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Superfluidity in exciton bilayer systems: Josephson effect and collective modes as definitive identification-markers
Full text linkQuesta tesi indaga le principali impronte digitali della superfluidità nei sistemi a doppio
strato di eccitoni. La superfluidità a doppio strato di eccitoni è un fenomeno quantis-
tico innovativo nei sistemi con due strati semiconduttivi molto sottili, uno drogato con
elettroni e l’altro con lacune, separati da soli pochi nanometri. Previsioni teoriche re-
centi hanno delineato un diagramma di fase molto ricco per i sistemi a doppio strato di
eccitoni, con fasi superfluide, supersolide, di solido normale di eccitoni e di cristallo di
Wigner.
Con l’interesse crescente per le fasi condensate eccitoniche insieme alle difficoltà pratiche
di stabilire la loro esistenza attraverso misurazioni di trasporto, è cruciale identificare
marcatori chiari che segnalino in modo inequivocabile la superfluidità e la condensazione
coerente in questi sistemi. Questa tesi esplora due fenomeni che possono identificare tali
marcatori: l’effetto Josephson e i modi collettivi di densità.
Per l’effetto Josephson, proponiamo una giunzione Josephson a doppio strato di ec-
citoni in diclogenuri di metalli di transizione a doppio monostrato. Proponiamo una
misurazione sperimentale della corrente Josephson di eccitoni utilizzando il metodo di
Shapiro, e proponiamo un approccio sperimentale fattibile per la fabbricazione del dis-
positivo con un’altezza della barriera potenziale regolabile. Esaminiamo il sistema nelle
regioni di barriera potenziale alta e bassa, trovando comportamenti distinti nell’energia
della gap e nella velocità critica superfluida. Questo può aiutare a delineare il confine tra
i regimi di crossover BEC e BCS-BEC. Nella regione di barriera potenziale bassa, il super-
fluido di eccitoni può fluire sopra la barriera. Nella regione di barriera potenziale alta,
tuttavia, il flusso superfluido è guidato puramente dal tunneling quantistico delle cop-
pie elettrone-lacuna attraverso la barriera. Troviamo che il flusso superfluido si connette
senza problemi dalle regioni di barriera potenziale alta a quelle basse.
Per le modalità collettive di densità, esploriamo il loro comportamento a bassa tem-
peratura per identificare impronte digitali inequivocabili della transizione normale- su-
perfluida in funzione della densità. Ad alta densità, il sistema è nello stato normale e
risponde a piccole perturbazioni esterne di densità con modalità ottiche e acustiche a
bassa energia. Diminuendo la densità per entrare nella fase superfluida, la risposta del
sistema cambia drasticamente. La propagazione delle modalità acustiche e ottiche è bloc-
cata dalla presenza del gap superfluido nella relazione di dispersione della particella sin-
gola. Ci aspettiamo che, a causa della natura a raggio finito dell’interazione di Coulomb,
modalità collettive di rottura delle coppie appaiano all’inizio della superfluidità di ec-
citoni considerando le sole fluttuazioni di densità. Ciò è in contrasto con la teoria BCS
standard dove l’interazione di contatto rende necessaria l’inclusione delle fluttuazioni
dell’ampiezza del parametro d’ordine superfluido per osservare i modi collettivi di rot-
tura di coppie.
La nostra indagine è condotta utilizzando il campo medio. Il modello teorico, svilup-
pato da un approccio di integrazione sui cammini e dall’approssimazione di Hartree-
Fock, include gli effetti di screening e le correlazioni intra-strato. Le equazioni della
gap e del numero di particelle che governano il comportamento della fase superfluida
sono calcolate, e dimostriamo che le correlazioni intra-strato incrementano lo screening.
L’incremento è particolarmente forte nel regime di crossover BCS-BEC. Il risultato è una riduzione della gap superfluida, uno spostamento del confine di crossover da BEC a BCS-
BEC a densità significativamente più basse e la scomparsa di un minimo previsto nella
dimensione della coppia elettrone-lacuna in funzione della densità.
Questo studio avanza la comprensione della superfluidità nei sistemi a doppio strato
di eccitoni e fornisce previsioni teoriche e proposte sperimentali per indagini future.
Identificando marcatori chiari della superfluidità, il lavoro contribuisce allo sforzo più
ampio di realizzare e caratterizzare le fasi condensate eccitoniche in sistemi realistici.This thesis investigates key fingerprints of superfluidity in exciton bilayer systems. Ex-
citon bilayer superfluidity is a novel quantum phenomenon in semiconductor systems
with two very thin conducting layers, one doped with electrons and the other with holes
and separated by only a few nanometers. Recent theoretical predictions have outlined a
very rich phase diagram for exciton bilayer systems, with superfluid, supersolid, exciton
normal solid, and Wigner crystal phases.
