jaroslav chovan - Academia.edu (original) (raw)
Papers by jaroslav chovan
An effective field theory is derived that describes the low-frequency spin dynamics in the low-te... more An effective field theory is derived that describes the low-frequency spin dynamics in the low-temperature orthorombic phase of La2CuO4. Restricted to a single CuO2 layer the effective theory is a simple generalization of the relativistic nonlinear σ model to include all spin interactions allowed by symmetry. Incorporating a weak interlayer interaction leads to two coupled nonlinear σ models which provide an efficient description of the complete bilayer dynamics. Particular attention is paid to the weak-ferromagnetic and spin-flop transitions induced by external magnetic fields. The main features of the observed (covert) weak ferromagnetism are thus accounted for in a straightforward manner but some of the finer theoretical predictions would require further experimental investigation. The derived framework is also suitable for the study of the structure and dynamics of magnetic domains in undoped La2CuO4.
arXiv: Strongly Correlated Electrons, 2019
Low-temperature magnetic resonance study of the quasi-two-dimensional antiferromagnet Cu(en)(H$_2... more Low-temperature magnetic resonance study of the quasi-two-dimensional antiferromagnet Cu(en)(H$_2$O)$_2$SO$_4$ (en = C$_2$H$_8$N$_2$) was performed down to 0.45 K. This compound orders antiferromagnetically at 0.9 K. The analysis of the resonance data within the hydrodynamic approach allowed to identify anisotropy axes and to estimate the anisotropy parameters for the antiferromagnetic phase. Dipolar spin-spin coupling turns out to be the main contribution to the anisotropy of the antiferromagnetic phase. The splitting of the resonance modes and its non-monotonous dependence on the applied frequency was observed below 0.6 K in all three field orientations. Several models were discussed to explain the origin of the nontrivial splitting and the existence of inequivalent magnetic subsystems in Cu(en)(H$_2$O)$_2$SO$_4$ was chosen as the most probable source.
Axonal swellings (AS) are the neuropathological hallmark of axonal injury in several disorders fr... more Axonal swellings (AS) are the neuropathological hallmark of axonal injury in several disorders from trauma to neurodegeneration. Current evidence proposes a role of perturbed Ca2+ homeostasis in AS formation, involving impaired axonal transport and focal distension of the axons. Mechanisms of AS formation, in particular moments following injury, however, remain unknown. Here we show that AS form independently from intra-axonal Ca2+ changes, which are required primarily for the persistence of AS in time. We further show that the majority of axonal proteins undergoing de/phosphorylation immediately following injury belong to the cytoskeleton. This correlates with an increase in the distance of the actin/spectrin periodic rings and with microtubule tracks remodeling within AS. Observed cytoskeletal rearrangements support axonal transport without major interruptions. Our results demonstrate that the earliest axonal response to injury consists in physiological adaptations of axonal struc...
Ultrafast Phenomena in Semiconductors and Nanostructure Materials XIII, 2009
We report a femtosecond response in photoinduced magnetization rotation in the ferromagnetic semi... more We report a femtosecond response in photoinduced magnetization rotation in the ferromagnetic semiconductor GaMnAs, which allows for detection of a four-state magnetic memory at the femtosecond time scale. The temporal profile of this cooperative magnetization rotation exhibits a discontinuity that reveals two distinct temporal regimes, marked by the transition from a highly non-equilibrium, carrier-mediated regime within the first 200 fs,
Physical Review Letters, 2006
2005 Quantum Electronics and Laser Science Conference, 2005
physica status solidi (c), 2006
ABSTRACT We develop a theory of photoinduced dynamical magnetic effects in III-Mn-V ferromagnetic... more ABSTRACT We develop a theory of photoinduced dynamical magnetic effects in III-Mn-V ferromagnetic semiconductors valid in presence of strong carrier spin relaxation and dephasing. We treat relaxation by using the Lindblad semigroup method and calculate the nonlinear response numerically. We predict Mn-spin relaxation and precession towards the direction determined by nonlinear optical polarization. These effects occur during the pulse. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Physica B: Condensed Matter, 2001
An effective field theory that describes the low-frequency spin dynamics in the low-temperature o... more An effective field theory that describes the low-frequency spin dynamics in the low-temperature orthorhombic phase of La2CuO4 is derived. The main features of the inherent covert weak ferromagnetism are thus accounted for in a straightforward manner but some of the finer theoretical predictions would require further experimental investigation. In particular, theory predicts the occurrence of magnetic stripes in undoped La2CuO4
The European Physical Journal B, 2000
An effective field theory is derived that describes the low-frequency spin dynamics in the low-te... more An effective field theory is derived that describes the low-frequency spin dynamics in the low-temperature orthorombic phase of La2CuO4. Restricted to a single CuO2 layer the effective theory is a simple generalization of the relativistic nonlinear σ model to include all spin interactions allowed by symmetry. Incorporating a weak interlayer interaction leads to two coupled nonlinear σ models which provide an efficient description of the complete bilayer dynamics. Particular attention is paid to the weak-ferromagnetic and spin-flop transitions induced by external magnetic fields. The main features of the observed (covert) weak ferromagnetism are thus accounted for in a straightforward manner but some of the finer theoretical predictions would require further experimental investigation. The derived framework is also suitable for the study of the structure and dynamics of magnetic domains in undoped La2CuO4.
