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Laboratory for
Quantum Magnetism
TP lab presentation 2009
Henrik M. Ronnow (EPFL since Jan. 2007)
How many body physics?
One of the simplest problems:
H = J Si Sj
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)
1023 spins: Antiferromagnet (Louis Neel 1932)
Fluctuating singlets (PW Anderson 1973,1987)
1023 – some electrons: High-Tc superconductivity
– THE enigma of modern solid state physics
CuO S= 1/22
A small contribution
There are fluctuating singlets in the
ground state !
Quantum Magnetism
Theoretical models Novel materials
theNeutron Bulk methods scattering big magnets,
low temperature,
high pressure
of physics
Physics of Interacting Systems
• A challenge on all length scales Classical n-body problem (from 3 to
galaxies)
Neural networks
Spin-models
= QUANTUM EFFECT ?
Maybe the “Big Bang” was powered by
“Vacuum Quantum Fluctuations” ?
(Hawkins et al.)
Novel electronic materials
• Strongly correlated electrons
• Often magnetism plays a (leading?) role - e.g.:High-Tc superconductors Colossal magnetoresistance
La2-xBaxCuO4 La2-2xSr1+2xMn2O7
Doped spin ½ antiferromagnets Intrinsic spin valves
Building models
• SpinsLength: |S|=1/2
Quantum / classical
Dimension: Ising, XY, Heisenberg
• Architecture
Dimension
Connectivity
• InteractionsCu2+ O 2px Cu 3dx2-y2
H = J Si Sj
Anti-/Ferromagnetic
• Extentions
Randomness
Charge, orbit, lattice...
HsatCuGeO3
(Hpip)2CuBr4
(d6-5CAP)2CuCl42DHAF
CuGeO3
Magnetic measurementsM
ag
ne
tization
S
usceptibili
ty
NM
R, μ
SR
etc
. S
pecific
heat
Neutron Scattering
intensity cross-section correlation function wave-function overlap
2
f
2
0)(),(f
fE
dEd qSfSdEd
dI Q
Experiment Theory
fi kkQ
m
k
m
k
22
2
f
22
i
2
(Crystal) momentum transfer
Energy transfer
Mais les Neutrons, ils sont où ?
All ways lead to Rome…
Reactor or spallation sources:
6-10 in Europe
~2008 next-generationin US & Japan
European Spallation
Source (ESS) ?
Last decade:x10 in fluxx10 in detection
Can study samples and phenomena not previously possible
ILL, Grenoble
EPFL
SINQ, PSI
Bern
Start: Villigen
Via: Lausanne
Ziel: Grenoble
400.2 km 3:04 h
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 ?
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)
– Theory and simulations
some stars of LQM:
Neutrons
Low-T & high-P
Susceptibility and Specific heat
Metal-organic and hydrothemal synth
Real-time Laue,
0.3K SQUID end mar
„Horizontal team strategy‟ – competence based
The laboratories:
• Halle Bernard Vittoz
– 9 tesla cryomagnet
– Dilution fridge
– Dip-stick, 3He
Susceptometry,
specific heat,
high-pressure
Future:
– New magnetometer
– 18 tesla system
– 400μW fridge
The laboratories:
• The abyss (hosting the SQUID magnetometer)
– SQUID magnetometer
– Synthesis lab.
Copper
acetate
Cu(C5D5NO)6(11BF4)2
Synthesis
Crystal growth
SamplesMeasurements
TP-projects
• 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 SrCu2(BO3)2
5. Simulation of novel neutron spectrometer
6. New iron-pnictide and dichalcogenide superconductors
7. Theoretical modeling of spin systems
8. Magnetometer design for Swiss company (non-disclosure restriction)
9. Nano-devices of correlated electron materials (collaboration with STI)
10. New materials synthesis and quantitative crystal growth
Visit lqm.epfl.ch/publications for examples of past reports
Example: High-pressure susceptometry
• Recent TPIV
project:
Laurent Cevey
Quantum phase transition
at 20-25kbar !
• magnetometer based on
commercial hall probes
• Measure SrCu2(BO3)2
• Compare to neutron and ESR
New cell:
30kbar
New materials synthesis
Simulation of Novel Neutron Spectrometer
Continuous Angle Multiple Energy Analysis (CAMEA)
Hybrid for mapping excitation spectectra:
• 60º continuous angle coverage x15(over conventional TAS)
• 5 successive analysers x 4.5
• Better resolution x 3
• Estimated improvement x 200 !
• Prove improvement
• Develop actual design
Sample analysers
detectors
TP-projects
• General philosophy:
– 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