Laboratory for Quantum Magnetism. TP lab presentation 2009 Henrik M. Ronnow (EPFL since Jan. 2007)

Size: px

Start display at page:

Download "Laboratory for Quantum Magnetism. TP lab presentation 2009 Henrik M. Ronnow (EPFL since Jan. 2007)"

Blaze Crawford
5 years ago
Views:

1 Laboratory for Quantum Magnetism TP lab presentation 2009 Henrik M. Ronnow (EPFL since Jan. 2007)

2 How many body physics? One of the simplest problems: CuO S= 1/2 2 H = J S i S j 1 spin: trivial 2 spins: singlet state - 4 spins: back-of-the-envelope calc. 16 spins: 10 seconds on computer 40 spins: World record! (A. Läuchli, EPFL) spins: Antiferromagnet (Louis Neel 1932) Fluctuating singlets (PW Anderson 1973,1987) some electrons: High-Tc superconductivity THE enigma of modern solid state physics

3 A small contribution There are fluctuating singlets in the ground state!

4 Quantum Magnetism Theoretical models Neutron scattering the of physics Novel materials Bulk methods big magnets, low temperature, high pressure

5 Physics of Interacting Systems A challenge on all length scales Maybe the Big Bang was powered by Classical Vacuum n-body problem Quantum (from Fluctuations 3 to? galaxies) (Hawkins et al.) Spin-models Neural networks = QUANTUM EFFECT?

6 Novel electronic materials Strongly correlated electrons Often magnetism plays a (leading?) role - e.g.: High-Tc superconductors Colossal magnetoresistance La 2-x Ba x CuO 4 La 2-2x Sr 1+2x Mn 2 O 7 Doped spin ½ antiferromagnets Intrinsic spin valves

7 Building models Spins Length: S =1/2 Quantum / classical Dimension: Ising, XY, Heisenberg Architecture Dimension Connectivity Interactions Cu 2+ O 2p x Cu 3d x2-y2 H = J S i S j Anti-/Ferromagnetic Extentions Randomness Charge, orbit, lattice...

8 Magnetization Susceptibility NMR, μsr etc. Specific heat Magnetic measurements CuGeO 3 H sat (d6-5cap) 2 CuCl 4 2DHAF CuGeO 3 (Hpip) 2 CuBr 4

9 Neutron Scattering intensity cross-section correlation function wave-function overlap 2 f 2 ) 0 ( ), ( f E f de d q S f S de d d I Q Experiment Theory k i k f Q m k m k f 2 2 i 2 (Crystal) momentum transfer Energy transfer

in US & Japan Bern SINQ, PSI European Spallation Source (ESS)?

10 Mais les Neutrons, ils sont où? All ways lead to Rome Reactor or spallation sources: 6-10 in Europe ~2008 next-generation in US & Japan Bern SINQ, PSI European Spallation Source (ESS)? EPFL Last decade: x10 in flux x10 in detection Can study samples and phenomena not previously possible ILL, Grenoble Start: Villigen Via: Lausanne Ziel: Grenoble km 3:04 h

11 Quantum Magnetism - Quo vadis? Entanglement & quantum information theory New notation or new resource? Quantum phase transitions: Different quantum phases, universal behaviour etc. Controlled quantum magnets: Pump dynamically to obtain and control new semiconductor Bulk Restricted geometries Finite size quantization devices? Driver of new theories and pictorial explanations Correlated Electron Technologies?

12 Quantum Phase Transitions QPT? Quantum phase transition quantum fluctuations (T 0) Coherence length ξ c Universal scaling? LiHoF 4 Classical phase transition thermal fluctuations Correlation length ξ T Transverse field Ising model The world s simplest QPT? (Sachdev 99) Power-law scaling Universality classes

13 Quantum Phase Transition in a Spin Bath! Expected soft-mode transition at Hc Incomplete softening Minimum gap at finite T closest to QCP QC-scaling is fragile Hyperfine coupling to nuclear-spin bath

14 Controlled Quantum Magnets Conventional parameters: Temperature, Field, Pressure Dynamical pumping : Laser orbitals (exchange, valence ) Light phonons Radio-frequency / microwaves nuclear & electronic spins Tuned systems, non-equilibrium physics, time-dependence NMR saturated nuclear spins in LiHoF 4 Recover world s simplest quantum critical point LiY 0.95 Ho 0.05 F 4 spin glass -hole burning Clusters Image with SANS

15 Correlated electron technologies Metals Semiconductors Insulators Bardeen s transistor Discover, understand and control new semiconductors

16 Quantum Magnetism Theoretical models Neutron scattering the of physics Novel materials Bulk methods big magnets, low temperature, high pressure

