Nu ser du det:Real-space observation af proton-tunneller med mange kroppe i vand-nanokluster

Der er mere ved kvantetunneling end man kan se - eller rettere, visualiseringsteknikken. (Kvantetunnel er et kvantemekanisk fænomen, hvor en partikel overgår gennem en klassisk forbudt energitilstand.) De fleste kvantetunnelsdiskussioner fokuserer på usammenhængende enkeltpartikeltunnel; på den anden side, kvantetunnel i forbindelse med protondynamik involverer normalt mange hydrogenbindinger på én gang, hvilket fører til det der er kendt som korreleret mangekropps-tunneling . (Mange-kropsproblemet refererer til egenskaberne ved mikroskopiske systemer, der er beskrevet af kvantemekanik, omfattende et stort antal interagerende partikler - det vil sige ≥ 3-som kan blive viklet ind.) Ulempen er, at mens enpartikeltunnel er godt forstået, tunneller med mange kroppe er stadig indhyllet i mystik. For nylig, imidlertid, forskere ved Peking University, Beijing rapporterede real-space observation af samordnet protontunnel i et cyklisk vandtetramer-et makromolekylært nanokluster bestående af fire vandmolekyler arrangeret i en sløjfe eller ring-ved hjælp af et kryogent scanningstunnelmikroskop (STM). Forskerne fandt ud af, at tilstedeværelsen af ​​Cl ^- chloranion (en negativt ladet chlorion) ved spidsen af ​​STM -spidsen kan enten forbedre eller undertrykke den samordnede tunneleringsproces baseret på koblingssymmetrien mellem ion og protoner, tilføjer, at deres arbejde muliggør kontrol af protonernes kvantetilstande med atomskala præcision.

Prof. Ying Jiang diskuterede papiret om, at han, Prof. En-Ge Wang og deres kolleger offentliggjort i Naturfysik , bemærker, at en af ​​de største udfordringer, de stødte på, var direkte at visualisere den samordnede tunneling af fire protoner i en individuel hydrogenbundet vandtetramer adsorberet på en guldunderstøttet halitfilm. "Et grundlæggende krav er at lokalisere protonernes position i det hydrogenbundne netværk i det virkelige rum, sådan at protonernes bevægelse kan spores, "Fortæller Jiang Phys.org . "Dette er ekstremt vanskeligt for alle mikroskoper på grund af protonernes lette masse og lille størrelse - og endnu værre er, at protonernes vandringsafstand over hydrogenbindinger er mindre end en ångström (10 ^-10 m). Som resultat, at angribe dette problem kræver ideelt set evnen til at få adgang til vandmolekylets indre frihedsgrad. Heldigvis, vi udviklede en ny submolekylær billeddannelsesteknik sidste år ¹ det giver os mulighed for at skelne mellem vandmolekylers orientering samt hydrogenbindingsretningen. "Denne teknik er det, der banede vejen for forskerne for at tage fat på protondynamikken i det hydrogenbundne netværk.

"Desuden, "Jiang fortsætter, "den samordnede tunneling eller mangekroppskorrelerede tunnel af protoner er ekstremt følsom over for atomskala-miljøet, og kan let blive forstyrret eller endda dræbt af sonderne. "Dette skyldes, at den samordnede tunneling af protoner er en sammenhængende kvanteproces, som kræver, at alle protoner har nøjagtig samme fase - og den asymmetriske kobling mellem proberne og protonerne kan ødelægge fasekorrelationen mellem protonerne og slukke den kollektive tunneling. "Med andre ord, man kan let ødelægge en sådan samordnet tunneleringsproces, bare fordi STM -spidsen ikke er i den rigtige position. Derfor, vi skal placere STM -spidsen meget præcist inde i vandtetrameren for at sikre den symmetriske kobling, hvor de fire protoner alle er lige forbundet med STM -spidsen. "At søge efter en så præcis position kræver stor omhu og tålmodighed:Hvis STM -spidsen kun er slukket med 10 picometers (10 ^-12 m), forskerne kan få helt andre resultater.

Et andet problem var at finde ud af, at tilstedeværelsen af ​​chloranionen ved spidsen af ​​STM -spidsen enten kan forstærke eller undertrykke den samordnede tunneleringsproces, afhængigt af detaljerne i koblingssymmetrien mellem Cl anionen og protonerne. "Jeg er nødt til at understrege, at det er langt vanskeligere at kontrollere den samordnede tunneling af protoner end blot at visualisere denne proces - det betyder, at man samtidig skal manipulere flere kvantepartikler i det virkelige rum." Det er, mens STM -spidsen ikke kun fungerer som en lokal sonde, men kan bruges til at manipulere de enkelte atomer eller molekyler på overfladen med atomskala præcision, at manipulere mange-kropstilstande er udfordrende. "Det er vigtigt altid at holde koblingsgeometrien mellem STM -spidsen og fire protoner symmetrisk under manipulationen, "Jiang påpeger." Ellers, den samordnede tunneling undertrykkes let eller endda slukkes. "

Ud over, Jiang fortsætter, Cl anionen ved spidsen er meget vigtig for at opnå en effektiv manipulation af den samordnede protontunnel. "For at være ærlig, vi lærte faktisk dette af en ulykke:I flere måneder, vi forsøgte at kontrollere den samordnede tunneleringsproces med et metalspids, men alle forsøg mislykkedes. En dag blev spidsen styrtet ned i det guldunderstøttede natriumchloridfilmsubstrat på grund af en defekt operation. Uventet, ved hjælp af dette 'dårlige' tip, Vi var i stand til at forbedre tunneleringshastigheden meget effektivt. "Forskerne fandt senere ud af, at dette skete, fordi spidsen opsamlede et chloratom fra natriumchloridoverfladen - og da chloratomet er elektronegativt, spidsen er negativt ladet. Den lange rækkevidde elektriske vekselvirkning mellem det negativt ladede chloratom og de positivt ladede protoner fører derefter til undertrykkelse af tunneleringsbarrieren.

