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Very one dimensional organic conductors – Less is more • J. S. B, M. Almeida, L.L. Lumata, P. Kuhns, A. Reyes, D. Graf, R. Henriques,L. Prettner (Green), J. Wright, and S. Brown First, some news from the Magnet Lab in Tallahassee and Los Alamos
25 T SPLIT RESISTIVE MAGNET Objective 25 T central field 28 MW dc power (2 supplies) 4 ports at mid-plane of 45° each Vertical or horizontal field 2 Sets of inner coil Jack Toth Project Leader Now Working! Please consider coming to us for magnetooptics studies! Steve McGill - femptosecond Dmitry Smirnov – visible/raman Jason Li - FTIR
Optics in the Split-Florida Helix 25 T Dewar 32mm bore ~25T 11.4° 4 x scattering cone (line of sight) aligned with cell vs. 45° 4 tapered access ports each: 11.4° x 45° ~ 1m Instrumentation: IR & THz cw optics Visible/Fast optics Bruker 66 FTIR spectrometer (roving cell-to-cell) Amplified Ti:Sapphire (2.5 mJ, 150 fs, 1 KHz) OPA, Streak camera, VIS and NIR detectors Sub-THz tunable sources: BWO (Backward wave oscillators), Mid-IR CO2 laser (11 μm) Ar+, He-Ne, He-Cd, and dye lasers for cw 0.75m McPherson spectrometer TriVista High-resolution spectrometer Mid-IR and Far-IR detectors Activities: Custom optical cryostat being purchased Selecting window materials Preparing implementation of inelastic light scattering experiments Near future: Transfer of existing techniques + Split-Helix = new capabilities: Fiber-free techniques expand possibilities forUV spectroscopy, polarization-resolved, & time-resolved experiments FTIR in Voigt geometry IR luminescence
* New High Magnetic Field Record 97.4 tesla confirmed via magneto quantum oscillations in poly-crystalline copper 97.4 tesla *World Record magnetic field intensity for a Non-Destructive Pulsed Magnet
Very one dimensional organic conductors – Less is more • Per2[M(mnt)2] (M = Au, Pt, Co): • Charge Density Wave Spin-Peierls Metal Agenda: Some History Part I: P = 0 (SP-CDW coupling) Part II: P ≠ 0 (Low Temp Metal and SC) • J. S. B, M. Almeida, L.L. Lumata, P. Kuhns, A. Reyes, D. Graf, R. Henriques,L. Prettner (Green), J. Wright, and S. Brown Supported by NSF DMR-0602859 & 1005293 (JSB), by FCT (Portugal) PTDC/FIS/113500/2009 (MA), by NSF DMR-0804625 (SEB), and performed at the National High Magnetic Field Lab (supported by NSF DMR-0654118, by the State of Florida, and the DOE).
Quasi-one-dimensional organic conductor Perylene2[M(mnt)2] “L. Alcácer Salts”: Mol. Cryst. Liq. Cryst. 120, 221(1985) (mnt =maleonitriledithiolate) a = 16.612 Å ; ta = 2 meV b = 4.1891 Å : tb=150 meV c = 26.583 Å; tc = 0 meV Canadell et al., Eur. Phys. B 42, R453(2004).
In case you want to sleep through the history, here is the message: p 1/4 filled band - conductor Tetramerization - Peierls (CDW) d ½ filled band - insulator Dimerization - spin Peierls when S = ½ M b Main Result: A CDW forms on the Perylene Chains A Spin Peierls state forms on the M(mnt)2 chains with S ≠ 0. The two transitions are coincident. Why? CDW SP 2b CDW 4b Tetramer Dimer
Ln(s) Ni Cu Pd 1/T x 103
M-I Transition Magnetic Transition s /sRT cp 10-4/mole T (K) T (K)
l Dimerization of spin 1/2 d-electron chain.
Identification of magnetic transition as Spin-Peierls associated with the Pt Spin ½ chains (consistent with XRD).
Per2[Pt(mnt)2]: Spin-Peierls + CDW system also shows similar B dependence. Au Pt
Eq. 2 Eq. 1
B // chains B chains CDW induces SP B influences CDW-SP coupling
Graf et al., Phys. Rev. B 69, 125113 (2004) High field phase diagram for Per2[Au(mnt)2] Finally, our group did something!
Graf et al., Phys. Rev. Lett. 93, 076406 (2004). Per2[Pt(mnt)2] Second high field, high resistance phase?
~ 10 K • Large difference in the nature of the T-B phase diagrams determined from transport studies at high fields indicates the possible role of SP chains in the suppression of the CDW state. M = Au M = Pt How can we independently monitor the field dependence of the Spin Peierls chain to see what it is doing?
two-chain Proton (1H) NMR – Strongly influenced by Pt spin state.
Strategy: Study the 1H and 195Pt NMR signals with field and temperature, and compare it with the transport data. 1H 195Pt p SP The localized spin ½ electron at the Pt(mnt)2 site gives rise to the spin-Peierls behavior. CDW Electrical conductivity probes the perylene stacks. CDWSP Protons on the perylene are the links to the Pt(mnt)2 anions.
1H spectra change dramatically at SP transition. T-Dep B-Dep Multiple spectral lines: paramagnetic Single spectral line: spin singlet (SP) E. L. Green et al., PRB (Rapid), in press.
1H NMR results for Per2[Pt(mnt)2] T-B Phase Diagram: CDW – Transport SP – 1H NMR SP Boundary Follows CDW Boundary to First Critical Field region ~ 20 T. Strong coupling. E. L. Green et al., PRB (Rapid), in press.
The larger picture: Phase diagram for both S=0 and S = ½ cases CDW Second moment analysis of high field spectra indicate that SP spin singlet state is breaking down and system is becoming spin polarized. Torque magnetization corroborates this process. B (T) Spin chain moment B (T)
Part I summary (P = 0) • There are several unsolved questions: • What is the mechanism for the coupling of the SP and CDW chains/order parameters? (only one (?) theory has treated it) • Who drives who? Is the CDW necessary for the SP to form? (Mostly based on interpretation of experimental results.) • What is the origin of the “FISDW” high field phase? (Several theories and speculations)
Theory: Dimerization induced by the RKKY interaction, J. C. Xavier, R. G. Pereira, E. Miranda, I. Affleck, Physical Review Letters, 90, 247204 (2003). Model: One dimensional S=1/2 Kondo model with L sites. is the conduction electron spin operator. Kondo coupling J > 0 Dimerization of the S=1/2 spin system at ¼ filling is determined from the order parameter:
Dimerization induced by the RKKY interaction, J. C. Xavier, R. G. Pereira, E. Miranda, I. Affleck, Physical Review Letters, 90, 247204 (2003). • Results of theory: • Relevant to Per2[Pt(mnt)2] • 1D suppresses SDW • Small energy scale consistent with suppression by field. • Opens a charge gap as well (i.e. like CDW) at ¼ filling. • RKKY drives the dimerized spin + charge gap (“SP+CDW”) transition. Need a two chain theory where S and s are on different chains.
2) Who drives who? • CDW and SP form at same Tsp-cdw • CDW driven: • CDW can form in absence of spin chain. • Coulomb interactions when CDW forms may drive dimerization in SP chains. • NMR & transport: SP order parameter seems to develop fully slightly later that CDW does. • SP driven: • Xavier et al. – RKKY • SP seems to “pull down” CDW transition: For M = Au, Tcdw = 12 K; for M = Pt, Tsp-cdw = 8 K.
3) What is the origin of the “FISDW” high field phase? • Nesting (after restoration of metallic phase) – but only weakly orbital • Lebed(JETP): (“Gorkov-Lebed”) - TFICDW ~ 0.1 K , but THF ~ 4 K • Lebed(PRL): Zeeman splitting of 4 bands where original CDW nesting condition is • restored – but why in M=Pt but not M=Au – higher fields? • Restoration of non-magnetic CDW system when SP is spin polarized – however, • SP and CDW order parameters appear to be attractive, not repulsive. R. McDonald, PPHMF & private communication.
Can’t detect this at low T by Fermiology due to CDW formation at higher temperatures. Part 2 (P ≠ 0) The Metal Try to get rid of CDW with Pressure Canadell et al., Eur. Phys. B 42, R453(2004). Pressure dependence in Per2[M(mnt)2] is non-trivial.
