Possible spin-orbit driven spin-liquid ground state in the double perovskite phase of Ba

We report the structural transformation of hexagonal Ba3YIr 2O9 to a cubic double perovskite form (stable in ambient conditions) under an applied pressure of 8 GPa at 1273 K. While the ambient pressure synthesized sample undergoes long-range magnetic ordering at ∼4 K, the high-pressure (HP) synthesized sample does not order down to 2 K as evidenced from our susceptibility, heat capacity, and nuclear magnetic resonance (NMR) measurements. Further, for the HP sample, our heat capacity data have the form γT+βT3 in the temperature (T) range of 2-10 K with the Sommerfeld coefficient γ=10 mJ/mol-Ir K2. The 89Y NMR shift has no T dependence in the range of 4-120 K and its spin-lattice relaxation rate varies linearly with T in the range of 8-45 K (above which it is T independent). Resistance measurements of both the samples confirm that they are semiconducting. Our data provide evidence for the formation of a 5d-based, gapless, quantum spin-liquid in the cubic (HP) phase of Ba3YIr 2O9. In this picture, the γT term in the heat capacity and the linear variation of 89Y 1/T1 arises from excitations out of a spinon Fermi surface. Our findings lend credence to the theoretical suggestion that strong spin-orbit coupling can enhance quantum fluctuations and lead to a QSL state in the double perovskite lattice. © 2013 American Physical Society.

Possible spin-orbit driven spin-liquid ground state in the double perovskite phase of Ba3YIr2O9

Arash Alahgholipour Omrani, Henrik Moodysson Rønnow

We report the structural transformation of hexagonal Ba3YIr2O9 to a cubic double perovskite form (stable in ambient conditions) under an applied pressure of 8 GPa at 1273 K. While the ambient pressure synthesized sample undergoes long-range magnetic ordering at similar to 4 K, the high-pressure (HP) synthesized sample does not order down to 2 K as evidenced from our susceptibility, heat capacity, and nuclear magnetic resonance (NMR) measurements. Further, for the HP sample, our heat capacity data have the form gamma T + beta T-3 in the temperature (T) range of 2-10 K with the Sommerfeld coefficient gamma = 10 mJ/mol-Ir K-2. The Y-89 NMR shift has no T dependence in the range of 4-120 K and its spin-lattice relaxation rate varies linearly with T in the range of 8-45 K (above which it is T independent). Resistance measurements of both the samples confirm that they are semiconducting. Our data provide evidence for the formation of a 5d-based, gapless, quantum spin-liquid in the cubic (HP) phase of Ba3YIr2O9. In this picture, the gamma T term in the heat capacity and the linear variation of Y-89 1/T-1 arises from excitations out of a spinon Fermi surface. Our findings lend credence to the theoretical suggestion [Chen, Pereira, and Balents, Phys. Rev. B 82, 174440 (2010)] that strong spin-orbit coupling can enhance quantum fluctuations and lead to a QSL state in the double perovskite lattice.

Amer Physical Soc2013

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

J1−J2 square-lattice Heisenberg antiferromagnets with 4d1 spins: AMoOPO4Cl (A=K,Rb)

Peter Babkevich, Henrik Moodysson Rønnow

Magnetic properties of AMoOPO4Cl (A=K,Rb) with Mo5+ ions in the 4d1 electronic configuration are investigated by magnetization, heat capacity, and nuclear magnetic resonance (NMR) measurements on single crystals, combined with powder neutron diffraction experiments. The magnetization measurements reveal that they are good model compounds for the spin-1/2 J1−J2 square-lattice magnet with the first and second nearest-neighbor interactions. Magnetic transitions are observed at around 6 and 8 K in the K and Rb compounds, respectively. In contrast to the normal Néel-type antiferromagnetic order, the NMR and neutron diffraction experiments find a columnar antiferromagnetic order for each compound, which is stabilized by a dominant antiferromagnetic J2. Both compounds realize the unusual case of two interpenetrating J2 square lattices weakly coupled to each other by J1.

2017

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