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Electrically charged droplet: Case study of a simple generator
http://hdl.handle.net/2268/183623
Title: Electrically charged droplet: Case study of a simple generator
<br/>
<br/>Author, co-author: Brandenbourger, Martin; Dorbolo, StéphaneFri, 03 Jul 2015 16:28:55 GMTImproving CKM Unitary Limits via Low-Energy Nuclear Physics
http://hdl.handle.net/2268/183595
Title: Improving CKM Unitary Limits via Low-Energy Nuclear Physics
<br/>
<br/>Author, co-author: Finlay, Paul; Ball, G.; Bastin, Thierry; Bissel, Marc; Ettenauer, S.; Glover, Rohan; Naviliat-Cuncic, O.; Neyens, Gerda; Rand, E.; Severijns, Nathal; Svensson, C.; Velten, P.; Zákoucký, D.Fri, 03 Jul 2015 09:20:46 GMTMultiqubit symmetric states with maximally mixed one-qubit reductions
http://hdl.handle.net/2268/183481
Title: Multiqubit symmetric states with maximally mixed one-qubit reductions
<br/>
<br/>Author, co-author: Baguette, Dorian; Bastin, Thierry; Martin, John
<br/>
<br/>Abstract: We present a comprehensive study on the remarquable properties shared by maximally entangled symmetric states of arbitrary numbers of qubits in the sense of the maximal mixedness of the one-qubit reduced density operator. Such states are of great interest in quantum information as they maximize several measures of entanglement, such as Meyer-Wallach entropy [1] and any entanglement monotone based on linear homogenous positive functions of pure state within their SLOCC classes of states [2, 3]. When they exist, they are unique up to local unitaries within their SLOCC classes [3, 4]. They play a specific role in the determination of the local unitary equivalence of multiqubit states [5]. Moreover, they are maximally fragile (in the sense that they are the states which are the most sensitive to noise) and have therefore been proposed as ideal candidates for ultrasensitive sensors [6]. They appear in the litterature under various names : maximally entangled states [6], 1-uniform states [7], normal forms [3, 4] and nongeneric states [5].
We present a general criterion to easily identify whether given symmetric states are maximally entangled or not [9]. We show that these maximally entangled symmetric (MES) states are the only symmetric states for which the expectation value of the associated collective spin S of the system vanishes, which coincides with the definition of anticoherence to order one of spin states. This definition also coincides with the cancellation of the dipole moment of the Husimi function of the state. We then generalize these properties and show that a state is anticoherent to order t, <(S.n)^k> is independent of n for k = 1, . . . , t, where n is a unit vector, iff it has maximally mixed t-qubit reductions or iff all moments up to order 2t of its Husimi function vanish. We also establish the equivalence between anticoherent states to order t and unpolarized light states to order t [8], thereby encompassing various state characterizations under the same banner [9, 10].
We provide a nonexistence criterion allowing us to know immediately whether SLOCC classes of symmetric states can contain MES states or not. We show in particular that the symmetric Dicke state SLOCC classes never contain such MES states, with the only exception of the balanced Dicke state class for even numbers of qubits. We analyze the 4-qubit system exhaustively and identify and characterize all MES states of this system as well as the only 4-qubit state anticoherent to order 2. Finally, we analyze the entanglement content of MES states with respect to the geometric [11] and barycentric [12] measures of entanglement.
[1] D. A. Meyer and N. R. Wallach, J. Math. Phys. 43, 4273 (2002).
[2] Classes of states equivalent through stochastic local operations with classical communication.
[3] F. Verstraete, J. Dehaene, and B. De Moor, Phys. Rev. A 68, 012103 (2003).
[4] G. Gour and N. Wallach, N. J. Phys. 13, 073013 (2011).
[5] B. Kraus, Phys. Rev. Lett. 104, 020504 (2010).
[6] N. Gisin and H. Bechmann-Pasquinucci, Phys. Lett. A 246, 1 (1998).
[7] A. J. Scott, Phys. Rev. A 69, 052330 (2004).
[8] L. L. Sánchez-Soto, A. B. Klimov, P. de la Hoz, and G. Leuchs J. Phys. B : At. Mol. Opt. Phys. 46, 104011 (2013).