With the expanding interest in the excitonic condensed phases together with the dif-
ficulties in practice of establishing their existence through transport measurements, it is
crucial to identify clear markers that unambiguously signal superfluidity and coherent
condensation in these systems. This thesis explores two phenomena that can identify
such markers: the Josephson effect and the density collective modes.
For the Josephson effect, we propose an exciton bilayer Josephson junction in double
monolayer Transition Metal Dichalcogenides. We propose an experimental measurement
for the exciton Josephson current using the Shapiro method, and we propose a feasible
experimental approach for fabricating the device with a tunable potential-barrier height.
We investigate the system in the high and low potential-barrier regions, finding distinct
behaviour in the gap energy and superfluid critical velocity. This can help delineate
the boundary between the BEC and BCS-BEC crossover regimes. In the low potential-
barrier region, the exciton superfluid can flow over the barrier. In the high potential-
barrier region, however, the superfluid flow is driven purely by quantum tunnelling of
the electron-hole pairs through the barrier. We find that the superfluid flow smoothly
connects from the high to the low potential-barrier regions.
For the density collective modes, we explore their low-temperature behaviour to iden-
tify unambiguous fingerprints of the normal-superfluid transition as a function of den-
sity. At high density, the system is in the normal state and it responds to small external
density perturbations with low energy optic and acoustic modes. Decreasing the density
to enter the superfluid phase, the system-response changes dramatically. The propaga-
tion of the acoustic and optic modes is blocked by the presence of the superfluid gap in
the single-particle dispersion relation. We expect that due to the finite-range nature of
the Coulomb interaction, pair-breaking collective modes will appear at the onset of the
exciton superfluidity considering the only density fluctuations. This is in contrast with
the standard BCS theory where the contact interaction makes necessary the inclusion of
the amplitude fluctuations of the superfluid order parameter to observe pair-breaking
collective modes.
Our investigation is carried out using mean-field. The theoretical model, developed
from a path-integral approach and the Hartree-Fock approximation, includes the effects
of screening and intralayer correlations. The gap and number equations governing the
superfluid phase behaviour are calculated, and we demonstrate that intralayer correla-
tions enhance screening. The enhancement is particularly strong in the BCS-BEC crossover
regime. The result is a reduction in the superfluid gap, a shift of the BEC to BCS-BEC
crossover boundary to significantly lower densities, and the vanishing of a predicted
minimum in the electron-hole pair size as a function of density. This study advances the understanding of superfluidity in exciton bilayer systems
and provides theoretical predictions and experimental proposals for future investiga-
tions. By identifying clear markers of superfluidity, the work contributes to the broader
effort of realizing and characterizing excitonic condensed phases in realistic systems.Deze thesis onderzoekt de belangrijkste kenmerken van superfluiditeit in excitoon dubbel-
laag systemen. Excitoon bilayer superfluiditeit is een nieuw kwantumfenomeen in halfgelei-
dersystemen met twee zeer dunne geleidende lagen, waarvan één gedoteerd is met elek-
tronen en de andere met gaten, gescheiden door slechts enkele nanometers. Recente the-
oretische resultaten voorspellen een zeer rijk fasediagram voor excitoon bilayersystemen,
met superfluïde, supersolid, excitoon normale vaste stof en Wigner kristalfasen.
Met de groeiende interesse in de excitonische gecondenseerde fasen, samen met de
moeilijkheden om hun bestaan vast te stellen door middel van transportmetingen, is het
cruciaal om duidelijke markers te identificeren die ondubbelzinnig superfluiditeit en co-
herente condensatie in deze systemen signaleren. Deze thesis onderzoekt twee fenome-
nen die dergelijke markers kunnen identificeren: het Josephson-effect en de dichtheid-
scollectieve modi.
Voor het Josephson-effect stellen we een excitoon bilayer Josephson-junctie voor in
dubbele monolayer overgangsmetaaldichalcogeniden. We stellen een experimentele met-
ing voor van de excitoon Josephson-stroom met behulp van de Shapiro-methode, en we
stellen een haalbare experimentele benadering voor om het apparaat te fabriceren met
een regelbare potentiaalbarrièrehoogte. We onderzoeken het systeem in de hoge en lage
potentiaalbarrière-regio’s en vinden distinct gedrag in de gapenergie en de superfluïde
kritische snelheid. Dit kan helpen om de grens tussen de BEC- en BCS-BEC crossover-
regimes af te bakenen. In de lage potentiaalbarrière-regio kan het excitoon superfluïdum
over de barrière stromen. Echter, in de hoge potentiaalbarrière-regio, wordt de super-
fluïde stroom puur aangedreven door kwantumtunneling van de elektron-gatenparen
door de barrière. We vinden dat de superfluïde stroom vloeiend overgaat van de hoge
naar de lage potentiaalbarrière-regio’s.
Voor de dichtheidscollectieve modi onderzoeken we hun gedrag bij lage temperatuur
om ondubbelzinnige kenmerken van de normaal-superfluïde overgang als functie van
de dichtheid te identificeren. Bij hoge dichtheid bevindt het systeem zich in de normale
staat en reageert het op kleine externe dichtheidsstoornissen met lage energie optische
en akoestische modi. Bij het verlagen van de dichtheid om de superfluïde fase binnen
te gaan, verandert de systeemrespons drastisch. De propagatie van de akoestische en
optische modi wordt geblokkeerd door de aanwezigheid van de superfluïde gap in de
enkelvoudige deeltjesdispersierelatie. We verwachten dat, door de eindige afstands-
bereik aard van de Coulomb-interactie, paarbrekende collectieve modi zullen verschij-
nen bij het begin van de exciton-superfluiditeit, waarbij alleen rekening wordt gehouden
met de dichtheidsfluctuaties. Dit is in contrast met de standaard BCS-theorie, waar de
contactinteractie het noodzakelijk maakt om de amplitude-fluctuaties van de superfluïde
ordeparameter op te nemen om paarbrekende collectieve modi te kunnen waarnemen.
Ons onderzoek wordt uitgevoerd met behulp van mean-field. Het theoretische model,
ontwikkeld vanuit een padintegraalmethode en de Hartree-Fock benadering, omvat de
effecten van afscherming en intralaagcorrelaties. De gap- en nummervergelijkingen die
het gedrag van de superfluïde fase bepalen, worden berekend, en we tonen aan dat in-
tralaagcorrelaties de afscherming versterken. De versterking is bijzonder sterk in het
BCS-BEC crossover-regime. Het resultaat is een vermindering van de superfluïde gap, een verschuiving van de BEC naar BCS-BEC crossover-grens naar aanzienlijk lagere dichthe-
den, en het verdwijnen van een voorspeld minimum in de elektron-grootte van het gaten-
paar als functie van dichtheid.
Deze studie bevordert het begrip van superfluiditeit in excitoon bilayersystemen en
biedt theoretische voorspellingen en experimentele voorstellen voor toekomstig onder-
zoek. Door duidelijke markers van superfluiditeit te identificeren, draagt dit werk bij aan
de bredere inspanning om excitonische gecondenseerde fasen in realistische systemen te
realiseren en te karakteriseren
International Advisory Board for International Conference on STRONGLY COUPLED COULOMB SYSTEMS 2011, Budapest, Hungary
No full textThe STRONGLY COUPLED COULOMB SYSTEMS 2011 international conference held in Budapest, Hungary, between 24 July and 29 July 2011 with 250 participants on scientific topics covering a wide range of phenomena taking place in many-body systems characterised by strong long-range interactions
Selection Committee for the Feenberg Memorial Medal in Many-Body Physics awarded at the Fifteenth International Conference on Recent Progress in Many-Body Theories - RPMBT15, Columbus, Ohio, USA from 27 to 31 July, 2009
No full textInternational Advisory Committee of the Seventeenth International Conference on Recent Progress in Many-Body Theories - RPMB17, Rostock, Germany, 2013
No full textProgram chairman, STRONGLY COUPLED COULOMB SYSTEMS 2011 international conference, Budapest, Hungary
No full textThe STRONGLY COUPLED COULOMB SYSTEMS 2011 international conference held in Budapest, Hungary, between 24 July and 29 July 2011 with 250 participants on scientific topics covering a wide range of phenomena taking place in many-body systems characterised by strong long-range interactions
Proceedings of the Australian Institute of Physics 15th Biennial Congress 2002
No full textNot availabl
Program Committee of the Fourteenth International Conference on Recent Progress in Many-Body Theories - RPMBT14, Barcelona, Spain, 2007
No full textChairman of the Selection Committee for the Feenberg Memorial Medal in Many-Body Physics awarded at the Sixteenth International Conference on Recent Progress in Many-Body Theories - RPMBT16, Bariloche, Argentina, Nov. 28 to Dec. 2 2011
No full textThe Feenberg Medal, or Feenberg Memorial Medal in Many-Body Physics is awarded for an important contribution to the research filed of many-body theory. The Feenberg Medal is awarded for work that is firmly established and that can be demonstrated to have significantly advanced the field of many-body physics. The Medal is awarded every two years. The ceremony for awarding the Medal is organised by the Selection Committee chaired by David Neilson and is held during the International Conference on Recent Progress in Many-Body Theories - RPMB
International Advisory Committee of the Sixteenth International Conference on Recent Progress in Many-Body Theories - RPMBT16, Bariloche, Argentina, Nov. 28 to Dec. 2 2011
No full textThe Conference continues the series initiated in Trieste, Italy in 1978 and is devoted to new developments in the field of many-body theories. A major aim of the conference series is to foster the exchange of ideas among physicists working in such diverse areas as nuclear physics, quantum chemistry, complex systems, lattice Hamiltonians, quantum fluids and condensed matter physics
International Advisory Committee of the Fifteenth International Conference on Recent Progress in Many-Body Theories - RPMBT15, Columbus, Ohio, USA, 2009
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