17 The laboratory Activities: ~ 40% neutron scattering (at international facilities) ~ 60% in-house activities Sample synthesis and study of new materials Sub-kelvin measurements (susceptibility etc.) High-pressure cells (quantum phase transitions)

visitor, Frustrated quantum magnets Ivica Zivkovic, Pdoc, Ruthanates,

Mohamed Zayed, PhD, SrCu 2 (BO 3 ) 2, high pressure neutron Goran Nilsen, PhD,

phase transitions Neda Nikseresht, PhD LiReF 4, quantum phase transitions

Arash Omrani, PhD, nano-transport devices of novel electronic materials Julian

18 People! Henrik M Ronnow (that s me) Caroline Pletscher, secretary Mark de Vries, visitor, Frustrated quantum magnets Ivica Zivkovic, Pdoc, Ruthanates, non-linear susceptibility Julio Larrea, Pdoc, high-pressure measurements Mohamed Zayed, PhD, SrCu 2 (BO 3 ) 2, high pressure neutron Goran Nilsen, PhD, Chemistry, new system synthesis Conradin Kraemer, PhD (PSI) LiReF 4, quantum phase transitions Neda Nikseresht, PhD LiReF 4, quantum phase transitions Martin Mourigal, PhD (ILL), low-d quantum magnets, neutron scatt. Arash Omrani, PhD, nano-transport devices of novel electronic materials Julian Piatek, TP4, Masters, PhD, low-t susceptibility, Li(Ho/Er)F 4 Bastien DallaPiazza TP4, Masters, PhD, inhomogenous meanfield theory Laurent Cevey Staggiere TP4, Master novel superconductors

19 Halle Bernard Vittoz 9 tesla cryomagnet Dilution fridge Dip-stick, 3 He The laboratories: Susceptometry, specific heat, high-pressure Future: New magnetometer 18 tesla system 400μW fridge

20 The laboratories: The abyss (hosting the SQUID magnetometer) SQUID magnetometer Synthesis lab. Copper acetate Synthesis Crystal growth Cu(C 5 D 5 NO) 6 ( 11 BF 4 ) 2 Measurements Samples

21 General philosophy: TP-projects Foreseeable outcome in one semester Related to real research (linked to ongoing projects) Can be extended to Dimploma/Master s project Defined together with student Past projects and present suggestions: 1. Synthesis of spin-dimer systems (Farley) 2. Adiabatic cooling for magnetometer (dalla Piazza) 3. Low-T susceptometer (Piatek) 4. High-pressure susceptometry of SrCu 2 (BO 3 ) 2 (Cevey) 5. Simulation of novel neutron spectrometer 6. New iron-pnictide superconductors 7. Quantum criticality under pressure in CeCoGe 3-x Si x 8. Magnetometer design for Swiss company (non-disclosure restriction) 9. Nano-devices of correlated electron materials (collaboration with STI) 10. Quantitative crystal growth

22 Example: High-pressure susceptometry Current TPIV project: Laurent Cevey Quantum phase transition at 20-25kbar! design and build coils for high-pressure susceptometer Measure SrCu 2 (BO 3 ) 2 Compare to neutron and ESR New cell: 30kbar

23 Square lattice antiferromagnet

Quantitative crystal growth Single crystals are prerequisite to most projects Crystal growth often a chemist s secret fingerspitzgefühl Apply physical approach: measure and

24 Quantitative crystal growth Single crystals are prerequisite to most projects Crystal growth often a chemist s secret fingerspitzgefühl Apply physical approach: measure and control Growth by evaporating solvent from saturated solution Controlled temperature gradient crystal grows on cold finger Optical monitoring transmission decrease towards saturation

25 Parallel processing simulation Last year Bastien dalla Piazza wrote inhomogenous meanfield simulation of quantum magnets To speed up, we want to use modern graphics card for parallelized simulation Need soemone with good computing skills

26 Simulation of Novel Neutron Spectrometer Continuous Angle Multiple Energy Analysis (CAMEA) Hybrid for mapping excitation spectectra: 60º continuous angle coverage (over conventional TAS) x15 5 successive analysers x 4.5 Better resolution x 3 Estimated improvement x 200! Prove improvement detectors Develop actual design Sample analysers

27 General philosophy: TP-projects Foreseeable outcome in one semester Related to real research (linked to ongoing projects) Can be extended to Dimploma/Master s project Defined together with student Any questions? If interested, schedule a discussion henrik.ronnow@epfl.ch

Laboratory for Quantum Magnetism. TP lab presentation 2009 Henrik M. Ronnow (EPFL since Jan. 2007)

Laboratory for Quantum Magnetism TP lab presentation 2009 Henrik M. Ronnow (EPFL since Jan. 2007) How many body physics? One of the simplest problems: CuO S= 1/2 2 H = J S i S j 1 spin: trivial 2 spins: