The topic of lattice quantum spin systems (or 'spin systems' for
short) is a f- cinating branch of theoretical physics and one of
great pedigree, although many
importantquestionsstillremaintobeanswered.
The'spins'areatomic-sizedm-
netsthatarelocalisedtopointsonalatticeandtheyinteractviathelawsofquantum
mechanics.
Thisintrinsicquantummechanicalnatureandthelarge(usuallyeff-
tivelyin
nite)numberofspinsleadstostrikingresultswhichcanbequitedifferent
fromclassicalresultsandareoftenunexpectedandindeedcounter-intuitive.
Spinsystemsconstitutethebasicmodelsofquantummagneticinsulatorsandso
arerelevanttoawholehostofmagneticmaterials.
Furthermore,theyareimportant
asprototypicalmodelsofquantumsystemsbecausetheyareconceptuallysimple
and yet stilldemonstrate surprisingly rich physics. Low dimensional
systems, in
2Dandespecially1D,havebeenparticularlyfruitfulbecausetheirsimplicityhas
enabledexactsolutionstobefoundwhichstillcontainmanyhighlynon-trivialf-
tures.
Spinsystemsoftendemonstratephasetransitionsandsowecanusethemto
studytheinterplayofthermalandquantum
uctuationsindrivingsuchtransitions.
Ofcoursetherearemanycasesinwhichwecan ndnoexactsolutionandinthese
casestheycanbeusedasatestinggroundforapproximatemethodsofmodern-day
quantummechanics. Thesequantumsystemsthusprovideagreatvarietyofint-
estinganddif cultchallengestothemathematicianorphysicalscientist.
Thisbookwaspromptedbyaseriesoftalksgivenbyoneoftheauthors(JBP)at
asummerschoolinJyvaskyla,Finland.
Thesetalksprovidedadetailedviewofhow
onegoesaboutsolvingthebasicproblemsinvolvedintreatingandunderstanding
spinssystemsatzerotemperature.
Itwasthislevelofdetail,missingfromothertexts
inthearea,thatpromptedtheotherauthor(DJJF)tosuggestthattheselecturesbe
broughttogetherwithsupplementarymaterialinordertoprovideadetailedguide
whichmightbeofuse,perhapstoagraduatestudentstartingworkinthisarea.
Thebookisorganisedintochaptersthatdeal rstlywiththenatureofquantum
mechanicalspinsandtheirinteractions.
Thefollowingchaptersthengiveadetailed
guidetothesolutionoftheHeisenbergandXYmodelsatzerotemperatureusing
theBetheAnsatzandtheJordan-Wignertransformation,respectively.
Approximate methodsarethenconsideredfromChap.
7onwards,dealingwithspin-wavet-
oryandnumericalmethods(suchasexactdiagonalisationsandMonteCarlo).
The
coupledclustermethod(CCM),apowerfultechniquethathasonlyrecentlybeen
vii viii Preface appliedtospinsystemsisdescribedinsomedetail. The
nalchapterdescribesother
work,someofitveryrecent,toshowsomeofthedirectionsinwhichstudyofthese
systemshasdeveloped.
Theaimofthetextistoprovideastraightforwardandpracticalaccountofall
of the steps involved in applying many of the methods used for
spins systems, especiallywherethisrelatestoexactsolutionsforin
nitenumbersofspinsatzero temperature.
Inthisway,wehopetoprovidethereaderwithinsightintothesubtle
natureofquantumspinproblems. Manchester,UK JohnB. Parkinson
January2010 DamianJ. J. Farnell Contents 1 Introduction ...1
References...5 2 Spin Models...7 2. 1 SpinAngularMomentum...7 2. 2
CoupledSpins...10 1 2. 3 TwoInteractingSpin- 'areatomic-sizedm-
netsthatarelocalisedtopointsonalatticeandtheyinteractviathelawsofquantum
mechanics.
Thisintrinsicquantummechanicalnatureandthelarge(usuallyeff-
tivelyin
nite)numberofspinsleadstostrikingresultswhichcanbequitedifferent
fromclassicalresultsandareoftenunexpectedandindeedcounter-intuitive.
Spinsystemsconstitutethebasicmodelsofquantummagneticinsulatorsandso
arerelevanttoawholehostofmagneticmaterials.
