Fast and accurate algorithm for the simulation of NMR spectra of large spin systems

The computational cost for the simulation of NMR spectra grows exponentially with the number of nuclei. Today, the memory available to store the Hamiltonian limits the size of the system that can be studied. Modern computers enable to tackle systems containing up to 13 spins [1], which obviously does not allow to study most molecules of interest in research. This issue can be addressed by identifying groups of spins or fragments that are not or only weakly interacting together, i.e., that only share weakly coupled spin pairs. Such a fragmentation is only permitted in the weak coupling regime, i.e., when the coupling interaction is weak compared to the difference in chemical shift of the coupled spins. Here, we propose a procedure that removes weak coupling interactions in order to split the spin system efficiently and to correct a posteriori for the effect of the neglected couplings. This approach yields accurate spectra when the adequate interactions are removed, i.e., between spins only involved in weak coupling interactions, but fails otherwise. As a result, the computational time for the simulation of 1D spectra grows linearly with the size of the spin system. (C) 2010 Elsevier Inc. All rights reserved.

Probing the interaction of nanoparticles with small molecules in real time via quartz crystal microbalance monitoring

Stefan Guldin, Nikolaos Nianias, Runzhang Qi, Ye Yang

Despite extensive advances in the field of molecular recognition, the real-time monitoring of small molecule binding to nanoparticles (NP) remains a challenge. To this end, we report on a versatile approach, based on quartz crystal microbalance with dissipation monitoring, for the stepwise in situ quantification of gold nanoparticle (AuNPs) immobilisation and subsequent uptake and release of binding partners. AuNPs stabilised by thiol-bound ligand shells of prescribed chemical composition were densely immobilised onto gold surfaces via dithiol linkers. The boronate ester formation between salicylic acid derivatives in solution and boronic acids in the AuNP ligand shell was then studied in real time, revealing a drastic effect of both ligand architecture and Lewis base concentration on the interaction strength. The binding kinetics were analysed with frequency response modelling for a thorough comparison of binding parameters including relaxation time as well as association rate constant. The results directly mirror those from previously reported in-depth studies using nuclear magnetic resonance spectroscopy. By achieving quantitative characterisation of selective binding of analytes with molecular weight below 300 Da, this new method enables rapid, low cost, rational screening of AuNP candidates for molecular recognition.

ROYAL SOC CHEMISTRY2019

Emergence and Detection of Exotic Phases in Spin-1 Systems

Frédéric Alexis Michaud

This work is devoted to the study of spin S = 1 systems, and more precisely to the emergence of exotic quantum phases in such systems, and to the establishment of tools to observe such phases. It is split in four main chapters. In the first chapter, we show how spin S = 1 systems can emerge from microscopic models, and which kinds of interaction might appear in the effective spin model. We start from a two-orbital Hubbard model, and by a strong coupling development to fourth order, we derive an effective model. We will see that three types of interaction appear beyond the Heisenberg interaction : a plaquette interaction, a biquadratic interaction and a three-spin interaction. In the second chapter, we study Raman scattering on systems with quadrupolar order to show that it can be used to probe such order. We first start by deriving an effective light scattering operator following Shastry and Shraiman calculation on spin S = 1/2 systems. Using this effective operator, we compute the Raman spectra with exact diagonalization and linear flavor-wave theory. We show that two different regimes appear depending on the incoming photon energy, and that combining this to different polarizations accessible with Raman scattering, the presence of quadrupolar order can be established with this probe. The third chapter is devoted to the study of the three-spin interaction that appeared in the first chapter on a chain. We start by establishing the classical and the mean field phase diagram of this system. We then turn to the quantum case. We show that, whatever the value of the spin is, the ground state is perfectly dimerized for a particular value of the three-spin interaction. The presence of such a point in the phase diagram implies the existence of a quantum phase transition when increasing the three-spin interaction. By an intensive numerical study, we show that this transition is continuous, and that its critical behavior is the one of a SU(2)k=2S Wess-Zumino-Witten model, at least for spins S = 1/2,1,3/2,2. In the last section of this chapter, we study the phase diagram of the chain for spin S = 1 under a magnetic field. We conclude this work with a study of the three-spin interaction on a square lattice. The classical and mean field phase diagram are established. It is shown that for a large three-spin interaction, the classical ground state is highly degenerate. This degeneracy is lifted in the quantum case by a process of order by disorder. We compute the quantum fluctuations with linear spin-wave theory, and show that some phases are selected over others. We confirm these results by an exact diagonalization study of the system.

EPFL2013

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

Emergence and Detection of Exotic Phases in Spin-1 Systems

Frédéric Alexis Michaud

EPFL2013

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