[9] D. Baguette, T. Bastin, and J. Martin, Phys. Rev. A 90, 032314 (2014).
[10] O. Giraud, D. Braun, D. Baguette, T. Bastin, and J. Martin, arXiv :1409.1106.
[11] T.-C. Wei and P. M. Goldbart, Phys. Rev. A 68, 042307 (2003).
[12] W. Ganczarek, M. Kus, and K. Zyczkowski, Phys. Rev. A 85, 032314 (2012).Wed, 01 Jul 2015 11:24:00 GMTTowards precision β-decay measurements with laser cooled 35Ar
http://hdl.handle.net/2268/183438
Title: Towards precision β-decay measurements with laser cooled 35Ar
<br/>
<br/>Author, co-author: Glover, Rohan; Lenaers, Florence; Velten, Philippe; Finlay, Paul; Couratin, C.; Hayen, L.; Severijns, Nathal; Bastin, ThierryTue, 30 Jun 2015 12:01:21 GMTTest of atmospheric neutrons effects on FPGA at Centre Spatial de Liège (CSL) – Université de Liège (ULg)
http://hdl.handle.net/2268/183403
Title: Test of atmospheric neutrons effects on FPGA at Centre Spatial de Liège (CSL) – Université de Liège (ULg)
<br/>
<br/>Author, co-author: Carapelle, AlainTue, 30 Jun 2015 08:43:40 GMTOptical collimation of an atomic beam using a white light molasses
http://hdl.handle.net/2268/183355
Title: Optical collimation of an atomic beam using a white light molasses
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<br/>Author, co-author: Glover, Rohan; Bastin, ThierryMon, 29 Jun 2015 14:52:50 GMTOperational entanglement families of symmetric mixed N-qubit states
http://hdl.handle.net/2268/183354
Title: Operational entanglement families of symmetric mixed N-qubit states
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<br/>Author, co-author: Bastin, Thierry; Mathonet, Pierre; Solano, EnriqueMon, 29 Jun 2015 14:49:21 GMTAnticoherence of multiqubit symmetric states
http://hdl.handle.net/2268/183348
Title: Anticoherence of multiqubit symmetric states
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<br/>Author, co-author: Baguette, Dorian; Bastin, Thierry; Martin, JohnMon, 29 Jun 2015 14:16:51 GMTThermoelectric properties of two stacking sequences of crystalline GST-225
http://hdl.handle.net/2268/183312
Title: Thermoelectric properties of two stacking sequences of crystalline GST-225
<br/>
<br/>Author, co-author: Ibarra Hernandez, Wilfredo; Raty, Jean-Yves
<br/>
<br/>Abstract: Pseudobinary GeTe-Sb2Te3 compounds are widely used as phase-change
optical materials for DVD-RAM.[3] Ge2Sb2Te5 (GST-225) is used for
this propose but the stacking sequence of the stable crystal structure
is motive of debate. Pseudobinary compounds there are claimed to be
good thermoelectric materials due the large number of intrinsic structural
vacancies.[4] Thermoelectric properties for two proposed stacking sequences of GST-225 are computed using DFT[5, 6] and Boltzmann transport equation in the constant relaxation time approximation. After phonon calculations, no dynamic instabilities were found in the Irreducible Brillouin Zone for either of the proposed stacking sequences. One of the stacking sequences shows semiconductor-like density of states (DOS) with a computed gap of 190 meV unlike the other stacking sequence which has a metallic-like DOS. Thermoelectric properties calculation
reveals that semiconductor-like structure has the highest value of Seebeck coeffcient (SC).Mon, 29 Jun 2015 10:44:43 GMTAb Initio DFT study of electronic and thermoelectric properties of crystalline Ge2Sb2Te5
http://hdl.handle.net/2268/183311
Title: Ab Initio DFT study of electronic and thermoelectric properties of crystalline Ge2Sb2Te5
<br/>
<br/>Author, co-author: Ibarra Hernandez, Wilfredo; Raty, Jean-Yves
<br/>
<br/>Abstract: Pseudo-binary phase change materials such as (GeTe)n/(Sb2Te3)m have been recently considered for thermoelectric applications. Among these, Ge2Sb2Te5 (GST225, n=2 and m=1) is very popular as it is the leading candidate for non-volatile memory devices such as phase change random access memory. It is well know that the stable crystal structure of GST225 is hexagonal, with atomic layers stacked in the c direction. The stacking sequence is however still under some debate, and structures varying from conventional semiconductor to Dirac semimetal have been claimed to differ only by the nature of the stacking sequence. Here we present electronic, dynamic and thermoelectric calculations on three different stacking sequences of crystalline GST225. We use ab-initio DFT calculations together with Boltzmann transport equations to access thermoelectric properties within the constant relaxation time approximation. Our results show that all three proposed stacking sequences are (meta-)stable. From the density of states we determine that two structures are metallic while the most stable structure has a 0.35 eV band gap. Above 100K, the computed Seebeck coefficient seems to indicate that the experimentally observed structure is the Dirac semimetal one, the doping level being of the order of 1 × 1020 cm−3.Mon, 29 Jun 2015 10:43:55 GMTleidenfrost drops
http://hdl.handle.net/2268/183302
Title: leidenfrost drops
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<br/>Author, co-author: sobac, benjamin; rednikov, alexey; Dorbolo, Stéphane; colinet, pierreMon, 29 Jun 2015 09:17:02 GMTStability and Aging of Phase Change Materials : An Ab Initio Perspective
http://hdl.handle.net/2268/183300
Title: Stability and Aging of Phase Change Materials : An Ab Initio Perspective
<br/>
<br/>Author, co-author: Raty, Jean-Yves
<br/>
<br/>Abstract: Data recording with Phase Change Materials is a much studied topic as the writing/erasing characteristics, cyclability and downscaling properties of these materials allow for efficient data storage in future generations of devices. Nevertheless, some aspects of phase change materials are limiting their performances and delaying their wider technological application.
First, aging phenomena are common to all amorphous structures, but of special importance PCMs since it impedes the realization of multi-level memories. Different interpretations have been proposed, but we focus here on the structural relaxation of amorphous GeTe, chosen because it is the simplest system that is representative of the wider class of GST alloys, lying along the GeTe-Sb2Te3 composition line of the GeSbTe phase diagram. One difficulty encountered in the simulation of these amorphous systems is that the direct generation of an amorphous structure by quenching a liquid using Density Functional Theory (DFT) based Molecular Dynamics leads to one sample with a small number of atoms, and, hence of small number of atomic environments. Here we sample a large number of local atomic environments, corresponding to different bonding schemes, by chemically substituting different alloys, selected to favor different local atomic structures. This enables spanning a larger fraction of the configuration space relevant to aging. Our results support a model of the amorphous phase and its time evolution that involves an evolution of the local (chemical) order towards that of the crystal. On the other hand its electronic properties drift away from those of the crystal, driven by an increase of the Peierls-like distortion of the local environments in the amorphous, as compared to the crystal [1].
A second problem faced by PCMs is the fact that data recording is limited at high temperature due to the increased propensity to recrystallize. One approach to counter this is to stabilize the PCM using impurity atoms such as C or N. Using DFT and the analysis of the mechanical properties (constraints theory), we demonstrate how these impurity atoms modify the rigidity of the network, which is experimentally correlated with the activation energy for crystallization [2].
Finally, the crystal phase itself has been shown to have variable conductivities depending on the thermal history and annealing conditions. If this could be used profitably for multi-level recording, it also indicates that the crystal is undergoes some temporal evolution. Using DFT, we clarify the stability behavior of GST crystal and show that the metal-insulator transition is driven by the migration of intrinsic vacancies and an Anderson localization transition [3].
