-
Boundary shape engineering for the spatial control of confined active particles
Authors:
Roberto Di Leonardo,
András Búzás,
Lóránd Kelemen,
Dávid Tóth,
Szilvia Tóth,
Pál Ormos,
Gaszton Vizsnyiczai
Abstract:
Unlike an equilibrium gas, the distribution of active particles can be very sensitive to what happens at the boundaries of their container. Experiments and simulations have previously highlighted the possibility of exploiting this behavior for the geometric control of active particles, although a general theoretical framework is lacking. Here we propose a boundary method based on the flux transfer…
▽ More
Unlike an equilibrium gas, the distribution of active particles can be very sensitive to what happens at the boundaries of their container. Experiments and simulations have previously highlighted the possibility of exploiting this behavior for the geometric control of active particles, although a general theoretical framework is lacking. Here we propose a boundary method based on the flux transfer formalism typical of radiometry problems, where surface elements transmit and receive "rays" of active particles with infinite persistence length. As in the case of blackbody radiation, a Lambert scattering law results in a uniform distribution of active particles within the cavity, while other scattering laws result in specific patterns of particle accumulation in the bulk or over the boundary walls. We validate our method's predictions with numerical simulations and demonstrate its practical utility by spatially controlling swimming microalgae confined in light-defined arenas. The presented boundary method offers a simple and efficient way to predict particle distributions when both the geometry of the boundaries and the scattering law are known. In addition, it provides a general design principle for engineering container shapes optimized for transport, accumulation, and sorting of self-propelled colloids and microorganisms.
△ Less
Submitted 2 October, 2024;
originally announced October 2024.
-
Nature of the field-induced magnetic incommensurability in multiferroic Ni$_3$TeO$_6$
Authors:
J. Lass,
Ch. Røhl Andersen,
H. K. Leerberg,
S. Birkemose,
S. Toth,
U. Stuhr,
M. Bartkowiak,
Ch. Niedermayer,
Zhilun Lu,
R. Toft-Petersen,
M. Retuerto,
J. Okkels Birk,
K. Lefmann
Abstract:
Using single crystal neutron scattering we show that the magnetic structure Ni$_3$TeO$_6$ at fields above 8.6 T along the $c$ axis changes from a commensurate collinear antiferromagnetic structure with spins along c and ordering vector $Q_C$= (0 0 1.5), to a conical spiral with propagation vector $Q_{IC}$= (0 0 1.5$\pmδ$),$δ\sim$0.18, having a significant spin component in the ($a$,$b$) plane. We…
▽ More
Using single crystal neutron scattering we show that the magnetic structure Ni$_3$TeO$_6$ at fields above 8.6 T along the $c$ axis changes from a commensurate collinear antiferromagnetic structure with spins along c and ordering vector $Q_C$= (0 0 1.5), to a conical spiral with propagation vector $Q_{IC}$= (0 0 1.5$\pmδ$),$δ\sim$0.18, having a significant spin component in the ($a$,$b$) plane. We determine the phase diagram of this material in magnetic fields up to 10.5 T along $c$ and show the phase transition between the low field and conical spiral phases is of first order by observing a discontinuous jump of the ordering vector. $Q_{IC}$ is found to drift both as function of magnetic field and temperature. Preliminary inelastic neutron scattering reveals that the spin wave gap in zero field has minima exactly at $Q_{IC}$ and a gap of about 1.1 meV consisting with a cross-over around 8.6 T. Our findings excludes the possibility of the inverse Dzyaloshinskii-Moriya interaction as a cause for the giant magneto-electric coupling earlier observed in this material and advocates for the symmetric exchangestriction as the origin of this effect.
△ Less
Submitted 3 December, 2019; v1 submitted 30 September, 2019;
originally announced September 2019.
-
Liquid-phase exfoliated indium-selenide flakes and their application in hydrogen evolution reaction
Authors:
Elisa Petroni,
Emanuele Lago,
Sebastiano Bellani,
Danil W. Boukhvalov,
Antonio Politano,
Bekir Gurbulak,
Songul Duman,
Mirko Prato,
Silvia Gentiluomo,
Reinier Oropesa-Nunez,
Jaya-Kumar Panda,
Peter S. Toth,
Antonio Esau Del Rio Castillo,
Vittorio Pellegrini,
Francesco Bonaccorso
Abstract:
Single- and few-layered InSe flakes are produced by the liquid-phase exfoliation of beta-InSe single crystals in 2-propanol, obtaining stable dispersions with a concentration as high as 0.11 g/L. Ultracentrifugation is used to tune the morphology, i.e., the lateral size and thickness of the as-produced InSe flakes. We demonstrate that the obtained InSe flakes have maximum lateral sizes ranging fro…
▽ More
Single- and few-layered InSe flakes are produced by the liquid-phase exfoliation of beta-InSe single crystals in 2-propanol, obtaining stable dispersions with a concentration as high as 0.11 g/L. Ultracentrifugation is used to tune the morphology, i.e., the lateral size and thickness of the as-produced InSe flakes. We demonstrate that the obtained InSe flakes have maximum lateral sizes ranging from 30 nm to a few um, and thicknesses ranging from 1 to 20 nm, with a max population centred at ~ 5 nm, corresponding to 4 Se-In-In-Se quaternary layers. We also show that no formation of further InSe-based compounds (such as In2Se3) or oxides occurs during the exfoliation process. The potential of these exfoliated-InSe few-layer flakes as a catalyst for hydrogen evolution reaction (HER) is tested in hybrid single-walled carbon nanotubes/InSe heterostructures. We highlight the dependence of the InSe flakes morphologies, i.e., surface area and thickness, on the HER performances achieving best efficiencies with small flakes offering predominant edge effects. Our theoretical model unveils the origin of the catalytic efficiency of InSe flakes, and correlates the catalytic activity to the Se vacancies at the edge of the flakes.
△ Less
Submitted 21 March, 2019;
originally announced March 2019.
-
Structural, thermodynamic, and local probe investigations of a honeycomb material Ag$_{3}$LiMn$_{2}$O$_{6}$
Authors:
R. Kumar,
Tusharkanti Dey,
P. M. Ette,
K. Ramesha,
A. Chakraborty,
I. Dasgupta,
R. Eremina,
Sándor Tóth,
A. Shahee,
S. Kundu,
M. Prinz-Zwick,
A. A. Gippius,
H. A. Krug von Nidda,
N. Büttgen,
P. Gegenwart,
A. V. Mahajan
Abstract:
The system Ag[Li$_{1/3}$Mn$_{2/3}$]O$_{2}$ belongs to a quaternary 3R-delafossite family and crystallizes in a monoclinic symmetry with space group $C\,2/m$ and the magnetic Mn$^{4+}$($S=3/2$) ions form a honeycomb network in the $ab$-plane. An anomaly around 50 K and the presence of antiferromagnetic (AFM) coupling (Curie-Weiss temperature $θ_{CW}\sim-51$ K) were inferred from our magnetic suscep…
▽ More
The system Ag[Li$_{1/3}$Mn$_{2/3}$]O$_{2}$ belongs to a quaternary 3R-delafossite family and crystallizes in a monoclinic symmetry with space group $C\,2/m$ and the magnetic Mn$^{4+}$($S=3/2$) ions form a honeycomb network in the $ab$-plane. An anomaly around 50 K and the presence of antiferromagnetic (AFM) coupling (Curie-Weiss temperature $θ_{CW}\sim-51$ K) were inferred from our magnetic susceptibility data. The magnetic specific heat clearly manifests the onset of magnetic ordering in the vicinity of 48\,K and the recovered magnetic entropy, above the ordering temperature, falls short of the expected value, implying the presence of short-range magnetic correlations. The (ESR) line broadening on approaching the ordering temperature $T_{\rm N}$ could be described in terms of a Berezinski-Kosterlitz-Thouless (BKT) scenario with $T_{\rm KT}=40(1)$ K. $^{7}$Li NMR line-shift probed as a function of temperature tracks the static susceptibility (K$_{iso}$) of magnetically coupled Mn$^{4+}$ ions. The $^{7}$Li spin-lattice relaxation rate (1/$T$$_{1}$) exhibits a sharp decrease below about 50 K. Combining our bulk and local probe measurements, we establish the presence of an ordered ground state for the honeycomb system Ag$_{3}$LiMn$_{2}$O$_{6}$.Our ab-initio electronic structure calculations suggest that in the $ab$-plane, the nearest neighbor (NN) exchange interaction is strong and AFM, while the next NN and the third NN exchange interactions are FM and AFM respectively. In the absence of any frustration the system is expected to exhibit long-range, AFM order, in agreement with experiment.
△ Less
Submitted 13 April, 2019; v1 submitted 20 March, 2019;
originally announced March 2019.
-
Unconventional magnetism in the 4d$^{4}$ based ($S=1$) honeycomb system Ag$_{3}$LiRu$_{2}$O$_{6}$
Authors:
R. Kumar,
Tusharkanti Dey,
P. M. Ette,
K. Ramesha,
Atasi Chakraborty,
I. Dasgupta,
J. C. Orain,
C. Baines,
Sándor Tóth,
A. Shahee,
S. Kundu,
M. Prinz-Zwick,
A. A. Gippius,
N. Büttgen,
P. Gegenwart,
A. V. Mahajan
Abstract:
We have investigated the thermodynamic and local magnetic properties of the Mott insulating system Ag$_{3}$LiRu$_{2}$O$_{6}$ containing Ru$^{4+}$ (4$d$$^{4}$) for novel magnetism. The material crystallizes in a monoclinic $C2/m$ structure with RuO$_{6}$ octahedra forming an edge-shared two-dimensional honeycomb lattice with limited stacking order along the $c$-direction. The large negative Curie-W…
▽ More
We have investigated the thermodynamic and local magnetic properties of the Mott insulating system Ag$_{3}$LiRu$_{2}$O$_{6}$ containing Ru$^{4+}$ (4$d$$^{4}$) for novel magnetism. The material crystallizes in a monoclinic $C2/m$ structure with RuO$_{6}$ octahedra forming an edge-shared two-dimensional honeycomb lattice with limited stacking order along the $c$-direction. The large negative Curie-Weiss temperature ($θ_{CW}$ = -57 K) suggests antiferromagnetic interactions among Ru$^{4+}$ ions though magnetic susceptibility and heat capacity show no indication of magnetic long-range order down to 1.8 K and 0.4 K, respectively. $^{7}$Li nuclear magnetic resonance (NMR) shift follows the bulk susceptibility between 120-300 K and levels off below 120 K. Together with a power-law behavior in the temperature dependent spin-lattice relaxation rate between 0.2 and 2 K, it suggest dynamic spin correlations with gapless excitations. Electronic structure calculations suggest an $S = 1$ description of the Ru-moments and the possible importance of further neighbour interactions as also bi-quadratic and ring-exchange terms in determining the magnetic properties. Analysis of our $μ$SR data indicates spin freezing below 5 K but the spins remain on the borderline between static and dynamic magnetism even at 20 mK.
△ Less
Submitted 5 February, 2019;
originally announced February 2019.
-
Materials knowledge system for nonlinear composites
Authors:
Marat I. Latypov,
Laszlo S. Toth,
Surya R. Kalidindi
Abstract:
In this contribution, we present a new Materials Knowledge System framework for microstructure-sensitive predictions of effective stress--strain responses in composite materials. The model is developed for composites with a wide range of combinations of strain hardening laws and topologies of the constituents. The theoretical foundation of the model is inspired by statistical continuum theories, l…
▽ More
In this contribution, we present a new Materials Knowledge System framework for microstructure-sensitive predictions of effective stress--strain responses in composite materials. The model is developed for composites with a wide range of combinations of strain hardening laws and topologies of the constituents. The theoretical foundation of the model is inspired by statistical continuum theories, leveraged by mean-field approximation of self-consistent models, and calibrated to data obtained from micromechanical finite element simulations. The model also relies on newly formulated data-driven linkages between micromechanical responses (phase-average strain rates and effective strength) and microstructure as well as strength contrast of the constituents. The paper describes in detail the theoretical development of the model, its implementation into an efficient computational plasticity framework, calibration of the linkages, and demonstration of the model predictions on two-phase composites with isotropic constituents exhibiting linear and power-law strain hardening laws. It is shown that the model reproduces finite element results reasonably well with significant savings of the computational cost.
△ Less
Submitted 13 December, 2018; v1 submitted 20 September, 2018;
originally announced September 2018.
-
Solution blending preparation of polycarbonate/graphene composite: boosting the mechanical and electrical properties
Authors:
Emanuele Lago,
Peter. S. Toth,
Giammarino Pugliese,
Vittorio Pellegrini,
Francesco Bonaccorso
Abstract:
We report on the preparation of polycarbonate-based graphene (PC/G) composites, by using a simple and scalable solution blending method to disperse single (SLG) and few layer (FLG) graphene flakes, prepared by liquid phase exfoliation (LPE), in the polymer matrix. A solvent-exchange process is carried out to re-disperse the exfoliated SLG/FLG flakes in an environmentally friendly solvent, i.e. 1,3…
▽ More
We report on the preparation of polycarbonate-based graphene (PC/G) composites, by using a simple and scalable solution blending method to disperse single (SLG) and few layer (FLG) graphene flakes, prepared by liquid phase exfoliation (LPE), in the polymer matrix. A solvent-exchange process is carried out to re-disperse the exfoliated SLG/FLG flakes in an environmentally friendly solvent, i.e. 1,3dioxolane, which is also used to dissolve the polycarbonate pellets, thus facilitating the mixing of the polymer dispersion with the SLG/FLG flakes. The loading of SLG/FLG flakes improves the mechanical and thermal properties, as well as the electrical conductivity of the polymer, reaching a +26% improvement of the elastic modulus at 1 wt% loading, and an electrical conductivity 10^-3 S m^-1 at 10 wt% with a percolation threshold achieved at 0.55 vol%. The as-prepared PC/G composite with the aforementioned reinforced properties can be a promising material for 3D printing-based applications.
△ Less
Submitted 4 May, 2018;
originally announced May 2018.
-
Spin dynamics and exchange interactions in CuO measured by neutron scattering
Authors:
H. Jacobsen,
S. M. Gaw,
A. J. Princep,
E. Hamilton,
S. Tóth,
R. A. Ewings,
M. Enderle,
E. M. Hétroy Wheeler,
D. Prabhakaran,
A. T. Boothroyd
Abstract:
The magnetic properties of CuO encompass several contemporary themes in condensed matter physics, including quantum magnetism, magnetic frustration, magnetically-induced ferroelectricity and orbital currents. Here we report polarized and unpolarized neutron inelastic scattering measurements which provide a comprehensive map of the cooperative spin dynamics in the low temperature antiferromagnetic…
▽ More
The magnetic properties of CuO encompass several contemporary themes in condensed matter physics, including quantum magnetism, magnetic frustration, magnetically-induced ferroelectricity and orbital currents. Here we report polarized and unpolarized neutron inelastic scattering measurements which provide a comprehensive map of the cooperative spin dynamics in the low temperature antiferromagnetic (AFM) phase of CuO throughout much of the Brillouin zone. At high energies ($E \gtrsim 100$\,meV) the spectrum displays continuum features consistent with the des Cloizeax--Pearson dispersion for an ideal $S=\frac{1}{2}$ Heisenberg AFM chain. At lower energies the spectrum becomes more three-dimensional, and we find that a linear spin-wave model for a Heisenberg AFM provides a very good description of the data, allowing for an accurate determination of the relevant exchange constants in an effective spin Hamiltonian for CuO. In the high temperature helicoidal phase, there are features in the measured low-energy spectrum that we could not reproduce with a spin-only model. We discuss how these might be associated with the magnetically-induced multiferroic behavior observed in this phase.
△ Less
Submitted 5 April, 2018; v1 submitted 29 January, 2018;
originally announced January 2018.
-
The Final Chapter In The Saga Of YIG
Authors:
A. J. Princep,
R. A. Ewings,
S. Ward,
S. Tóth,
C. Dubs,
D. Prabhakaran,
A. T. Boothroyd
Abstract:
The magnetic insulator Yttrium Iron Garnet can be grown with exceptional quality, has a ferrimagnetic transition temperature of nearly 600 K, and is used in microwave and spintronic devices that can operate at room temperature. The most accurate prior measurements of the magnon spectrum date back nearly 40 years, but cover only 3 of the lowest energy modes out of 20 distinct magnon branches. Here…
▽ More
The magnetic insulator Yttrium Iron Garnet can be grown with exceptional quality, has a ferrimagnetic transition temperature of nearly 600 K, and is used in microwave and spintronic devices that can operate at room temperature. The most accurate prior measurements of the magnon spectrum date back nearly 40 years, but cover only 3 of the lowest energy modes out of 20 distinct magnon branches. Here we have used time-of-flight inelastic neutron scattering to measure the full magnon spectrum throughout the Brillouin zone. We find that the existing model of the excitation spectrum, well known from an earlier work titled "The Saga of YIG", fails to describe the optical magnon modes. Using a very general spin Hamiltonian, we show that the magnetic interactions are both longer-ranged and more complex than was previously understood. The results provide the basis for accurate microscopic models of the finite temperature magnetic properties of Yttrium Iron Garnet, necessary for next-generation electronic devices.
△ Less
Submitted 18 May, 2017;
originally announced May 2017.
-
Strong exchange anisotropy in YbMgGaO$_4$ from polarized neutron diffraction
Authors:
Sándor Tóth,
Katharina Rolfs,
Andrew R. Wildes,
Christian Rüegg
Abstract:
We measured the magnetic correlations in the triangular lattice spin-liquid candidate material YbMgGaO$_4$ via polarized neutron diffraction. The extracted in-plane and out-of-plane components of the magnetic structure factor show clear anisotropy. We found that short-range correlations persist at the lowest measured temperature of 52 mK and neutron scattering intensity is centered at the $M$ midd…
▽ More
We measured the magnetic correlations in the triangular lattice spin-liquid candidate material YbMgGaO$_4$ via polarized neutron diffraction. The extracted in-plane and out-of-plane components of the magnetic structure factor show clear anisotropy. We found that short-range correlations persist at the lowest measured temperature of 52 mK and neutron scattering intensity is centered at the $M$ middle-point of the hexagonal Brillouin-zone edge. Moreover, we found pronounced spin anisotropy, with different correlation lengths for the in-plane and out-of-plane spin components. When comparing to a self-consistent Gaussian appoximation, our data clearly support a model with only first-neighbor coupling and strongly anisotropic exchanges.
△ Less
Submitted 16 May, 2017;
originally announced May 2017.
-
Crystal growth, structure and magnetic properties of Ca10Cr7O28
Authors:
Christian Balz,
Bella Lake,
Manfred Reehuis,
A. T. M. Nazmul Islam,
Yogesh Singh,
Oleksandr Prokhnenko,
Philip Pattison,
Sandor Toth
Abstract:
A detailed diffraction study of Ca10Cr7O28 is presented which adds significant new insights into the structural and magnetic properties of this compound. A new crystal structure type was used where the a and b axes are doubled compared to previous models providing a more plausible structure where all crystallographic sites are fully occupied. The presence of two different valences of chromium was…
▽ More
A detailed diffraction study of Ca10Cr7O28 is presented which adds significant new insights into the structural and magnetic properties of this compound. A new crystal structure type was used where the a and b axes are doubled compared to previous models providing a more plausible structure where all crystallographic sites are fully occupied. The presence of two different valences of chromium was verified and the locations of the magnetic Cr5+ and non-magnetic Cr6+ ions were identified. The Cr5+ ions have spin-1/2 and form distorted kagome bilayers which are stacked in an ABC arrangement along the c axis. These results lay the foundation for understanding of the quantum spin liquid behavior in Ca10Cr7O28 which has recently been reported in [C. Balz et al., Nature Physics, 12, 942 (2016)].
△ Less
Submitted 16 December, 2016;
originally announced December 2016.
-
Phase segregation facilitates exfoliation of franckeite crystals to a single unit cell thickness
Authors:
Matěj Velický,
Peter S. Toth,
Alexander M. Rakowski,
Aidan P. Rooney,
Aleksey Kozikov,
Artem Mishchenko,
Colin R. Woods,
Thanasis Georgiou,
Sarah J. Haigh,
Kostya S. Novoselov,
Robert A. W. Dryfe
Abstract:
Weak interlayer van der Waals interactions in bulk crystals facilitate their mechanical exfoliation to monolayer and few-layer two-dimensional (2D) materials, which exhibit striking physical phenomena absent in their bulk form. Here we study a 2D form of a mineral franckeite and show that phase segregation into discrete layers at the sub-nanometre scale facilitates its layered structure and basal…
▽ More
Weak interlayer van der Waals interactions in bulk crystals facilitate their mechanical exfoliation to monolayer and few-layer two-dimensional (2D) materials, which exhibit striking physical phenomena absent in their bulk form. Here we study a 2D form of a mineral franckeite and show that phase segregation into discrete layers at the sub-nanometre scale facilitates its layered structure and basal cleavage. This behaviour is likely to be common in a wider family of complex crystals and could be exploited for a single-step synthesis of van der Waals heterostructures, as an alternative to stacking of 2D materials. Mechanical exfoliation allowed us to produce crystals down to a single unit cell thickness and rationalise its basal cleavage by atomic-resolution scanning transmission electron microscopy (STEM). We demonstrate p-type electrical conductivity and remarkable electrochemical properties in exfoliated crystals, which shows promise for energy storage applications.
