Magnetic surprises on a triangular lattice collin broholm johns hopkins university nist
Download
1 / 27

Magnetic Surprises on a Triangular Lattice Collin Broholm Johns Hopkins University & NIST - PowerPoint PPT Presentation


  • 119 Views
  • Uploaded on

Magnetic Surprises on a Triangular Lattice Collin Broholm Johns Hopkins University & NIST. Introduction Frustration Neutron Scattering Triangular Lattice AFM Theoretical Status Experimental Status Triangular lattice systems Neel (RbFe(MoO 4 ) 2 ) Glassy (NiGa 2 S 4 )

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about ' Magnetic Surprises on a Triangular Lattice Collin Broholm Johns Hopkins University & NIST' - olympia-daugherty


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
Magnetic surprises on a triangular lattice collin broholm johns hopkins university nist

Magnetic Surprises on a Triangular Lattice Collin BroholmJohns Hopkins University & NIST

Introduction

Frustration

Neutron Scattering

Triangular Lattice AFM

Theoretical Status

Experimental Status

Triangular lattice systems

Neel (RbFe(MoO4)2)

Glassy (NiGa2S4)

Conclusions


Collaborators
Collaborators

NiGa2S4

S. Nakatsuji

Y. Nambu

H. Tonomura

O. Sakai

C. Stock

S. Jonas

Y. Qiu

Y. Maeno

J. Chung

RbFe(MoO4)2

G. Gasparovic

M. Kenzelmann

S. Park

A. Smirnov

L. N. Demianets

A. Ya. Shapiro

G. Lawes

A. P. Ramirez

La4Cu3MoO12

Y. Qiu

S. Ishiwata

M. Azuma

M. Takano

R. Bewley

W. J. L. Buyers

Rutgers Seminar


Frustrated condensed matter
Frustrated Condensed Matter

Lattice symmetry established by chemical bonding (large energy

scales) can “frustrate” weaker magnetic interactions

“Satisfied”

“Frustrated”

Frustration produces classically degenerate states of matter with

a potential for “emergent” quantum properties

Rutgers Seminar


MACS spectrometer

under construction at NIST

pi

pf

ħQ

MACS spectrometer under construction at NIST

Rutgers Seminar


La 4 cu 3 moo 12 a lattice of spin 1 2 trimers

z=3/4 CuMoO plane

La4Cu3MoO12: A lattice of spin-1/2 trimers

Magnetic susceptibility

Crystal Structure

(Azuma et. al., PRB 62 R3588)

Rutgers Seminar


Frustrated quantum spin triangles

J2

J1

J

J

J3

J

Frustrated quantum spin triangles

Rutgers Seminar

Yiming Qiu et al. PRB 2005


Spectroscopy of spin trimers

0.2

10 K

Transition to quartet

0.1

0.0

70 K

0.1

0.0

Spectroscopy of spin trimers

Phonons

Rutgers Seminar

Yiming Qiu et al. PRB 2005



Strongly fluctuating spin trimer afm
Strongly fluctuating spin trimer AFM

Yiming Qiu et al. PRB 2005

Rutgers Seminar


Neutron Scattering

Stone et al. (2003).

Exact two-spinon

cross-section

Karbach et al. 2000

Rutgers Seminar


Frustrated linkage of triangles

Kagome lattice

Frustrated Linkage of Triangles

Triangular lattice

Rutgers Seminar


A brief status of triangular lattice research
A brief status of Triangular lattice research

  • Theory

    • RVB state proposed (Anderson 1973)

    • Classical Heisenberg model has 120o LRO at T=0 (Huse, Rutenberg)

    • S=1/2 model has LRO for T=0 (Huse, Singh)

    • Finite temperature KT transition (Kawamura & Miyashita)

    • RVB state in Ising model (Moessner and Sondhi)

  • Experiments

    • Spin-1/2 insulators generally have some form of dimerization and no LRO

    • Spin-1/2 κ(ET)2Cu2(CN)3close to MIT shows no spin order. It superconducts under pressure

    • Spin-1/2 anisotropic system has de-confined excitations in a field (Cs2CuCl4)

Rutgers Seminar


Rbfe moo 4 2 spin 5 2 triangular afm
RbFe(MoO4)2:spin-5/2 triangular AFM

J1’

J’

J

J2’

Kenzelmann, Gasparovic et al. (2005)

Rutgers Seminar


Field dependent long range order
Field Dependent Long Range Order

Kenzelmann, Gasparovic et al. (2005)

  • 120o structure

  • Field dependent Incommensurate stacking

  • Spins in triangular plane

  • Moment reduced to 75(1)%

  • Incommensurate state is ferroelectric!

Rutgers Seminar


Niga 2 s 4 spin 1 triangular lattice afm
NiGa2S4 : Spin-1 Triangular Lattice AFM

Nakatsuji et al. Science (2005)

J1

1 x 97o

2 x 2.42 Å

J2

2 x 100o

2 x 2.42 Å

1 x 3.21 Å

J3

2 x 138.5o

2 x 2.42Å

1 x 3.63

Rutgers Seminar


No phase transition as t q cw 0
No phase transition as T/QCW → 0

  • QCW=80 K so AFM

  • Cusp in c for T≈7 K

  • No conventional phase transition

  • Two maxima in C/T

  • Finite T entropy plateau

Nakatsuji et al. (2005)

Rutgers Seminar


Spin freezing without c t anomaly

Single crystal

Spin freezing without C(T) anomaly

Nakatsuji et al. (2005)

Rutgers Seminar


Short range magnetic correlations
Short range magnetic correlations

Nakatsuji et al. (2005)

Surprises:

Å

Rutgers Seminar


Single crystals confirm 2d incom sro
Single crystals confirm 2D Incom. SRO

C. Stock et al. (2005)

FM inter-plane correlations

Planar spin structure

Å

Rutgers Seminar


Excitations from low t state
Excitations from low T state

“Early” dispersion relation

What is clear so far:

  • Spin wave like modes at low T

  • A “slow” low E mode throughout zone

  • + A highly dispersive mode

Rutgers Seminar


Classical short range fluctuations at high t
Classical Short range fluctuations at high T

Correlations at entropy plateau:

  • Relaxation rate ħG<kBT

  • Correlation length ~ a

Rutgers Seminar


The weak response to h a
The weak response to H//a*

With increasing field:

  • Intra plane correlations unchanged

  • Reduce Inter-plane correlations

  • Moments more to plane

Rutgers Seminar


Frozen spin structure
Frozen spin structure

  • ≈120o structure on 2a super-lattice (colors)

  • Incommensurate modulation

  • Spins in triangular plane

  • Moment reduced to 75(8)%

  • Weak ferromagnetic inter-plane correlations

Nakatsuji et al. (2005)

Rutgers Seminar


J 1 j 3 model on triangular lattice
J1-J3 model on triangular lattice?

  • Accounts for

    • no peak

    • incommensurability

  • Plausible given likely exchange paths

  • Further confirmation needed

J1=0

J1=-0.2J3

Rutgers Seminar


Thought on quantum glassy phase
Thought on quantum glassy phase

  • Not conventional disordered glass:

    • Coherent wave propagation

    • No forward scattering

  • Impurities involved but unlikely to define short length scale

  • Puddles of AFM around impurities in isolated singlet ground state system

  • Gapped triplet sector gapless singlet sector pinned by impurities

Rutgers Seminar


Conclusions
Conclusions

  • Glassy phases endemic to real spin systems near quantum critical point

  • Distinguishing characteristics of glassy phase

    • Coherent modes despite short range spin-correlations

    • Absence of forward scattering

    • Gapless triplet spectrum

  • RbFe(MoO4)2

    • 120o structure for sufficient inter-plane coupling

    • Competing interplane interactions yield incommensurate ferroelectric phase

  • NiGa2S4

    • spin-1 system with potential for J1-J3 model

    • Glassy incommensurate phase at low T suggest weak interlayer coupling and weak or absent 2D Neel order

    • Double peak structure in C(T) indicates a gapped phase may be present in this problem

Rutgers Seminar


Future plans
Future plans

  • RbFe(MoO4)2

    • Explore multiferroic properties in particular electric field induced handedness of magnetic state

    • Establish exchange constants through spin wave measurements

  • NiGa2S4

    • Carrier doping may be possible.

    • Effects of uni-axial stress and pressure

    • Spin wave measurements to determine hamiltonian

    • Increase disorder (Zn, S)

  • Theoretical issues:

    • Spin-1 triangular lattice AFM: spin liquid?

    • J1-J3 triangular lattice model: entropy plateau?

    • Phenomenology of glassy quantum magnetism

Rutgers Seminar


ad