[1] J.Y Raty, W. Zhang, J. Luckas, C. Chen, R. Mazzarello, C. Bichara and M. Wuttig, Nat. Comm. (2015)
[2] G. Ghezzi, J.Y. Raty, S. Maitrejean, A. Roule, E. Elkaim and F. Hippert, Applied Physics Letters, 99 (2011) 151906
[3] W. Zhang, A. Thiess, P. Zalden, R. Zeller, P. H. Dederichs, J-Y. Raty, M.Wuttig, S. Blügel et R. Mazzarello, Nature Materials 11 (2012) 952Mon, 29 Jun 2015 09:10:22 GMTAGING MECHANISMS IN AMORPHOUS PHASE CHANGE MATERIALS
http://hdl.handle.net/2268/183299
Title: AGING MECHANISMS IN AMORPHOUS PHASE CHANGE MATERIALS
<br/>
<br/>Author, co-author: Raty, Jean-Yves
<br/>
<br/>Abstract: Aging phenomena are common to all amorphous structures, but of special importance in phase change materials (PCM) since it impedes the realization of multi-level memories. Different interpretations have been proposed, but we focus here on the structural relaxation of amorphous GeTe, chosen because it is the simplest system that is representative of the wider class of GST alloys, lying along the GeTe-Sb2Te3 composition line of the GeSbTe phase diagram.
The direct melt-and-quench DFT based Molecular Dynamics approach leads to models with a few hundred atoms, and, hence a small number of atomic environments. Here we sample a large number of local atomic environments, and bonding schemes, by chemically substituting different alloys to favor different local atomic structures. This enables spanning a larger fraction of the configuration space relevant to aging.
GST alloys are known to display complex bonding that does not follow the chemist’s “octet-rule”. This lead to many controversies, especially concerning the local structure around Ge atoms. We overcome this problem by using state of the art non local DFT-MD, including the so-called van der Waals corrections. This leads to more clearly defined environments that are thoroughly analyzed. We can then identify their fingerprints in the available structural experimental data and assess their stability to find the driving forces leading to the structural relaxation. The calculated electronic properties nicely match the most recent photothermal deflection spectroscopy experiments.
Our results support a model of the amorphous phase and its time evolution that involves an evolution of the local (chemical) order towards that of the crystal (by getting rid of homopolar bonds), and an evolution of its electronic properties that drift away from those of the crystal, driven by an increase of the Peierls-like distortion of the local environments in the amorphous [1].
[1] J.Y Raty, W. Zhang, J. Luckas, C. Chen, R. Mazzarello, C. Bichara and M. Wuttig, Nature Comm. To appear.Mon, 29 Jun 2015 09:06:07 GMTAging mechanisms in amorphous phase-change materials
http://hdl.handle.net/2268/183298
Title: Aging mechanisms in amorphous phase-change materials
<br/>
<br/>Author, co-author: Raty, Jean-Yves
<br/>
<br/>Abstract: Aging phenomena are common to all amorphous structures, but of special importance in phase change materials (PCM) since it impedes the realization of multi-level memories. Different interpretations have been proposed, but we focus here on the structural relaxation of amorphous GeTe, chosen because it is the simplest system that is representative of the wider class of GST alloys, lying along the GeTe-Sb2Te3 composition line of the GeSbTe phase diagram.
The direct melt-and-quench DFT based Molecular Dynamics approach leads to models with a few hundred atoms, and, hence a small number of atomic environments. Here we sample a large number of local atomic environments, and bonding schemes, by chemically substituting different alloys to favor different local atomic structures. This enables spanning a larger fraction of the configuration space relevant to aging.
GST alloys are known to display complex bonding that does not follow the chemist’s “octet-rule”. This lead to many controversies, especially concerning the local structure around Ge atoms. We overcome this problem by using state of the art non local DFT-MD, including the so-called van der Waals corrections. This leads to more clearly defined environments that are thoroughly analyzed. We can then identify their fingerprints in the available structural experimental data and assess their stability to find the driving forces leading to the structural relaxation. The calculated electronic properties nicely match the most recent photothermal deflection spectroscopy experiments.
Our results support a model of the amorphous phase and its time evolution that involves an evolution of the local (chemical) order towards that of the crystal (by getting rid of homopolar bonds), and an evolution of its electronic properties that drift away from those of the crystal, driven by an increase of the Peierls-like distortion of the local environments in the amorphous [1].
[1] J.Y Raty, W. Zhang, J. Luckas, C. Chen, R. Mazzarello, C. Bichara and M. Wuttig, Nature Comm. To appear.Mon, 29 Jun 2015 08:54:19 GMTStructural properties of densified GexSe1-x glasses
http://hdl.handle.net/2268/183297
Title: Structural properties of densified GexSe1-x glasses
<br/>
<br/>Author, co-author: Yildirim, Can; Micoulaut, Matthieu; Raty, Jean-Yves
<br/>
<br/>Abstract: The evolution in structural properties of liquid and densified
glassy GexSe1-x is investigated by use of First Principles Molecular
Dynamics (FPMD) combined with X-ray absorption (XAS) experiments
for the glassy state. Four different compositions (x=10%, 16%,
18% and 33%) representing the flexible and stressed rigid sides of the
reversibility window are the focus of the study. The target systems
were studied at pressures up to ~ 11 GPa under both annealed and
cold compression conditions. We examine the structure factors,
pair distribution functions, bond angle distributions, coordination
numbers and neighbor distributions. The results show that the real
and reciprocal space properties are in very good agreement with the
experimental findings. The structural evolution during densification
reveals the edge sharing tetrahedra is maintained upon compression
whereas Ge-Ge homopolar bonds tend to increase in number.
Ge-Se-Ge bond angular distributions show a transformation from
tetrahedral octahedral geometry. We discuss the effect of thermal
history on structural properties during densification.Mon, 29 Jun 2015 08:48:42 GMTAging mechanisms in amorphous GeTe
http://hdl.handle.net/2268/183296
Title: Aging mechanisms in amorphous GeTe
<br/>
<br/>Author, co-author: Raty, Jean-Yves; Zhang, Wei; Luckas, Jennifer; Mazzarello, Riccardo; Bichara, Christophe; Wuttig, Matthias
<br/>
<br/>Abstract: Aging phenomena are common to all amorphous structures, but
of special importance in phase change materials (PCM) since it
impedes the realization of multi-level memories. Different interpretations
have been proposed, but we focus here on the structural
relaxation of amorphous GeTe, chosen because it is the simplest
system that is representative of the wider class of GST alloys, lying
along the GeTe-Sb2Te3 composition line of the GeSbTe phase
diagram. We investigate the structure of amorphous GeTe using
Density Functional Theory based Molecular Dynamics, using either
the standard Generalized Gradient Approximation, or more elaborate
Van der Waals approximation. New insight is provided on the
stability of homopolar GeGe bonds and tetrahedral Ge bonding,
in relation with the resistance drift phenomenon, that is investigated
experimentally using photothermal deflection spectroscopy
experimentsMon, 29 Jun 2015 08:42:56 GMTAging Mechanisms in Amorphous GeTe
http://hdl.handle.net/2268/183295
Title: Aging Mechanisms in Amorphous GeTe
<br/>
<br/>Author, co-author: Raty, Jean-Yves; Zhang, Wei; Luckas, Jennifer; Mazzarello, Riccardo; Bichara, Christophe; Wuttig, Matthias
<br/>
<br/>Abstract: We investigate the structure of amorphous GeTe using Density Functional Theory based Molecular Dynamics, using either the standard Generalized Gradient Approximation, or the more elaborate Van der Waals approximation. New insight is provided on the stability of homopolar GeGe bonds and tetrahedral Ge bonding, in relation with the resistance drift phenomenon, that is investigated experimentally using photothermal deflection spectroscopy experiments.
Aging phenomena are common to all amorphous structures, but of special importance in phase change materials (PCM) since it impedes the realization of multi-level memories. Different interpretations have been proposed, but we focus here on the structural relaxation of amorphous GeTe, chosen because it is the simplest system that is representative of the wider class of GST alloys, lying along the GeTe-Sb2Te3 composition line of the GeSbTe phase diagram.
Since the structural relaxations concerned with the drift take place on long time scales, the task of understanding them to limit their consequences is not a simple one. We successfully achieved this goal by developing new approaches to overcome a series of hurdles.
A first problem is that directly generating an amorphous structure by quenching a liquid using Density Functional Theory (DFT) based Molecular Dynamics leads to one sample with a small number of atom (typically a few hundreds), and, hence of small number of atomic environments. Here we sample a large number of local atomic environments, corresponding to different bonding schemes, by chemically substituting different alloys, selected to favor different local atomic structures. This enables spanning a larger fraction of the configuration space relevant to aging.
A second aspect is that GST alloys are known to display complex bonding mechanisms, for which the simple chemist’s “octet-rule” does not apply, leading a long series of controversies, concerning in particular the local structure around Ge atoms. We overcome this problem by using state of the art non local DFT-MD, including the so-called van der Waals corrections. This leads to more clearly defined environments that are thoroughly analyzed.
We can then identify their fingerprints in the available structural experimental data and assess the stability of these local environments to obtain information of the driving forces leading to the structural relaxation. The calculated electronic properties nicely match the most recent photothermal deflection spectroscopy experiments that are presented here.
Our results support a model of the amorphous phase and its time evolution that involves an evolution of the local (chemical) order towards that of the crystal (by getting rid of homopolar bonds), and an evolution of its electronic properties that drift away from those of the crystal, driven by an increase of the Peierls-like distortion of the local environments in the amorphous, as compared to the crystal.Mon, 29 Jun 2015 08:31:42 GMTImportance of Dispersion Forces for the Simulation of Amorphous Phase Change Materials
http://hdl.handle.net/2268/183294
Title: Importance of Dispersion Forces for the Simulation of Amorphous Phase Change Materials
<br/>
<br/>Author, co-author: Raty, Jean-Yves; Wuttig, Matthias; Bichara, Christophe
<br/>
<br/>Abstract: GeTe and GST amorphous materials are re-investigated using ab initio dft molecular dynamics to compare the effect of various functionals that include the treatment of dispersion (Van der Waals) forces on the structural and dynamical properties of the final amorphous structures. We show that the proportion of tetrahedral Ge to other types of environments as well as the ratio of 3-fold and 2-fold bonded Te atoms is much dependent on the choice of functional. The different functionals yield variable agreement with the available structural experimental data. Properties such as the diffusion coefficient and vibrational densities of states are computed, indicating that models that are extremely close in energy may have very different experimental signaturesMon, 29 Jun 2015 08:27:02 GMTAging mechanisms in amorphous GeTe
http://hdl.handle.net/2268/183293
Title: Aging mechanisms in amorphous GeTe
<br/>
<br/>Author, co-author: Raty, Jean-Yves; Zhang, wei; Luckas, Jennifer; Mazzarello, Riccardo; Bichara, Christophe; Wuttig, Matthias
<br/>
<br/>Abstract: We investigate the structure of amorphous GeTe using Density Functional Theory based Molecular Dynamics, using either the standard Generalized Gradient Approximation, or the more elaborate van der Waals approximation that proves more accurate in this system. New insight is provided on the stability of homopolar GeGe bonds and tetrahedral Ge bonding, in relation with the resistance drift phenomenon, that is investigated experimentally using photothermal deflection spectroscopy experiments.Mon, 29 Jun 2015 08:15:37 GMTAging mechanisms in amorphous phase-change materials
http://hdl.handle.net/2268/183286
Title: Aging mechanisms in amorphous phase-change materials
<br/>
<br/>Author, co-author: Raty, Jean-Yves; Zhang, wei; Luckas, Jennifer; Chen, Chao; Mazzarello, Riccardo; Bichara, Christophe; Wuttig, Matthias
<br/>
<br/>Abstract: Aging is a ubiquitous phenomenon in glasses. In the case of phase-change materials, it leads to a drift in the electrical resistance, which hinders the development of ultrahigh density storage devices. Here we elucidate the aging process in amorphous GeTe, a prototypical phase-change material, by advanced numerical simulations, photothermal deflection spectroscopy and impedance spectroscopy experiments. We show that aging is accompanied by a progressive change of the local chemical order towards the crystalline one. Yet, the glass evolves towards a covalent amorphous network with increasing Peierls distortion, whose structural and electronic properties drift away from those of the resonantly bonded crystal. This behaviour sets phase-change materials apart from conventional glass-forming systems, which display the same local structure and bonding in both phasesMon, 29 Jun 2015 07:03:03 GMT