Laboratory for Quantum Magnetism - EPFL · Quantum Magnetism – an arena for quantum phenomena 1) Model and Materials Spin, interactions dimension frustration 2) Theoretical methods

Henrik M. Ronnow – EPFL 2011 Slide 1

Laboratory for Quantum Magnetism Quantum spins and correlated electrons

Neutron Scattering and low-temperature physics

Henrik Moodysson Rønnow

Laboratory for Quantum Magnetism (LQM)

EPFL, Switzerland

Henrik M. Ronnow – EPFL 2011 Slide 2

Complexity of many-body systems

• Structure of a protein

• Pop2p-subunit Jonstrup et al (2007)

• Mega-Dalton:

~1’000’000 atoms (5 colors?)

~3’000’000 numbers needed

to describe the structure

Ground state of a magnet H = J Si Sj

1 spin: trivial

2 spins: singlet state |↑↓ - |↓↑

4 spins: back-of-the-envelope calc.

16 spins:10 seconds on computer (4GB)

40 spins: World record:1’099’511’627’776 coefficients needed to describe a state

Classical: 3N Quantum: 2N

1023 spins:

1D: analytic solution (Bethe 1931)

2D: antiferromagnet (Néel 1932) or

fluctuating singlets? (Anderson 1973,1987)

1023 ±some electrons:High-Tc superconductivity

– THE enigma of modern solid state physics

CuO S= 1/22

Henrik M. Ronnow – EPFL 2011 Slide 3

Spin – the drosophila of quantum physics

Spin: an atomic scale magnetic moment

• Quantization: S=0, 1/2, 1, 3/2,…..∞

• Superposition: |ψ = |↑ + |↓

likelihood of up: ρ(↑) = |↑|ψ|2 = 2

• Quantum fluctuations

average moment Sz = 0

imagine that spins fluctuate in ‘imaginary time’

• Quantum correlations e.g. two spins ‘entangled’ |ψ = ( |↑↓ - |↓↑ ) /√2 this is why 2N, not N

+1/2

- 1/2 S = 1/2

Henrik M. Ronnow – EPFL 2011 Slide 4

Quantum Magnetism – an arena for quantum phenomena

1) Model and Materials

Spin, interactions

dimension

frustration

2) Theoretical methods analytic approximations

numerical simulations

The

of many body physics

3) Experimental tools:

Bulk probes

Neutron scattering

Phenomena: Order, phase transitions,

quantum fluctuations, collective excitations, entanglement …

Henrik M. Ronnow – EPFL 2011 Slide 5

Hsat CuGeO3

(Hpip)2CuBr4

(d6-5CAP)2CuCl4

2DHAF

CuGeO3

Magnetic measurements M

ag

netization

S

usceptibili

ty

NM

R,

μS

R e

tc. S

pecific

heat

Henrik M. Ronnow – EPFL 2011 Slide 6

Neutron scattering – an intense future European Spallation Source (ESS)

1.5b€, almost certain to happen

(now: design update phase)

Switzerland will contribute 3-4%

Increased Swiss neutron scattering

CH-DK design 5 instruments and will

bid for constructing and operating 2

CAMEA: 102 -104 over swiss best now

SNS J-Parc ESS

• 1st generation facilities:

– General purpose research reactors

• 2nd generation facilities:

– Dedicated to neutron scattering:

– ILL, France, FRM2 Munich, SINQ CH, ISIS, UK etc.

• 3rd generation facilities:

– SNS, US 1.4b$, commission 2006

– J-Parc, Japan 150b¥, commission 2008

– ESS, Sweden 1.5b€, start 2013, commission 2019

– China Spallation, start 2011, commission 2018

• 2nd to 3rd generation gains of 10-1000 times !

– Faster experiments, smaller samples, better data

– Time resolved physics, new fields of science

– New opportunities for EPFL life, mat-sci, chem

Henrik M. Ronnow – EPFL 2011 Slide 7

A unique tool: Neutron scattering

Conservation rules:

Sample Neutron

Momentum ħQ = ħki – ħkf

Energy ħω = Ei – Ef = ħ(ki2 – kf

2)/2mn

Spin S = σi – σf

We can control and measure these quantities !

scattering and conservation rules

Henrik M. Ronnow – EPFL 2011 Slide 8

Magnetic neutron scattering

Dipole interaction – electron spin and orbit moment

dipole factor

spin-spin

correlation function

magnetic

form factor

Fourier transform

pre factor

cross-section

Henrik M. Ronnow – EPFL 2011 Slide 9

Dynamic structure factor

Spin-spin correlation function

Dynamic structure factor

Fluctuation dissipation theorem gen. susceptibility

intrinsic dynamics response to perturbation

Theory !

