Collective modes in quasi-one-dimensional charge-density wave systems probed by femtosecond time-resolved optical studies

Schaefer, Hanjo; Kabanov, V. V.; Demšar, Jure

doi:10.1103/physrevb.89.045106

Cited by 64 publications

(122 citation statements)

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“…It was shown recently [22,25] be well explained in terms of a linear coupling between the electronic part of the order parameter and q CDW phonons of the high-T phase. In the simplest case of an incommensurate CDW with inversion symmetry, the electronic part of the order parameter may be represented by a complex Δ, whereby Δð−q CDW Þ ¼ Δ Ã ðq CDW Þ.…”

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“…This is expected if the phase transition is driven by the electronic instability such as in the CDW scenario [22] as opposed to by simple zone folding (adaptive martensite scenario). Indeed, in standard CDW systems [22,24,25] numerous Raman active (q ¼ 0) modes appear in the CDW phase, showing comparable softening upon approaching the CDW ordering temperature T CDW .…”

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“…Alloying with cobalt results in a shift of T M far above room temperature and is thus of vital importance for technological applications [20]. While STM provides direct access to static structural modulation, the bulk sensitive femtosecond optical spectroscopy is particularly useful for studying low-energy collective excitations, like Raman active phonons [21] or amplitude modes of charge-density wave (CDW) systems [22][23][24][25].In this Letter, we demonstrate the existence of collective modes in the 14M phase that stem from the coupling of the lattice to the underlying electronic instability. We show that the stacking sequence recorded by STM, which is close to the structure proposed by the adaptive martensite scenario [17,18], can be consistently explained by the higher-order coupling between the electronic order and the lattice.…”

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Collective Modes and Structural Modulation in Ni-Mn-Ga(Co) Martensite Thin Films Probed by Femtosecond Spectroscopy and Scanning Tunneling Microscopy

et al. 2015

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The origin of the martensitic transition in the magnetic shape memory alloy Ni-Mn-Ga has been widely discussed. While several studies suggest it is electronically driven, the adaptive martensite model reproduced the peculiar nonharmonic lattice modulation. We used femtosecond spectroscopy to probe the temperature and doping dependence of collective modes, and scanning tunneling microscopy revealed the corresponding static modulations. We show that the martensitic phase can be described by a complex charge-density wave tuned by magnetic ordering and strong electron-lattice coupling. DOI: 10.1103/PhysRevLett.115.076402 PACS numbers: 71.45.Lr, 78.47.-p, 81.30.Kf, 81.70.Fy Magnetic shape memory alloys present a new type of multifunctional materials, which display strong coupling between the magnetic and structural degrees of freedom. The ferromagnetic Ni-Mn-Ga alloy serves as a prototype system, displaying a giant 12% magnetic-field-induced strain in its low-temperature (T) martensitic phase (M phase) [1,2]. Since in Ni-Mn-Ga the M-phase transition can be tuned far above the room temperature by changing stoichiometry or doping [3,4], this alloy is of special technological interest [1,2,5].The rich phase diagram of Ni 2þxþy Mn 1−x Ga 1−y is characterized by a complex sequence of phase transitions. In its high-T (austenite) phase, Ni-Mn-Ga has a cubic L2 1 Heusler structure. At the M-phase transition temperature T M , the lattice undergoes a transformation, which can be described by a periodic shuffling of (011) planes along the ½011 direction [6]. Depending on the stoichiometry and the residual stress, the resulting low-T M phase is commonly found to have either tetragonal or orthorhombic symmetry, with a modulation period of ten (10M) or 14 (14M) atomic layers [6,7], respectively. For specific stoichiometries, e.g., Ni 2þx Mn 1−x Ga with x ≲ 0.1, a premartensitic phase (PM phase) is observed above T M [3] with the transition temperature T PM up to 50 K above T M [3,8]. In the PM phase, the lattice displays a harmonic threefold modulation with the wave vector q PM ¼ q max ð 1 3 ; 1 3 ; 0Þ. Inelastic neutron scattering studies of the high-T cubic phase [9] showed a dramatic softening of the TA-2 phonon branch at q PM , indicative of a Kohn anomaly. These observations [9][10][11][12], together with the observed opening of a pseudogap at T PM [8,13], suggest an electronic instability within the Peierls scenario to be driving the PM-phase transition. The observation of a phase mode in the 10M phase [14] also suggested the electronic instability. In fact, the electronic band structure studies [11,15,16] revealed the possible Fermi surface nesting conditions for all of the observed structural modulations. However, for the case of the 14M martensite, it was argued that the modulated phase can be constructed from nonmodulated martensitic unit cells [17,18]. This adaptive martensite scenario [17][18][19] recently received considerable attention.To determine which theory gives a more appropriate physical picture of the pha...

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Collective Modes and Structural Modulation in Ni-Mn-Ga(Co) Martensite Thin Films Probed by Femtosecond Spectroscopy and Scanning Tunneling Microscopy

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“…Recently, this type of collective mode has attracted much attention thanks to experimental observations in various condensed-matter and quantum-gas systems, such as superconductors NbSe 2 [3][4][5] and Nb 1−x Ti x N [6][7][8][9], quantum antiferromagnets TlCuCl 3 [10,11] and KCuCl 3 [12], charge-density-wave materials K 0.3 MoO 3 [13,14] and TbTe 3 [15,16], superfluid 3 He B-phase [17,18], and superfluid Bose gases in optical lattices [19,20].…”

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Localized Higgs modes of superfluid Bose gases in optical lattices: A Gutzwiller mean-field study

Danshita

Tsuchiya

2017

Phys. Rev. A

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We study effects of a potential barrier on collective modes of superfluid Bose gases in optical lattices. We assume that the barrier is created by local suppression of the hopping amplitude. When the system is in a close vicinity of the Mott transition at commensurate fillings, where an approximate particle-hole symmetry emerges, there exist bound states of Higgs amplitude mode that are localized around the barrier. By applying the Gutzwiller mean-field approximation to the Bose-Hubbard model, we analyze properties of normal modes of the system with a special focus on the Higgs bound states. We show that when the system becomes away from the Mott transition point, the Higgs bound states turn into quasi-bound states due to inevitable breaking of the particlehole symmetry. We use a stabilization method to compute the resonance energy and line width of the quasi-bound states. We compare the results obtained by the Gutzwiller approach with those by the Ginzburg-Landau theory. We find that the Higgs bound states survive even in a parameter region far from the Mott transition, where the Ginzburg-Landau theory fails.

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Finding all‐nonmetal transition‐metal‐like superatom and its magnetic building block

Jiang

Liu

et al. 2017

Int J of Quantum Chemistry

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In novel superatom chemistry, it is very attractive that all-metal clusters can mimic the behaviors of nonmetal atoms and simple nonmetal molecules. Wizardly all-metal halogen-like superatom Al 13 with 2P 5 sub shell (corresponding to the 3p 5 of chlorine) is the most typical example. In contrast, how to mimic the behaviors of magnetic transition-metal atom using all-nonmetal cluster is an intriguing challenge for superatom chemistry. In response to this based on human intuition, using quantum chemistry methods and extending jellium model from metal cluster to all-nonmetal cluster, we have found out that all-nonmetal octahedral B 6 cluster with characteristic jellium electron configuration 1S 2 1P 6 2S 2 1D 8 in the triplet ground state can mimic the behaviors of transitionmetal Ni atom with electron configuration 3s 2 3p 6 4s 2 3d 8 in electronic configuration, physics and chemistry. Interestingly, the characteristic order of 1S1P2S1D for the B 6 nonmetal cluster with short B-B lengths is different from that of the traditional jellium model-1S1P1D2S for metal clusters with long M-M lengths, which exhibits a novel size effect of nonmetal cluster on jellium orbital ordering. Based on the jellium electron configuration, the B 6 with the spin moment value of 2l B is a new all-nonmetal transition-metal nickel-like superatom exhibiting a new kind of allnonmetal magnetic superatom. Finding the application of the all-nonmetal magnetic superatom, we encapsulate the magnetic superatom B 6 inside fully hydrogenated fullerene forming a clathrate B 6 @C 60 H 60 with the spin moment value of 2l B . As the C 60 H 60 cage as a polymerization unit can conserve the spin moment of endohedral B 6 , the clathrate B 6 @C 60 H 60 is a new all-nonmetal magnetic superatom building block. Naturally, magnetic superatom structures of the B 6 and B 6 @C 60 H 60 may be metastable. K E Y W O R D S all-nonmetal magnetic superatom, all-nonmetal octahedral cluster, clathrate B 6 @C 60 H 60 , spin moment, transition-metal Int J Quantum Chem. 2018;118:e25570.

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Collective modes in quasi-one-dimensional charge-density wave systems probed by femtosecond time-resolved optical studies

Cited by 64 publications

References 48 publications

Collective Modes and Structural Modulation in Ni-Mn-Ga(Co) Martensite Thin Films Probed by Femtosecond Spectroscopy and Scanning Tunneling Microscopy

Collective Modes and Structural Modulation in Ni-Mn-Ga(Co) Martensite Thin Films Probed by Femtosecond Spectroscopy and Scanning Tunneling Microscopy

Localized Higgs modes of superfluid Bose gases in optical lattices: A Gutzwiller mean-field study

Finding all‐nonmetal transition‐metal‐like superatom and its magnetic building block

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