laboratory for quantum magnetism · laboratory for quantum magnetism ... scattering big magnets,...
TRANSCRIPT
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
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 ?
Quantum Phase Transitions
Classical phase
transition –
thermal fluctuations
Correlation
length ξT
Power-law scaling
Universality classes
Quantum phase transition
– quantum fluctuations (T 0)
Coherence length ξc
Universal scaling ?
QPT?
LiHoF4
Transverse field
Ising model
The world‟s
simplest QPT ?
(Sachdev ‟99)
Quantum Phase Transition in a Spin Bath !
Expected soft-mode transition at Hc
Incomplete softening
QC-scalingis fragile
Hyperfine coupling to nuclear-spin „bath‟
Minimum
gap at
finite T
„closest‟
to QCP
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 LiHoF4
Recover
“world‟s simplest
quantum critical point”
LiY0.95Ho0.05F4
spin glass
-hole burning
Clusters
Image with SANS
Correlated electron technologies
Metals Insulators
Discover, understand and control “new semiconductors”
Semiconductors
Bardeen‟s transistor
Quantum Magnetism
Theoretical models Novel materials
theNeutron Bulk methods scattering big magnets,
low temperature,
high pressure
of physics
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)
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, SrCu2(BO3)2, high pressure neutron
Goran Nilsen, PhD, Chemistry, new system synthesis
Conradin Kraemer, PhD (PSI) LiReF4, quantum phase transitions
Neda Nikseresht, PhD LiReF4, 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)F4
Bastien DallaPiazza TP4, Masters, PhD, inhomogenous meanfield theory
Laurent Cevey Staggiere TP4, Master novel superconductors
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• 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
• 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 (Cevey)
5. Simulation of novel neutron spectrometer
6. New iron-pnictide superconductors
7. Quantum criticality under pressure in CeCoGe3-xSix
8. Magnetometer design for Swiss company (non-disclosure restriction)
9. Nano-devices of correlated electron materials (collaboration with STI)
10. Quantitative crystal growth
Example: High-pressure susceptometry
• Current TPIV
project:
Laurent Cevey
Quantum phase transition
at 20-25kbar !
• design and build coils for
high-pressure susceptometer
• Measure SrCu2(BO3)2
• Compare to neutron and ESR
New cell:
30kbar
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
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
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