Henrik M. Ronnow – EPFL 2011 Slide 10

Laboratory for Quantum Magnetism

• Brief overview of our activities:

Neutron scattering

Materials Discovery & Crystal Growth

In-house experiments

Modeling

Henrik M. Ronnow – EPFL 2011 Slide 11

LQM: Low temperature physics

AC-susceptibility

• Low-T0.03K to 18T

• Macroscopic dynamics:

spin-glass & domain-walls

• Superconductivity and

vortex dynamics

• 0.03K squid setup

Specific heat – under high

pressure

• Phase transitions

• Density of states

• Normally adiabatic

• We set up AC-Cp to

30kbar and 0.3K

DC-magnetization

• Basic characterization

• In-situ E-field

1keV/200 = 5meV/nm

• low-T 0.3K

• high-P 1-5GPa

J. White J. Piatek

I1 ()

I2 ()

Tac1 (2)

Tac2 (2) S. Zabihzadeh J. Larrea

S. Gerber

Henrik M. Ronnow – EPFL 2011 Slide 12

Mais les Neutrons, ils sont où ?

The future of neutron scattering

Reactor or spallation sources:

6-10 in Europe

1.4b$ SNS 2007

150bJPY J-parc 2009

1.48b€ ESS 2011-2019

European Spallation Source

This decade: x10 in flux x10 in detection

LQM instruments at PSI

Eiger, TASP

CAMEA ILL, Grenoble

EPFL

SINQ, PSI

Bern

Start: Villigen

Via: Lausanne

Ziel: Grenoble

400.2 km 3:04 h

Henrik M. Ronnow – EPFL 2011 Slide 13

Science examples Quantum magnets

• 2D square lattice the

(,0) anomaly

Neutrons+theory

• SrCu2(BO3)2:

new high-pressure

quantum phase

Superconductors

• Cuprates

• Iron-based

• Universal properties

• A hypothesis for new

high-Tc families

Dipolar magnets

• LiReF4

• Spin-bath in Lihof4

• AFM 2D criticality in

LiErF4

• Spin-glass, thermal

runaway

Cp(T,P)

Henrik M. Ronnow – EPFL 2011 Slide 14

Shastry-Sutherland model SrCu2(BO3)2

Shastry-Sutherland model SrCu2(BO3)2 Pressure reduces the gap

Frustration ‘decouples’ dimers

Singlet ground state for J’/J<0.7

exact solvable at 0.5

Possible intermediate

phase above 0.7

SrCu2(BO3)2 realizes this model

is close to critical ratio

Henrik M. Ronnow – EPFL 2011 Slide 15

• Specific heat: - Cp under pressure

in absolute units !

- Bump at 0 kbar

sharp at QPT

- New low-T

phase transition

valence bond solid

New quantum phase: plaquette singlet state

• 2 new excitations

– Correspond to excitations of a

‘plaquette singlet state’

T

P 0 kbar

20 kbar

2-triplon is catching up with 1-triplon

At 22 kbar gap seems unchanged, but there is a new low-

energy excitation.

Intensity is consistent

with a ‘plaquette-

singlet’ phase

But, there are two

ways of filling lattice

with plaquette singlets

M. Zayed PhD thesis 2010 J. Larrea et al. in preparation

Henrik M. Ronnow – EPFL 2011 Slide 16

DallaPiazza

S=1/2 square lattice – spinons in 2D ! Physical realizations

Variational Neel + Valence-Bond states

(collaboration D. Ivanov)

Hubbard heritage

B. Dalla Piazza et al arxiv 1104.4224, collaboration w. Prof. Grioni

Project onto Mott insulator

unified fit to neutron & RIXS spin wave data, achieve

most accurate Hubbard parameters for cuprates

Consolidates: Neutron, RIXS, ARPES and Raman results

spin wave excitations

Next: spinons in 2D

Get wave-function from mean-field

Hamiltonian, minimize and excite with

real Hamiltonain

La2CuO4 CFTD CAPCuBr CAPCuCl

J [K] 1500 73.3 8.5 1.2

J’/J 5 x 10-5 4 x 10-5 ~ 0.1 ~ 0.1

TN [K] 325 16.4 5.1 0.64

HS [T] 4500 220 24 3.4

Henrik M. Ronnow – EPFL 2011 Slide 17

hourglass in FeTe0.7Se

• Iron based superconductors show

same hourglass dispersion as

cuprates

• Follow the same phenomenology:

commensuration, gap, resonance

• Hourglass is necessary condition

for high-Tc superconductivity

Henrik M. Ronnow – EPFL 2011 Slide 18

dipolar antiferromagnet

LiErF4

• Simple exact model:

• Surprising 2D criticality LiHoF4

Appeared Friday 15th June:

Henrik M. Ronnow – EPFL 2011 Slide 19

DallaPiazza

LQM: Modeling LiREF4:

• quantum dipoles

• Ising (Ho) and

• XY (Er)