△ Less
Submitted 8 December, 2016;
originally announced December 2016.
-
Electromagnon dispersion probed by inelastic x-ray scattering
Authors:
Sándor Tóth,
Björn Wehinger,
Katharina Rolfs,
Turan Birol,
Uwe Stuhr,
Hiroshi Takatsu,
Kenta Kimura,
Tsuyoshi Kimura,
Henrik M. Rønnow,
Christian Rüegg
Abstract:
Inelastic x-ray scattering with meV energy resolution (IXS) is an ideal tool to measure collective excitations in solids and liquids. In non-resonant scattering condition, the cross section is strongly dominated by lattice vibrations (phonons). However, it is possible to probe additional degrees of freedom such as magnetic fluctuations that are strongly coupled to the phonons. The IXS spectrum of…
▽ More
Inelastic x-ray scattering with meV energy resolution (IXS) is an ideal tool to measure collective excitations in solids and liquids. In non-resonant scattering condition, the cross section is strongly dominated by lattice vibrations (phonons). However, it is possible to probe additional degrees of freedom such as magnetic fluctuations that are strongly coupled to the phonons. The IXS spectrum of the coupled system will contain not only the phonon dispersion (majority component) but also the so far undetected magnetic correlation function (minority component). Here we report the discovery of strong magnon-phonon coupling in LiCrO$_2$ that enables the measurement of magnetic correlations throughout the Brillouin-zone via IXS. We found electromagnon excitations and electric dipole active two-magnon excitations in the magnetically ordered phase and paraelectromagnons in the paramagnetic phase of LiCrO$_2$. We predict that the numerous group of (frustrated) magnets with dominant direct exchange and non-collinear magnetism shows similarly strong coupling and surprisingly large and measurable IXS cross section for magnons and multi-magnon processes.
△ Less
Submitted 23 June, 2016;
originally announced June 2016.
-
Frustrated Ising chains on the triangular lattice in Sr$_3$NiIrO$_6$
Authors:
S. Toth,
W. Wu,
D. T. Adroja,
S. Rayaprol,
E. V. Sampathkumaran
Abstract:
Inelastic neutron scattering study on the spin-chain compound Sr$_3$NiIrO$_6$ reveals gapped quasi-1D magnetic excitations. The observed one-magnon band between 29.5 and 39 meV consists of magnon modes of the Ni$^{2+}$ ions. The fitting of the spin wave spectrum reveals strongly coupled Ising-like chains along the $c$-axis that are weakly coupled into a frustrated triangular lattice in the $ab$-pl…
▽ More
Inelastic neutron scattering study on the spin-chain compound Sr$_3$NiIrO$_6$ reveals gapped quasi-1D magnetic excitations. The observed one-magnon band between 29.5 and 39 meV consists of magnon modes of the Ni$^{2+}$ ions. The fitting of the spin wave spectrum reveals strongly coupled Ising-like chains along the $c$-axis that are weakly coupled into a frustrated triangular lattice in the $ab$-plane. The magnetic excitations survive up to 200 K well above the magnetic ordering temperature of $T_N \sim 75$ K, also indicating a quasi-1D nature of the magnetic interactions in Sr$_3$NiIrO$_6$. Our microscopic model is in good agreement with ab initio electronic structure calculations and explains the giant spin flip field observed in bulk magnetization measurements.
△ Less
Submitted 22 April, 2016;
originally announced April 2016.
-
Jahn-Teller versus quantum effects in the spin-orbital material LuVO3
Authors:
M. Skoulatos,
S. Toth,
B. Roessli,
M. Enderle,
K. Habicht,
D. Sheptyakov,
A. Cervellino,
P. G. Freeman,
M. Reehuis,
A. Stunault,
G. J. McIntyre,
L. D. Tung,
C. Marjerrison,
E. Pomjakushina,
P. J. Brown,
D. I. Khomskii,
Ch. Rueegg,
A. Kreyssig,
A. I. Goldman,
J. P. Goff
Abstract:
We report on combined neutron and resonant x-ray scattering results, identifying the nature of the spin-orbital ground state and magnetic excitations in LuVO3 as driven by the orbital parameter. In particular, we distinguish between models based on orbital Peierls dimerization, taken as a signature of quantum effects in orbitals, and Jahn-Teller distortions, in favor of the latter. In order to sol…
▽ More
We report on combined neutron and resonant x-ray scattering results, identifying the nature of the spin-orbital ground state and magnetic excitations in LuVO3 as driven by the orbital parameter. In particular, we distinguish between models based on orbital Peierls dimerization, taken as a signature of quantum effects in orbitals, and Jahn-Teller distortions, in favor of the latter. In order to solve this long-standing puzzle, polarized neutron beams were employed as a prerequisite in order to solve details of the magnetic structure, which allowed quantitative intensity-analysis of extended magnetic excitation data sets. The results of this detailed study enabled us to draw definite conclusions about classical vs quantum behavior of orbitals in this system and to discard the previous claims about quantum effects dominating the orbital physics of LuVO3 and similar systems.
△ Less
Submitted 20 May, 2015; v1 submitted 24 March, 2015;
originally announced March 2015.
-
Spin-Orbit-Induced Orbital Excitations in Sr2RuO4 and Ca2RuO4: A Resonant Inelastic X-ray Scattering Study
Authors:
C. G. Fatuzzo,
M. Dantz,
S. Fatale,
P. Olalde-Velasco,
N. E. Shaik,
B. Dalla Piazza,
S. Toth,
J. Pelliciari,
R. Fittipaldi,
A. Vecchione,
N. Kikugawa,
J. S. Brooks,
H. M. Rønnow,
M. Grioni,
Ch. Rüegg,
T. Schmitt,
J. Chang
Abstract:
High-resolution resonant inelastic X-ray scattering (RIXS) at the oxygen K-edge has been used to study the orbital excitations of Ca2RuO4 and Sr2RuO4. In combination with linear dichroism X-ray absorption spectroscopy, the ruthenium 4d-orbital occupation and excitations were probed through their hybridization with the oxygen p-orbitals. These results are described within a minimal model, taking in…
▽ More
High-resolution resonant inelastic X-ray scattering (RIXS) at the oxygen K-edge has been used to study the orbital excitations of Ca2RuO4 and Sr2RuO4. In combination with linear dichroism X-ray absorption spectroscopy, the ruthenium 4d-orbital occupation and excitations were probed through their hybridization with the oxygen p-orbitals. These results are described within a minimal model, taking into account crystal field splitting and a spin-orbit coupling λ_{so}=200~meV. The effects of spin-orbit interaction on the electronic structure and implications for the Mott and superconducting ground states of (Ca,Sr)2RuO4 are discussed.
△ Less
Submitted 7 April, 2015; v1 submitted 5 March, 2015;
originally announced March 2015.
-
Giant gap in the magnon excitations of the quasi-1D chain compound Sr3NiIrO6
Authors:
W. Wu,
D. T. Adroja,
S. Toth,
S. Rayaprol,
E. V. Sampathkumaran
Abstract:
Inelastic neutron scattering on the spin-chain compound Sr$_3$NiIrO$_6$ reveals gapped quasi-1D magnetic excitations. The observed one-magnon band between 29.5 and 39 meV consists of two dispersive modes. The spin wave spectrum could be well fitted with an antiferromagnetic anisotropic exchange model and single ion anisotropy on the Ni site. The extracted dominant anisotropic antiferromagnetic int…
▽ More
Inelastic neutron scattering on the spin-chain compound Sr$_3$NiIrO$_6$ reveals gapped quasi-1D magnetic excitations. The observed one-magnon band between 29.5 and 39 meV consists of two dispersive modes. The spin wave spectrum could be well fitted with an antiferromagnetic anisotropic exchange model and single ion anisotropy on the Ni site. The extracted dominant anisotropic antiferromagnetic intra-chain exchange interaction between Ir and Ni ions are $J_z=19.5$ meV and $J_{xy}=12.1$ meV. These values justify previous electronic structure calculations, showing the importance of Ir spin orbit coupling on the electron correlations. The magnetic excitations survive up to 200 K well above the magnetic ordering temperature of $T_N \sim 70$ K, also indicating a quasi-1D nature of the magnetic interactions in Sr$_3$NiIrO$_6$. Our results not only support the idea of the existence of a new temperature scale well above $T_N$, but also emphasize the need to consider new exchange paths complicated by the SOC, resulting in additional characteristic temperatures in such spin-chain systems.
△ Less
Submitted 23 January, 2015;
originally announced January 2015.
-
Nonconventional magnetic order in frustrated diamond lattice antiferromagnet CoAl2O4 studied by neutron diffraction and classical Monte-Carlo simulation
Authors:
O. Zaharko,
S. Tóth,
O. Sendetskyi,
A. Cervellino,
A. Wolter-Giraud,
T. Dey,
A. Maljuk,
V. Tsurkan
Abstract:
CoAl2O4 spinel with magnetic Co2+ ions on the diamond A-lattice is known to be magnetically frustrated. We compare neutron single crystal diffraction patterns measured in zero and applied magnetic fields with the ones obtained from classical Monte-Carlo models. In simulations we test the influence of various parameters on diffraction patterns: the ratio of nearest-, J1, and next-nearest, J2, neigh…
▽ More
CoAl2O4 spinel with magnetic Co2+ ions on the diamond A-lattice is known to be magnetically frustrated. We compare neutron single crystal diffraction patterns measured in zero and applied magnetic fields with the ones obtained from classical Monte-Carlo models. In simulations we test the influence of various parameters on diffraction patterns: the ratio of nearest-, J1, and next-nearest, J2, neighbor interactions, magnetic field applied along the principal crystallographic directions, and random disorder on the A(Co2+)- and B(Al3+)- sites. We conclude that the models considered so far explain the broadening of magnetic Bragg peaks in zero magnetic field and their anisotropic response to applied magnetic field only partly. As bulk properties of our single crystal are isotropic, we suggest that its microstructure, specifically <111>-twin boundaries, could be a reason of the nonconventional magnetic order in CoAl2O4.
△ Less
Submitted 5 October, 2014;
originally announced October 2014.
-
Linear spin wave theory for single-Q incommensurate magnetic structures
Authors:
S. Toth,
B. Lake
Abstract:
Linear spin wave theory provides the leading term in the calculation of the excitation spectra of long-range ordered magnetic systems as a function of $1/\sqrt{S}$. This term is acquired using the Holstein-Primakoff approximation of the spin operator and valid for small $δS$ fluctuations of the ordered moment. We propose an algorithm that allows magnetic ground states with general moment direction…
▽ More
Linear spin wave theory provides the leading term in the calculation of the excitation spectra of long-range ordered magnetic systems as a function of $1/\sqrt{S}$. This term is acquired using the Holstein-Primakoff approximation of the spin operator and valid for small $δS$ fluctuations of the ordered moment. We propose an algorithm that allows magnetic ground states with general moment directions and single-Q incommensurate ordering wave vector using a local coordinate transformation for every spin and a rotating coordinate transformation for the incommensurability. Finally we show, how our model can determine the spin wave spectrum of the magnetic C-site langasites with incommensurate order.
△ Less
Submitted 23 February, 2015; v1 submitted 25 February, 2014;
originally announced February 2014.
-
Exciton-magnon transitions in the frustrated chromium antiferromagnets CuCrO2, alpha-CaCr2O4, CdCr2O4, and ZnCr2O4
Authors:
M. Schmidt,
Zhe Wang,
Ch. Kant,
F. Mayr,
S. Toth,
A. T. M. N. Islam,
B. Lake,
V. Tsurkan,
A. Loidl,
J. Deisenhofer
Abstract:
We report on optical transmission spectroscopy of the Cr-based frustrated triangular antiferromagnets CuCrO2 and alpha-CaCr2O4, and the spinels CdCr2O4 and ZnCr2O4 in the near-infrared to visible-light frequency range. We explore the possibility to search for spin correlations far above the magnetic ordering temperature and for anomalies in the magnon lifetime in the magnetically ordered state by…
▽ More
We report on optical transmission spectroscopy of the Cr-based frustrated triangular antiferromagnets CuCrO2 and alpha-CaCr2O4, and the spinels CdCr2O4 and ZnCr2O4 in the near-infrared to visible-light frequency range. We explore the possibility to search for spin correlations far above the magnetic ordering temperature and for anomalies in the magnon lifetime in the magnetically ordered state by probing exciton-magnon sidebands of the spin-forbidden crystal-field transitions of the Cr3+ ions (spin S = 3/2). In CuCrO2 and alpha-CaCr2O4 the appearance of fine structures below T_N is assigned to magnon sidebands by comparison with neutron scattering results. The temperature dependence of the line width of the most intense sidebands in both compounds can be described by an Arrhenius law. For CuCrO2 the sideband associated with the 4A2 -> 2T2 transition can be observed even above T_N. Its line width does not show a kink at the magnetic ordering temperature and can alternatively be described by a Z2 vortex scenario proposed previously for similar materials. The exciton-magnon features in alpha-CaCr2O4 are more complex due to the orthorhombic distortion. While for CdCr2O4 magnon sidebands are identified below T_N and one sideband excitation is found to persist across the magnetic ordering transition, only a weak fine structure related to magnetic ordering has been observed in ZnCr2O4.
△ Less
Submitted 17 June, 2013; v1 submitted 15 October, 2012;
originally announced October 2012.
-
Magnetic soft modes in the locally distorted triangular antiferromagnet alpha-CaCr2O4
Authors:
S. Toth,
B. Lake,
K Hradil,
T. Guidi,
K. C. Rule,
M. B. Stone,
A. T. M. N. Islam
Abstract:
In this paper we explore the phase diagram and excitations of a distorted triangular lattice antiferromagnet. The unique two-dimensional distortion considered here is very different from the 'isosceles'-type distortion that has been extensively investigated. We show that it is able to stabilize a 120° spin structure for a large range of exchange interaction values, while new structures are found f…
▽ More
In this paper we explore the phase diagram and excitations of a distorted triangular lattice antiferromagnet. The unique two-dimensional distortion considered here is very different from the 'isosceles'-type distortion that has been extensively investigated. We show that it is able to stabilize a 120° spin structure for a large range of exchange interaction values, while new structures are found for extreme distortions. A physical realization of this model is α-CaCr2O4 which has 120° structure but lies very close to the phase boundary. This is verified by inelastic neutron scattering which reveals unusual roton-like minima at reciprocal space points different from those corresponding to the magnetic order.
△ Less
Submitted 8 June, 2012;
originally announced June 2012.
-
Softened magnetic excitations in the s = 3/2 distorted triangular antiferromagnet alpha-CaCr2O4
Authors:
Dirk Wulferding,
Kwang-Yong Choi,
Peter Lemmens,
Alexey N. Ponomaryov,
Johan van Tol,
A. T. M. Nazmul Islam,
Sandor Toth,
Bella Lake
Abstract:
The spin dynamics and magnetic excitations of the slightly distorted triangular s = 3/2 system alpha-CaCr2O4 are investigated by means of Raman spectroscopy and electron spin resonance (ESR) to elucidate its peculiar magnetic properties. Two-magnon excitations in circular RL symmetry show a multi-maximum structure with a dominant spectral weight at low energies. The temperature dependence of the E…
▽ More
The spin dynamics and magnetic excitations of the slightly distorted triangular s = 3/2 system alpha-CaCr2O4 are investigated by means of Raman spectroscopy and electron spin resonance (ESR) to elucidate its peculiar magnetic properties. Two-magnon excitations in circular RL symmetry show a multi-maximum structure with a dominant spectral weight at low energies. The temperature dependence of the ESR linewidth is described by a critical broadening DeltaHpp(T) ~ (T - T_N)^{-p} with the exponent p = 0.30(3) - 0.38(5) for temperatures above T_N = 42.6 K. The exponent is much smaller than that of other s = 3/2 triangular lattices. This is ascribed to soft roton-like modes, indicative of the instability of a helical 120° phase. As an origin we discuss a complex spin topology formed by four inequivalent nearest neighbor and sizable next-nearest neighbor interactions.
△ Less
Submitted 10 October, 2012; v1 submitted 16 May, 2012;
originally announced May 2012.
-
Modeling of Deformation and Texture Development of Copper in a 120° ECAE Die
Authors:
Roxane Arruffat Massion,
L. S. Toth,
Jean-Philippe Mathieu
Abstract:
A flow line function is proposed to describe the material deformation in ECAE for a 120\degree die. This new analytical approach is incorporated into a viscoplastic self-consistent polycrystal code to simulate the texture evolution in Route A of copper and compared to experimental textures as well as to those corresponding to simple shear.
A flow line function is proposed to describe the material deformation in ECAE for a 120\degree die. This new analytical approach is incorporated into a viscoplastic self-consistent polycrystal code to simulate the texture evolution in Route A of copper and compared to experimental textures as well as to those corresponding to simple shear.
△ Less
Submitted 12 May, 2012;
originally announced May 2012.
-
120° Helical Magnetic Order in the Distorted Triangular Antiferromagnet alpha-CaCr2O4
Authors:
S. Tóth,
B. Lake,
S. A. J. Kimber,
O. Pieper,
M. Reehuis,
A. T. M. N. Islam,
O. Zaharko,
C. Ritter,
A. H. Hill,
H. Ryll,
D. N. Argyriou,
A. J. Williams
Abstract:
α-CaCr2O4 is a distorted triangular antiferromagnet. The magnetic Cr3+ ions which have spin-3/2 and interact with their nearest neighbors via Heisenberg direct exchange interactions, develop long-range magnetic order below T_N=42.6 K. Powder and single-crystal neutron diffraction reveal a helical magnetic structure with ordering wavevector k=(0,~1/3,0) and angles close to 120° between neighboring…
▽ More
α-CaCr2O4 is a distorted triangular antiferromagnet. The magnetic Cr3+ ions which have spin-3/2 and interact with their nearest neighbors via Heisenberg direct exchange interactions, develop long-range magnetic order below T_N=42.6 K. Powder and single-crystal neutron diffraction reveal a helical magnetic structure with ordering wavevector k=(0,~1/3,0) and angles close to 120° between neighboring spins. Spherical neutron polarimetry unambiguously proves that the spins lie in the ac plane perpendicular to k. The magnetic structure is therefore that expected for an ideal triangular antiferromagnet where all nearest neighbor interactions are equal, in spite of the fact that α-CaCr2O4 is distorted with two inequivalent Cr3+ ions and four different nearest neighbor interactions. By simulating the magnetic order as a function of these four interactions it is found that the special pattern of interactions in α-CaCr2O4 stabilizes 120° helical order for a large range of exchange interactions.
△ Less
Submitted 4 May, 2011; v1 submitted 14 February, 2011;
originally announced February 2011.
-
Enhanced thermal stability and spin-lattice relaxation rate of N@C60 inside carbon nanotubes
Authors:
S. Toth,
D. Quintavalle,
B. Nafradi,
L. Korecz,
L. Forro,
F. Simon
Abstract:
We studied the temperature stability of the endohedral fullerene molecule, N@C60, inside single-wall carbon nanotubes using electron spin resonance spectroscopy. We found that the nitrogen escapes at higher temperatures in the encapsulated material as compared to its pristine, crystalline form. The temperature dependent spin-lattice relaxation time, T_1, of the encapsulated molecule is significa…
▽ More
We studied the temperature stability of the endohedral fullerene molecule, N@C60, inside single-wall carbon nanotubes using electron spin resonance spectroscopy. We found that the nitrogen escapes at higher temperatures in the encapsulated material as compared to its pristine, crystalline form. The temperature dependent spin-lattice relaxation time, T_1, of the encapsulated molecule is significantly shorter than that of the crystalline material, which is explained by the interaction of the nitrogen spin with the conduction electrons of the nanotubes.
△ Less
Submitted 30 January, 2008;
originally announced January 2008.
-
Charge-Generated Changes in Graphite Structures
Authors:
I. Pocsik,
M. Veres,
M. Fule,
S. Toth,
M. Koos
Abstract:
It will be shown that a negatively charged carbon atom in a graphite sheet has an electron structure that is unusual in carbons, viz. the lone pair of electrons. This atom, similarly to the positively charged ones, is no longer able to participate in the (pi) bond network: both pop out from the sheet because of their elongated single bonds and tetrahedral structure. The lone-pair structure local…
▽ More
It will be shown that a negatively charged carbon atom in a graphite sheet has an electron structure that is unusual in carbons, viz. the lone pair of electrons. This atom, similarly to the positively charged ones, is no longer able to participate in the (pi) bond network: both pop out from the sheet because of their elongated single bonds and tetrahedral structure. The lone-pair structure localizes the extra electron. Forbidden gaps open above and below the Fermi level as a consequence of the destroyed (pi) bonds. This model offers an explanation for many anomalous experimental data.
△ Less
Submitted 18 July, 2003;
originally announced July 2003.