Superlattice material to filer electrons with high selectivity

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A chiral molecular intercalation superlattice (CMIS) designed by Duan’s group achieves a remarkable spin polarization ratio of more than 60%, among the highest selectivity achieved in molecular spin tunneling junctions.

Chirality and chiral-induced spin selectivity (CISS)

Chirality describes a molecule that can’t be superimposed on its own mirror image. Two geometrically different chiral molecules of the same formula, distinguished by the R- and S- configuration, exhibit different optical properties. More intriguingly, a material block made of the same chiral molecules can function like a security gate when electrons swarm through, only granting access to electrons with the same spin identity. That is, electrons in spin up state will make their way through the chiral molecules that favors spin up state, while electrons in the spin down state will get blocked and deflected, or vice versa. This intrinsic filtering effect known as chiral-induced spin selectivity (CISS) is of great interest for quantum information processing, where information is stored as spin charge. 

In this research published in Nature, researchers in Duan’s group designed a spin tunneling junction made of chiral molecular intercalation superlattices (CMIS), a structure that brings out the best of CISS. 

Unique structure: chiral molecular intercalation superlattices (CMIS)

A spin tunneling junction is a spin filter that researchers assemble to evaluate CISS and the performance of their chosen chiral material. The basic setup includes a metal electrode to conduct electricity, a ferromagnetic material that selectively controls the incoming current to be only in 1 spin state: either spin up or spin down. A block of chiral superlattice is sandwiched in between, of what design is the research ground for many. 

Traditionally, the filter structure is made from self-assembled molecular layers, which have chiral molecules (the “studs” in figure 1) spin coat directly onto the ferromagnetic material. The resulting quality is largely degraded by defects known as pinholes, which let opposite spin slip pass. Pinholes permeate as the number of studs increases, which limits the reach of maximal  spin selectivity.

Given the case, Duan’s group takes an innovative approach to make chiral molecular intercalation superlattices (CMIS) as the filter instead. Different from the traditional structure, a superlattice is a high order, periodic structure made of alternating layers of multiple materials. For their CMIS, the team has either a left handed R-α-methylbenzylamine (R-MBA) or the right handed S-α-methylbenzylamine (S-MBA) inserted in between the host layer of tantalum disulfide (TaS2) sheet, a synthetic process known as intercalation. 

“The superlattice works like stacking lego bricks on each other to make a multi-stage filter, this structure brings its spin selectivity to the next level.” Co-author Dr. Huaying Ren said. “It greatly minimize pinholes through the 2D protection layer”.

Figure 1. Components of spin tunneling junction with chiral molecular intercalation superlattices
(credit: Nature: News and Views)

Evaluation of filtering effect

Such a device creates an unprecedented plot of current vs. magnetic field that marks the breakage in electron filtering limit (Figure 2).

In Figure 2a, the superlattice is made of chiral molecule R-MBA intercalated into H- phase TaS2.  During the field sweep scan, when the magnetic field is greater than the coercive field of the Cr3Te4, the out-of-plane ferromagnetic ordering in Cr3Te4 switches abruptly, causing an abrupt change of the spin polarization and, thus, an abrupt change in the tunneling probability through the CMIS, resulting two extreme current states. Similar but opposite behavior is also observed when S-MBA chiral molecule was used as the chiral molecule. 

Figure 2. Magnetic-field dependent tunneling current measured in the a) R-MBA/H-TaS2 and b)  S-MBA/H-TaS2   (credit: the paper) 

By calculating the spin polarization ratio, the ratio between the two extreme currents and a key criteria to evaluate the performance of the device, 63% is reached. Considering the traditional approach can only reach a ratio of single digit, the current work is remarkably among the highest spin selectivity achieved. 

This exciting experimental result invites more investigation in the application of chiral molecular intercalation superlattices.

“The performance is highly specific to the materials we used, our next plan is to explore other possible chiral materials, 2D host material, and ferromagnet with further improved performance to enable practical applications.” Co-author Dr. Qi Qian said. 

About the Lead Authors

Dr. Huaying Ren received her Ph.D. degree in physical chemistry from Peking University in 2018. She is now a postdoctoral scholar in the Duan group. Her research at UCLA has focused on the synthesis and fundamental investigation of functional two-dimensional atomic crystal based superlattices, including the brand new family of chiral molecular superlattices. Ren recenty received the department’s 2022 Postdoctoral Research Award.

Dr. Qi Qian received her Ph.D. degree in condensed matter physics from Purdue University in 2018. She then worked as a postdoctoral research scholar at UCLA in the Duan group. Her research interests mainly focus on low-temperature spin and charge transport properties of van der Waals heterostructures and superlattices, III-V semiconductors and perovskite materials. 

Article by Zhuoying Lin (Duan Group), UCLA Department of Chemistry & Biochemistry, Lin is a chemistry graduate student and science writer who joined our program in Fall 2021.  Read more of Lin’s UCLA Chemistry & Biochemistry articles here.