Physical Review B, 2002
The magnetic compound Ba2CuGe2O7 has recently been shown to be an essentially two-dimensional spi... more The magnetic compound Ba2CuGe2O7 has recently been shown to be an essentially two-dimensional spiral antiferromagnet that exhibits an incommensurate-to-commensurate phase transition when a magnetic field applied along the c-axis exceeds a certain critical value Hc. The T = 0 dynamics is described here in terms of a continuum field theory in the form of a nonlinear σ model. We are thus in a position to carry out a complete calculation of the low-energy magnon spectrum for any strength of the applied field throughout the phase transition. In particular, our spin-wave analysis reveals field-induced instabilities at two distinct critical fields H1 and H2 such that H1 < Hc < H2. Hence we predict the existence of an intermediate phase whose detailed nature is also studied to some extent in the present paper.
We report on the first observation of femtosecond response in the photoinduced cooperative magnet... more We report on the first observation of femtosecond response in the photoinduced cooperative magnetization rotation in ferromagnetic semiconductor GaMnAs, which enables the detection of four-state magnetic memory at the femtosecond time scale.
Physical Review B
Theoretical and experimental studies of a quasi-two-dimensional quantum antiferromagnet Cu(en)(H ... more Theoretical and experimental studies of a quasi-two-dimensional quantum antiferromagnet Cu(en)(H 2 O) 2 SO 4 (en = C 2 H 8 N 2) were performed. ab initio calculations of exchange interactions confirmed that the system represents a realization of a spatially anisotropic zigzag square lattice. Corresponding quantum Monte Carlo calculations of thermodynamic quantities were realized and the results were applied in the analysis of experimental susceptibility, magnetization, and specific heat studied at temperatures ranging from nominally 300 mK up to 8 K and magnetic fields up to 14 T. The analysis of experimental results provided the estimates of intralayer exchange couplings, J /k B = 3.5 ± 0.2 K and J = 0.35J. Theoretical analysis of spin symmetries in Cu(en)(H 2 O) 2 SO 4 structure predicted the presence of symmetric exchange anisotropies (out-of plane and in-plane spin anisotropy) and a spin-flop transition within the easy plane induced by the magnetic field applied along the easy axis. Isothermal magnetization measurements indicated the expected transition in the field 200 mT applied along the b axis which was finally identified as the easy axis lying within the easy plane bc. Magnetic phase diagrams with saturation fields about 6.5 T show nearly identical behavior in all studied directions. Differences appear only in weak magnetic fields as a result of the presence of weak exchange anisotropies ≈10 −3 J. The present analysis suggests that Cu(en)(H 2 O) 2 SO 4 can be a model system for exploring the interplay of quantum fluctuations, exchange anisotropies, and magnetic field in the two-dimensional lattice space.
Submitted for the MAR08 Meeting of The American Physical Society Magnetic Memory Effects in Coher... more Submitted for the MAR08 Meeting of The American Physical Society Magnetic Memory Effects in Coherent Magnetization Dynamics of GaMnAs: From Non-equilibrium to Thermal Regime INGRID COTOROS, University of California at Berkeley, California, JIGANG WANG, Lawrence Berkeley National Laboratory, California, XINYU LIU, JACEK FURDYNA, University of Notre Dame, Indiana, JAROSLAV CHOVAN, ILIAS PERAKIS, University of Crete, Heraklion, Greece, DANIEL CHEMLA, University of California at Berkeley, California — III-Mn-V heterostructures exhibit rich spin memory effects and their magnetic properties show strong responses to external stimuli (light, electrical gate and current) via carrier density tuning. Here we report on the coherent magnetization dynamics due to laser excitation of transient carriers in GaMnAs, distinctly depending on the initial magnetic state the system is prepared in. We identify two distinct temporal regimes that reveal a complex scenario of spin reorientation, marked by the...
The layered magnetic compound Ba_2CuGe_2O_7 exhibits spiral antiferromagnetic order thanks to a D... more The layered magnetic compound Ba_2CuGe_2O_7 exhibits spiral antiferromagnetic order thanks to a Dzyaloshinskii-Moriya (DM) anisotropy that is allowed by crystal symmetry. Here we theoretically examine some finer issues such as the experimentally observed lattice pinning of the propagation vector of helical magnetic domains. We find that DM anisotropy alone would lead to incorrect pinning, but agreement with experiment is restored upon including an additional exchange anisotropy that is also consistent with symmetry. The present results shed light on the so-called bisection rule which has been abstracted from experiment in presence of an in-plane magnetic field.
Physical Review B, 2008
We develop a theory of collective spin dynamics triggered by ultrafast optical excitation of ferr... more We develop a theory of collective spin dynamics triggered by ultrafast optical excitation of ferromagnetic semiconductors. Using the density matrix equations of motion in the mean field approximation and including magnetic anisotropy and hole spin dephasing effects, we predict the development of a light-induced magnetization tilt during ultra-short time intervals comparable to the pulse duration. This femtosecond dynamics in the coherent temporal regime is governed by the interband nonlinear optical polarizations and is followed by a second temporal regime governed by the magnetic anisotropy of the Fermi sea. We interpret our numerical results by deriving a Landau-Gilbert-like equation for the collective spin, which demonstrates an ultrafast correction to the magnetic anisotropy effective field due to second order coherent nonlinear optical processes. Using the Lindblad semigroup method, we also derive a contribution to the interband polarization dephasing determined by the Mn spin and the hole spin dephasing. Our predicted magnetization tilt and subsequent nonlinear dynamics due to the magnetic anisotropy can be controlled by varying the optical pulse intensity, duration, and helicity and can be observed with pump-probe magneto-optical spectroscopy.
Physical Review B, 2008
... Bragg reflectors,32 or by creating microcavities based on a micropillar-type geometry.33 ACKN... more ... Bragg reflectors,32 or by creating microcavities based on a micropillar-type geometry.33 ACKNOWLEDGMENTS This work was supported by the European Commission via the 5th Framework research training network HYTEC Contract No. HPRN-CT-2002-00315. *ilias@ ...
We report a femtosecond response in photoinduced magnetization rotation in the ferromagnetic semi... more We report a femtosecond response in photoinduced magnetization rotation in the ferromagnetic semiconductor GaMnAs, which allows for detection of a four-state magnetic memory at the femtosecond time scale. The temporal profile of this cooperative magnetization rotation exhibits a discontinuity that reveals two distinct temporal regimes, marked by the transition from a highly non-equilibrium, carrier-mediated regime within the first 200 fs, to a thermal, lattice-heating picosecond regime.
Acta Physica Polonica Series a
The Dzyaloshinskii-Moriya anisotropy, responsible for helimagnetism in Ba2CuGe2O7, has recently b... more The Dzyaloshinskii-Moriya anisotropy, responsible for helimagnetism in Ba2CuGe2O7, has recently been shown to comprise also a weak-ferromagnetic component. We theoretically demonstrate the signatures of weakferromagnetism in the magnon spectrum when a magnetic eld is applied perpendicular to the z axis. We also anticipate the occurrence of unconventional domain walls in the so-called intermediate phase, and briey elucidate the importance of the weak ferromagnetic component of the Dzyaloshinskii-Moriya anisotropy for the dynamics of driven domain walls.
An effective field theory is derived that describes the low-frequency spin dynamics in the low-te... more An effective field theory is derived that describes the low-frequency spin dynamics in the low-temperature orthorombic phase of La2CuO4. Restricted to a single CuO2 layer the effective theory is a simple generalization of the relativistic nonlinear σ model to include all spin interactions allowed by symmetry. Incorporating a weak interlayer interaction leads to two coupled nonlinear σ models which provide an efficient description of the complete bilayer dynamics. Particular attention is paid to the weak-ferromagnetic and spin-flop transitions induced by external magnetic fields. The main features of the observed (covert) weak ferromagnetism are thus accounted for in a straightforward manner but some of the finer theoretical predictions would require further experimental investigation. The derived framework is also suitable for the study of the structure and dynamics of magnetic domains in undoped La2CuO4.
arXiv: Strongly Correlated Electrons, 2019
Low-temperature magnetic resonance study of the quasi-two-dimensional antiferromagnet Cu(en)(H$_2... more Low-temperature magnetic resonance study of the quasi-two-dimensional antiferromagnet Cu(en)(H$_2$O)$_2$SO$_4$ (en = C$_2$H$_8$N$_2$) was performed down to 0.45 K. This compound orders antiferromagnetically at 0.9 K. The analysis of the resonance data within the hydrodynamic approach allowed to identify anisotropy axes and to estimate the anisotropy parameters for the antiferromagnetic phase. Dipolar spin-spin coupling turns out to be the main contribution to the anisotropy of the antiferromagnetic phase. The splitting of the resonance modes and its non-monotonous dependence on the applied frequency was observed below 0.6 K in all three field orientations. Several models were discussed to explain the origin of the nontrivial splitting and the existence of inequivalent magnetic subsystems in Cu(en)(H$_2$O)$_2$SO$_4$ was chosen as the most probable source.
Axonal swellings (AS) are the neuropathological hallmark of axonal injury in several disorders fr... more Axonal swellings (AS) are the neuropathological hallmark of axonal injury in several disorders from trauma to neurodegeneration. Current evidence proposes a role of perturbed Ca2+ homeostasis in AS formation, involving impaired axonal transport and focal distension of the axons. Mechanisms of AS formation, in particular moments following injury, however, remain unknown. Here we show that AS form independently from intra-axonal Ca2+ changes, which are required primarily for the persistence of AS in time. We further show that the majority of axonal proteins undergoing de/phosphorylation immediately following injury belong to the cytoskeleton. This correlates with an increase in the distance of the actin/spectrin periodic rings and with microtubule tracks remodeling within AS. Observed cytoskeletal rearrangements support axonal transport without major interruptions. Our results demonstrate that the earliest axonal response to injury consists in physiological adaptations of axonal struc...
Ultrafast Phenomena in Semiconductors and Nanostructure Materials XIII, 2009
We report a femtosecond response in photoinduced magnetization rotation in the ferromagnetic semi... more We report a femtosecond response in photoinduced magnetization rotation in the ferromagnetic semiconductor GaMnAs, which allows for detection of a four-state magnetic memory at the femtosecond time scale. The temporal profile of this cooperative magnetization rotation exhibits a discontinuity that reveals two distinct temporal regimes, marked by the transition from a highly non-equilibrium, carrier-mediated regime within the first 200 fs,
Physical Review Letters, 2006
2005 Quantum Electronics and Laser Science Conference, 2005
physica status solidi (c), 2006
ABSTRACT We develop a theory of photoinduced dynamical magnetic effects in III-Mn-V ferromagnetic... more ABSTRACT We develop a theory of photoinduced dynamical magnetic effects in III-Mn-V ferromagnetic semiconductors valid in presence of strong carrier spin relaxation and dephasing. We treat relaxation by using the Lindblad semigroup method and calculate the nonlinear response numerically. We predict Mn-spin relaxation and precession towards the direction determined by nonlinear optical polarization. These effects occur during the pulse. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Physica B: Condensed Matter, 2001
An effective field theory that describes the low-frequency spin dynamics in the low-temperature o... more An effective field theory that describes the low-frequency spin dynamics in the low-temperature orthorhombic phase of La2CuO4 is derived. The main features of the inherent covert weak ferromagnetism are thus accounted for in a straightforward manner but some of the finer theoretical predictions would require further experimental investigation. In particular, theory predicts the occurrence of magnetic stripes in undoped La2CuO4
The European Physical Journal B, 2000
An effective field theory is derived that describes the low-frequency spin dynamics in the low-te... more An effective field theory is derived that describes the low-frequency spin dynamics in the low-temperature orthorombic phase of La2CuO4. Restricted to a single CuO2 layer the effective theory is a simple generalization of the relativistic nonlinear σ model to include all spin interactions allowed by symmetry. Incorporating a weak interlayer interaction leads to two coupled nonlinear σ models which provide an efficient description of the complete bilayer dynamics. Particular attention is paid to the weak-ferromagnetic and spin-flop transitions induced by external magnetic fields. The main features of the observed (covert) weak ferromagnetism are thus accounted for in a straightforward manner but some of the finer theoretical predictions would require further experimental investigation. The derived framework is also suitable for the study of the structure and dynamics of magnetic domains in undoped La2CuO4.
