B}}=365

K nearest-neighbor antiferromagnetic exchange interactions and a moderate anisotropy of

t'/t=1.6

\mu

SR measurements reveal one-dimensional diffusive spin transport in EDT-BCO, and predominant fluctuations at the staggered component corresponding to the wave vector of

q=\pi/a

. For comparison, I investigate the antisymmetric exchange in a nearly isotropic triangular lattice organic QSL candidate,

\kappa

-(BEDT-TTF)

_2

_2

(CN)

_3

(

\kappa

-Ag,

J/k_{\text{B}}=175

t'/t=0.97

). The recently discovered sister compound of the first two-dimensional QSL (

\kappa

-(BEDT-TTF)

_2

_2

(CN)

_3

) displays a spin liquid-superconductor phase transition above a critical pressure of

p_{\text{c}}>0.9

GPa (

T_{\text{c}}=5.2

K). In

\kappa

-Ag, multi-frequency and angle-dependent ambient-pressure ESR studies found large antiferromagnetic fluctuations, and a staggered moment of

\mu_{\text{s}}=6\cdot 10^{-3}

\mu_{\text{B}}

at 2 K and at 15 T. Via globally fitting the field- and temperature-dependence of the ESR linewidth using the one-dimensional field theory of Oshikawa and Affleck, I assigned

\mu_{\text{s}}

p_{\text{DM}}=

0.3 GPa, as a result of moving away from the spiral-ordered phase. Above

p_{\text{DM}}

EPFL2018

Static and dynamic properties of non-collinear quantum antiferromagnets explored by neutron scattering

Luc Testa

EPFL2021

Thermopower as a Unique Tool to Probe Quantum Phase Transitions

Stevan Arsenijevic

EPFL2012