Furthermore,theyareimportant
asprototypicalmodelsofquantumsystemsbecausetheyareconceptuallysimple
and yet stilldemonstrate surprisingly rich physics. Low dimensional
systems, in
2Dandespecially1D,havebeenparticularlyfruitfulbecausetheirsimplicityhas
enabledexactsolutionstobefoundwhichstillcontainmanyhighlynon-trivialf-
tures.
Spinsystemsoftendemonstratephasetransitionsandsowecanusethemto
studytheinterplayofthermalandquantum
uctuationsindrivingsuchtransitions.
Ofcoursetherearemanycasesinwhichwecan ndnoexactsolutionandinthese
casestheycanbeusedasatestinggroundforapproximatemethodsofmodern-day
quantummechanics. Thesequantumsystemsthusprovideagreatvarietyofint-
estinganddif cultchallengestothemathematicianorphysicalscientist.
Thisbookwaspromptedbyaseriesoftalksgivenbyoneoftheauthors(JBP)at
asummerschoolinJyvaskyla,Finland.
Thesetalksprovidedadetailedviewofhow
onegoesaboutsolvingthebasicproblemsinvolvedintreatingandunderstanding
spinssystemsatzerotemperature.
Itwasthislevelofdetail,missingfromothertexts
inthearea,thatpromptedtheotherauthor(DJJF)tosuggestthattheselecturesbe
broughttogetherwithsupplementarymaterialinordertoprovideadetailedguide
whichmightbeofuse,perhapstoagraduatestudentstartingworkinthisarea.
Thebookisorganisedintochaptersthatdeal rstlywiththenatureofquantum
mechanicalspinsandtheirinteractions.
Thefollowingchaptersthengiveadetailed
guidetothesolutionoftheHeisenbergandXYmodelsatzerotemperatureusing
theBetheAnsatzandtheJordan-Wignertransformation,respectively.
Approximate methodsarethenconsideredfromChap.
7onwards,dealingwithspin-wavet-
oryandnumericalmethods(suchasexactdiagonalisationsandMonteCarlo).
The
coupledclustermethod(CCM),apowerfultechniquethathasonlyrecentlybeen
vii viii Preface appliedtospinsystemsisdescribedinsomedetail. The
nalchapterdescribesother
work,someofitveryrecent,toshowsomeofthedirectionsinwhichstudyofthese
systemshasdeveloped.
Theaimofthetextistoprovideastraightforwardandpracticalaccountofall
of the steps involved in applying many of the methods used for
spins systems, especiallywherethisrelatestoexactsolutionsforin
nitenumbersofspinsatzero temperature.
Inthisway,wehopetoprovidethereaderwithinsightintothesubtle
natureofquantumspinproblems. Manchester,UK JohnB. Parkinson
January2010 DamianJ. J. Farnell Contents 1 Introduction ...1
References...5 2 Spin Models...7 2. 1 SpinAngularMomentum...7 2. 2
CoupledSpins...10 1 2. 3 TwoInteractingSpin- 's...11 2 2. 4
CommutatorsandQuantumNumbers...14 2. 5 PhysicalPicture...16 2. 6 In
niteArraysofSpins...16 1 2. 7 1DHeisenbergChainwith S =
andNearest-Neighbour 2 Interaction...18 References...19 1 3 Quantum
Treatment of the Spin- Chain...21 2 3. 1 GeneralRemarks...21 3. 2
AlignedState...22 3. 3 SingleDeviationStates...23 3. 4
TwoDeviationStates...27 3. 4. 1 FormoftheStates ...33 3. 5
ThreeDeviationStates...36 Z N 3. 5. 1 BetheAnsatzforS = ?3...36 T 2
3. 6 StateswithanArbitraryNumberofDeviations...37 Reference...38 4
The Antiferromagnetic Ground State ...39 4. 1
TheFundamentalIntegralEquation...39 4. 2
SolutionoftheFundamentalIntegralEquation...43 4. 3
TheGroundStateEnergy...45 References...47 ix x Contents 5
Antiferromagnetic Spin Waves ...49 5. 1 TheBasicFormalism ...49 5.
2 MagneticFieldBehaviour ...
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