Counteriondimerisation effects in the two-chain compound (Per)2[Co(mnt)2]: structure and anomalous pressure dependence of the electrical transport properties M. Almeida, V. Gama, I. C. Santos, D. Graf and J. S. B., CrystEngComm, 2009, 11, 1103–1108 This anomalous behaviour can be understood as a consequence of a change of the perylene molecule overlap due to a transverse sliding of molecules along alternated directions of their planes imposed by the dimerised anion stacks. P (kbar)
T TMI& TR Au Pt Log(R/R0) P 1/T
Evolution of superconductivity from a charge density wave ground state in pressurized (Per)2[Au(mnt)2] D. Graf , J.S.B., M. Almeida, J.C. Dias, S. Uji, T. Terashimaand M. Kimata, Euro Physics Letters 85 27009/1-5(2009). Metallic at 5.3 Kbar – slow cooled! Bakrim and Bourbonnais, Supeconductivity close to the charge-density-wave instability, Euro Phys. Lett. 90, 27001(2010).
Superconductivity close to the charge-density-wave instability H. Bakrim and C. Bourbonnais, Euro Physics Letters 90, 27001(1-6)(2010).
D. Graf, J. S. Brooks, E. S. Choi, M. Almeida, R. T. Henriques, J. C. Dias, and S. Uji, Geometrical and orbital effects in a quasi-one-dimensional conductor, Physical Review B 80, 155104 (1-5)(2009). Per2[Au(mnt)2] 5 kbar Complex AMRO
Quantum Interference Orbits. D. Graf, J. S. Brooks, E. S. Choi, M. Almeida, R. T. Henriques, J. C. Dias, and S. Uji, Quantum interference in the quasi-one-dimensional organic conductor (Per)2Au(mnt)2 Phys. Rev. B 75, 245101/4(2007). (Per)2[Au(mnt)2]
Summary This two-chain highly one dimensional conductor comes in magnetic and non-magnetic flavors – Provides a huge variety of physical states and properties. – Surely there are many more surprises to come as theoretical and experimental methods advance. Immediate theoretical questions: SP-CDW coupling in a two-chain system. Step 1: B = 0. Step 2: B large. Chaikin: (TMTSF)2ClO4 = Quantum Gravity JSB: Per2[M(mnt)2] = Dark Energy
Per2[Pt(mnt)2] M B c q a SP- CDW
very one dimensional organic conductors – Less is more J. S. Brooks1* and M. Almeida2* 1NHMFL/Physics, 1800 E. Paul Dirac Dr., Tallahassee FL, 32310 USA 2Instituto Tecnológico e Nuclear / CFMCUL, Estrada Nacional no 10, P-2686-953 Sacavém, Portugal In this talk, we present a summary of recent work under “extreme conditions”, meaning high fields, low temperatures, and high pressure where organic conductors in the class (Per)2[M(mnt)2] do some pretty amazing things. Here M can be a spin = 0 (Au, Cu, Co), or a spin = 1/2 (Pt, Pd, Ni, Fe) metal ion. The work to be described, done by my group and collaborators, follows on nearly 30 years of previous, beautiful work by the Lisbon group and their collaborators that has been summarized in a relatively complete paper by Almeida and Henriques. Our more recent work has focused so far on the (Per)2[Au(mnt)2] S=0 , (Per)2[Pt(mnt)2] S=1/2 , and also (Per)2[Co(mnt)2] . In this presentation, for (Per)2[Au(mnt)2] and (Per)2[Pt(mnt)2], we will review the effects of high magnetic field on the charge density (CDW) and spin-Peierls (SP) ground states, the effects of pressure on these ground states, and the appearance of quantum interference, “magic angle effects”, and superconductivity (see also theory by Bakrim and Bourbonnais ) in (Per)2[Au(mnt)2] when the CDW is removed at high pressure. We will also review the unusual increase in the CDW transition temperature with pressure in (Per)2[Co(mnt)2]. The final topic in the presentation will focus on our most recent work involving 195Pt and 1H NMR in (Per)2[Pt(mnt)2] where we have tracked the spin-Peierls behavior of the [Pt(mnt)2] chains with field and temperature and have compared our results with previous electrical transport and magnetization studies of the CDW phase diagram under high magnetic fields. We will discuss these results in light of theoretical work that considers the interaction of itinerant conduction electrons and localized moments in quasi-one-dimensional systems. The overarching purpose of this presentation is to attract both the experimental and theoretical community to consider further work on these amazing systems that are clearly as rich in physical phenomena as the BEDT-TTF , TMTSF, and TMTTF materials. *Supported by NSF DMR-0602859 & 1005293 (JSB), by FCT (Portugal) PTDC/FIS/113500/2009 (MA), by NSF DMR-0804625 (SEB), and performed at the National High Magnetic Field Lab (supported by NSF DMR-0654118, by the State of Florida, and the DOE).