"Uden Cl ^- tip, protonen skal rejse en lang afstand fra det ene vandmolekyle til det andet, og energibarrieren er ret høj. Ved at indsætte en chloranion mellem vandmolekylerne etableres en 'bro' for protonerne. Attraktionen af ​​Cl ^- hjælper protoner, som det var, og hjælper derved protonoverførselsprocessen, "Jiang forklarer, "Det er den fysiske analogi af, hvorfor energibarrieren undertrykkes af spids/protonkoblingen."

After this accidental tip crashing, the researchers invested quite some time to explore a controllable and reproducible way to functionalize the tip apex with a single chlorine atom. "We discovered that chlorine atoms on the sodium chloride surface seemed very 'tip-friendly.'" Once they manipulated a bare tip to closely approach the NaCl(001) surface – that is, one in which crystalline cleavage occurs parallel to the faces of a cube – and then applied the proper voltage pulse, the chlorine atom readily translocated onto the tip end and became very stable.

I øvrigt, further lowering of the tip height leads to continuous decrease of the barrier because the electric interaction gets stronger – and if the tunneling barrier can be suppressed to such an extent that the zero-point energy of the protons exceeds the barrier height, an extreme quantum effect – that is, complete quantum delocalization – may occur. "In such a case, " Jiang notes, "the protons are shared by two nearest-neighboring water molecules, and the originally asymmetric hydrogen bond then becomes symmetric. This is a much stronger quantum effect than quantum tunneling, which we are still struggling to explore."

A third challenge was tuning the Cl ^- /proton electric coupling in three dimensions with picometer precision. "It's no exaggeration to say that tuning the coupling of protons to the atomic-scale environment in three dimensions with picometer precision is not possible with any technique other than STM. With the combination of the tip height z and tip lateral position x, y , we can actually achieve any coupling geometry between the Cl anion and the protons." Due to the high stability of their STM, the precision for tuning the dimensions can get down to one picometer or better, which is essential for controlling the many-body quantum states of protons. "We were very surprised to see that 10 picometer change in the tip height (z direction) can lead to almost one order of magnitude difference in the tunneling rate. This again shows the extreme sensitivity of the many-body tunneling to the atomic-scale environment, which has never been observed before."

The paper details how the scientists explored the role of individual chlorine anions in influencing the correlated tunneling process by using the Cl ^- -terminated tip, which if located at the exact center of the water tetramer, the Cl anion on the tip apex is equally coupled with the four protons and the cooperativity of the protons is reserved. ( Cooperativity is a phenomenon displayed by systems involving identical or near-identical elements, which act non-independently of each other, relative to a hypothetical standard non-interacting system in which the individual elements are acting independently.) "The tunneling probability can be greatly enhanced by the Cl ^- /proton electric attraction – but if the tip is slightly moved off the center at, for eksempel, the picometer scale, asymmetric coupling occurs. If that occurs, even if the Cl ^- /proton electric attraction is still present, the phase coherence between the protons can be easily destroyed due to inequivalent coupling between the protons and the chlorine anion. In such a case, the four protons can hardly move at the same pace and one would expect a rapid quenching of the correlated tunneling process."

When asked about the significant implications and potential applications of controlling the quantum states of protons with atomic-scale precision as made possible by their work, Jiang told Phys.org that the ability to control the quantum states of protons "can certainly improve our understanding of the role of quantum mechanics in proton dynamics, such as phase transition in ices of high-pressure phases. It may also provide completely new routes for the design of new energy, new medicine and new functional materials related to proton transfer."

Jiang adds that a less straightforward but very ambitious application is quantum computing. "The two many-body states of the four protons can be adopted to build a qubit, which is essential in quantum computing. If there is a way to decouple the water tetramer from the environment, we should be able to observe the superposition of the two many-body states. Imidlertid, the biggest challenge lies in how to realize coherent control on and readout of the two many-body states. Since scattering by tunneling electrons from the STM tip tends to destroy the quantum coherence of protons, it seems that we need to develop new techniques other than STM to realize such control."

Moving forward with their research, the scientists are now trying to build larger hydrogen-bonded water clusters on substrates to explore more novel correlated quantum behaviors of protons. "We're also curious about the upper limit of the number of protons at which cooperativity and tunnel collectively" – that is, correlated many-body tunneling – "can be maintained. Another thing we're planning to do is using an accurately-engineered STM tip to further suppress the tunneling barrier such that the zero-point motion of protons can surpass the energy barrier. We then expect to visualize the complete quantum delocalization at single proton level."

One innovation the researchers are interested in developing is achieving coherent control on the many-body quantum states of protons, as described above; another is improving the temporal resolution of their STM system, such that they can closely follow the coherent evolution of the many-body states in real time. "These new techniques may well make it possible to observe the Rabi oscillation of proton states, which is a common phenomenon for photon- or spin-based two-level systems." The Rabi oscillation, or Rabi cycle, is the cyclic behavior of a two-state (with non-equal energies) quantum system important in quantum optics, nuclear magnetic resonance and quantum computing that, in the presence of an oscillatory driving field, can become excited when it absorbs a quantum of energy.

As to other areas of research that might benefit from the study, Jiang tells Phys.org that "the improved understanding and the real-space control of correlated proton tunneling may have great impact in an extremely broad spectrum of research fields, such as phase transition, signal transduction, topological organic ferroelectrics, photosynthesis, and enzyme catalysis, to name just a few."

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