Physical Review B, 2002
The magnetic compound Ba2CuGe2O7 has recently been shown to be an essentially two-dimensional spi... more The magnetic compound Ba2CuGe2O7 has recently been shown to be an essentially two-dimensional spiral antiferromagnet that exhibits an incommensurate-to-commensurate phase transition when a magnetic field applied along the c-axis exceeds a certain critical value Hc. The T = 0 dynamics is described here in terms of a continuum field theory in the form of a nonlinear σ model. We are thus in a position to carry out a complete calculation of the low-energy magnon spectrum for any strength of the applied field throughout the phase transition. In particular, our spin-wave analysis reveals field-induced instabilities at two distinct critical fields H1 and H2 such that H1 < Hc < H2. Hence we predict the existence of an intermediate phase whose detailed nature is also studied to some extent in the present paper.
We report on the first observation of femtosecond response in the photoinduced cooperative magnet... more We report on the first observation of femtosecond response in the photoinduced cooperative magnetization rotation in ferromagnetic semiconductor GaMnAs, which enables the detection of four-state magnetic memory at the femtosecond time scale.
Physical Review B
Theoretical and experimental studies of a quasi-two-dimensional quantum antiferromagnet Cu(en)(H ... more Theoretical and experimental studies of a quasi-two-dimensional quantum antiferromagnet Cu(en)(H 2 O) 2 SO 4 (en = C 2 H 8 N 2) were performed. ab initio calculations of exchange interactions confirmed that the system represents a realization of a spatially anisotropic zigzag square lattice. Corresponding quantum Monte Carlo calculations of thermodynamic quantities were realized and the results were applied in the analysis of experimental susceptibility, magnetization, and specific heat studied at temperatures ranging from nominally 300 mK up to 8 K and magnetic fields up to 14 T. The analysis of experimental results provided the estimates of intralayer exchange couplings, J /k B = 3.5 ± 0.2 K and J = 0.35J. Theoretical analysis of spin symmetries in Cu(en)(H 2 O) 2 SO 4 structure predicted the presence of symmetric exchange anisotropies (out-of plane and in-plane spin anisotropy) and a spin-flop transition within the easy plane induced by the magnetic field applied along the easy axis. Isothermal magnetization measurements indicated the expected transition in the field 200 mT applied along the b axis which was finally identified as the easy axis lying within the easy plane bc. Magnetic phase diagrams with saturation fields about 6.5 T show nearly identical behavior in all studied directions. Differences appear only in weak magnetic fields as a result of the presence of weak exchange anisotropies ≈10 −3 J. The present analysis suggests that Cu(en)(H 2 O) 2 SO 4 can be a model system for exploring the interplay of quantum fluctuations, exchange anisotropies, and magnetic field in the two-dimensional lattice space.
Submitted for the MAR08 Meeting of The American Physical Society Magnetic Memory Effects in Coher... more Submitted for the MAR08 Meeting of The American Physical Society Magnetic Memory Effects in Coherent Magnetization Dynamics of GaMnAs: From Non-equilibrium to Thermal Regime INGRID COTOROS, University of California at Berkeley, California, JIGANG WANG, Lawrence Berkeley National Laboratory, California, XINYU LIU, JACEK FURDYNA, University of Notre Dame, Indiana, JAROSLAV CHOVAN, ILIAS PERAKIS, University of Crete, Heraklion, Greece, DANIEL CHEMLA, University of California at Berkeley, California — III-Mn-V heterostructures exhibit rich spin memory effects and their magnetic properties show strong responses to external stimuli (light, electrical gate and current) via carrier density tuning. Here we report on the coherent magnetization dynamics due to laser excitation of transient carriers in GaMnAs, distinctly depending on the initial magnetic state the system is prepared in. We identify two distinct temporal regimes that reveal a complex scenario of spin reorientation, marked by the...
The layered magnetic compound Ba_2CuGe_2O_7 exhibits spiral antiferromagnetic order thanks to a D... more The layered magnetic compound Ba_2CuGe_2O_7 exhibits spiral antiferromagnetic order thanks to a Dzyaloshinskii-Moriya (DM) anisotropy that is allowed by crystal symmetry. Here we theoretically examine some finer issues such as the experimentally observed lattice pinning of the propagation vector of helical magnetic domains. We find that DM anisotropy alone would lead to incorrect pinning, but agreement with experiment is restored upon including an additional exchange anisotropy that is also consistent with symmetry. The present results shed light on the so-called bisection rule which has been abstracted from experiment in presence of an in-plane magnetic field.
Physical Review B, 2008
We develop a theory of collective spin dynamics triggered by ultrafast optical excitation of ferr... more We develop a theory of collective spin dynamics triggered by ultrafast optical excitation of ferromagnetic semiconductors. Using the density matrix equations of motion in the mean field approximation and including magnetic anisotropy and hole spin dephasing effects, we predict the development of a light-induced magnetization tilt during ultra-short time intervals comparable to the pulse duration. This femtosecond dynamics in the coherent temporal regime is governed by the interband nonlinear optical polarizations and is followed by a second temporal regime governed by the magnetic anisotropy of the Fermi sea. We interpret our numerical results by deriving a Landau-Gilbert-like equation for the collective spin, which demonstrates an ultrafast correction to the magnetic anisotropy effective field due to second order coherent nonlinear optical processes. Using the Lindblad semigroup method, we also derive a contribution to the interband polarization dephasing determined by the Mn spin and the hole spin dephasing. Our predicted magnetization tilt and subsequent nonlinear dynamics due to the magnetic anisotropy can be controlled by varying the optical pulse intensity, duration, and helicity and can be observed with pump-probe magneto-optical spectroscopy.
Physical Review B, 2008
... Bragg reflectors,32 or by creating microcavities based on a micropillar-type geometry.33 ACKN... more ... Bragg reflectors,32 or by creating microcavities based on a micropillar-type geometry.33 ACKNOWLEDGMENTS This work was supported by the European Commission via the 5th Framework research training network HYTEC Contract No. HPRN-CT-2002-00315. *ilias@ ...
We report a femtosecond response in photoinduced magnetization rotation in the ferromagnetic semi... more We report a femtosecond response in photoinduced magnetization rotation in the ferromagnetic semiconductor GaMnAs, which allows for detection of a four-state magnetic memory at the femtosecond time scale. The temporal profile of this cooperative magnetization rotation exhibits a discontinuity that reveals two distinct temporal regimes, marked by the transition from a highly non-equilibrium, carrier-mediated regime within the first 200 fs, to a thermal, lattice-heating picosecond regime.
Acta Physica Polonica Series a
The Dzyaloshinskii-Moriya anisotropy, responsible for helimagnetism in Ba2CuGe2O7, has recently b... more The Dzyaloshinskii-Moriya anisotropy, responsible for helimagnetism in Ba2CuGe2O7, has recently been shown to comprise also a weak-ferromagnetic component. We theoretically demonstrate the signatures of weakferromagnetism in the magnon spectrum when a magnetic eld is applied perpendicular to the z axis. We also anticipate the occurrence of unconventional domain walls in the so-called intermediate phase, and briey elucidate the importance of the weak ferromagnetic component of the Dzyaloshinskii-Moriya anisotropy for the dynamics of driven domain walls.