• Nuclear spin order

• Quantum and classical phase transitions

• Re-entrant spin-glass in LiHo1-xYxF4 and LiHo1-xErxF4

• Inhomogeneous meanfield: GPU simulation to 2x106 sites (needed because dipole coupling is 3D and long ranged)

• Dynamics: neutron spectroscopy and random phase approx

Variational Neel + Valence-Bond states

(collaboration D. Ivanov)

Hubbard heritage

B. Dalla Piazza et al arxiv 1104.4224, collaboration w. Prof. Grioni

Project onto Mott insulator

unified fit to neutron & RIXS spin wave data, achieve

most accurate Hubbard parameters for cuprates

Consolidates: Neutron, RIXS, ARPES and Raman results

spin wave excitations

Next: spinons in 2D

Get wave-function from mean-field

Hamiltonian, minimize and excite with

real Hamiltonain Theory Exp 2008 Exp 2012

25eV gap

Henrik M. Ronnow – EPFL 2011 Slide 20

LQM: Materials discovery and crystal growth

Metal-organic material design (MOF) Exchange engineering:

New material: ‘C3PO’

ERC sub-project: E-field control of polar ligands

Rapid Hamiltonian change

quantum quench

collective out-of-equilibrium physics

x1000 in energy scale but realize same model:

2D S=1/2 square lattice antiferromagnet

• La2CuO4: Hubbard heritage

• CFTD: ZB quantum effect

• CAPCuCl: field-induced magnon decay

La2CuO4

J=1500K

Cu(DCO2)4D2O

J=70K Hs=220T

CAP2CuCl4

J=1K Hs=4T

GJ Nilsen

a

b

J

Aromatic overlap

Cu(3-methylpyridine-N-oxide)4(BF4)2

Different J’s 1.75:1:0.7:0.7

May be tunable to valence-bond solid

Henrik M. Ronnow – EPFL 2011 Slide 21

LQM: Materials discovery and crystal growth

Metal-oxide crystal growth:

• SrCu2(BO3)2

– Pressure tuned quantum phases

• BiCu2PO6

– Spin ladder, frustration

incomensurability

• Sr14-xCaxCu24O41

– Hole-doped spin ladder

superconductor for

x>12 & P>4GPa

• La2-2xSr1+2xMn2O7

– Intrinsic spin valves

– Micro-fab devices,

Shuang Wang, Image furnace growth

Collab.: Conder, Pomjakushina, Deng, PSI

Arash Omrani, collab. Prof. A. Kis, STI

0 50 100 150 200 250 3000

0.5

1

1.5

2

2.5

3

3.5

4

4.5x 10

8

T(K)

R(O

hm

s)

I=10nA

I=5nA

I=3nA

Ronnow et al. Nature 2006

Henrik M. Ronnow – EPFL 2011 Slide 22

Past TP4 and project students

• Julian Piatek: sub-K susceptometry Master PhD on Li(Ho/Er)F4

• Bastien Dalla Piazza: adiabatic cooling, spin-theory Master PhD

• Saba Zabihzadeh: high-P (T) and Cp Master LQM

• Laurent Cevey: high-P susceptometry Master in Beijing Swiss consulat

• P-F Duc: commercial M(H) solution Master in Beijing

• Sarah Debler: Hall-probe susceptometer Master (Munich Centre for Advanced Photonics)

• Visit lqm.epfl.ch→publications for examples of past reports

Henrik M. Ronnow – EPFL 2011 Slide 23

TP4 and student projects in LQM

• General philosophy:

– Foreseeable outcome in one semester

– Related to real research (linked to ongoing projects)

– Can be extended to Diploma/Master’s project

– Defined together with student: screwdriver / keyboard / paper preferences

• More info: – Check lqm.epfl.ch

– Under publications you can see old tp4 reports

• If interested,

schedule a discussion

– [email protected]

Henrik M. Ronnow – EPFL 2011 Slide 24

LQM: Caroline Pletscher Secretary Nikolay Tsyrulin Pdoc Jonathan White Pdoc Julian Piatek PhDMaster Bastien D-Piazza PhDMaster Neda Nikseresht PhD Arash Omrani PhD Shuang Wang PhD Saba Zabihzadeh Master Pierre-F. Duc Master Mark de Vries PdocLeccturer Uni Edinburgh Ivica Zivkovic PdocZagreb Institute of Physics Julio Larrea PdocUni Vienna Conradin Kramer PhD Banking Mohamed Zayed PhD Lecturer Uni Quatar Goran Nilsen PhD Fellow ISSP Tokyo Martin Mourigal PhD Fellow IQM John Hopkins Laurent Cevey MasterBeijing Collaborators: EPFL: Grioni, Forro, Kis, Ivanov, Ansermet, Mila PSI: many incl Ruegg, Conder, Schmitt, Mesot3

Geneva: Giannini, Renner

People

Enderle, Harrison ILL France McMorrow, Aeppli LCN & UCL

Neutron and synchrotron access:

Funding: