Encapsulating high-temperature superconducting twisted van der Waals heterostructures blocks detrimental eects of disorder Yejin Lee1 2Mickey Martini1 2Tommaso Confalone1 3Sanaz Shokri1 2

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Encapsulating high-temperature superconducting twisted van der Waals

heterostructures blocks detrimental eﬀects of disorder

Yejin Lee,1, 2 Mickey Martini,1, 2 Tommaso Confalone,1, 3 Sanaz Shokri,1, 2

Christian N. Saggau,1Daniel Wolf,1Genda Gu,4Kenji Watanabe,5Takashi Taniguchi,6

Domenico Montemurro,7Valerii M. Vinokur,8Kornelius Nielsch,1, 2, 9 and Nicola Poccia1

1Leibnitz Institute for Solid State and Materials Science Dresden (IFW Dresden), 01069 Dresden, Germany

2Institute of Applied Physics, Technische Universit¨at Dresden, 01062 Dresden, Germany

3Department of Physics, Sapienza University of Rome, 00185 Rome, Italy

4Condensed Matter Physics and Materials Science Department,

Brookhaven National Laboratory, Upton, NY 11973, USA

5Research Center for Functional Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan

6International Center for Materials Nanoarchitectonics,

National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan

7Department of Physics, University of Naples Federico II, 80125 Naples, Italy

8Terra Quantum AG, Kornhausstrasse 25, CH-9000 St. Gallen, Switzerland

9Institute of Materials Science, Technische Universit¨at Dresden, 01062 Dresden, Germany

(E-mail: y.lee@ifw-dresden.de)

High-temperature cuprate superconductors based van der Waals (vdW) heterostructures hold

high technological promise. One of the obstacles hindering the progress is the detrimental eﬀect

of disorder on the properties of the vdW devices-based Josephson junctions (JJ). Here we report

the new method of fabricating twisted vdW heterostructures made of Bi2Sr2CuCa2O8+δ, crucially

improving the JJ characteristics and pushing them up to those of the intrinsic JJs in bulk samples.

The method combines cryogenic stacking using a solvent-free stencil mask technique and covering

the interface by insulating hexagonal boron nitride crystals. Despite the high-vacuum condition

down to 10−6mbar in the evaporation chamber, the interface appears to be protected from wa-

ter molecules during the in-situ metal deposition only when fully encapsulated. Comparing the

current-voltage curves of encapsulated and unencapsulated interfaces, we reveal that the encapsu-

lated interfaces’ characteristics are crucially improved so that the corresponding JJs demonstrate

high critical currents and sharpness of the superconducting transition comparable to those of the

intrinsic JJs. Finally, we show that the encapsulated heterostructures are more stable over time.

Keywords: Van der Waals heterostructures, twisted high temperature superconductors, Josephson junctions, 2D

materials

I. INTRODUCTION

Layered quasi-two dimensional (2D) materials com-

prising the stack of monolayers held together by

van der Waals (vdW) forces can be cleaved via

a simple scotch tape exfoliation down to constitu-

tive monolayers [1]. High temperature superconduc-

tors (HTSC) provide a wide variety of such layered

correlated systems. Remarkably, even the atomically

thin Bi2Sr2CuCa2O8+δ(BSCCO) layers, i.e., the layers

containing a single or a few elementary cells, have been

found to possess the superconducting transition tem-

perature close to that of the bulk samples [2, 3] and

showed the superconductor-insulator transition driven by

the evolution of the density of states [4]. Because of

these properties, HTSCs can serve as starting building

blocks for the vdW heterostructures. However, isolat-

ing the cuprate single layers that hold superconductivity

remains a challenging task, especially if one wishes to re-

alize thin and crystalline-ordered interfaces. The point

is that the atomically thin BSCCO ﬂakes turn highly in-

sulating if contaminated with oxygen under the ambient

atmosphere [1, 5]. Raman measurements [5, 6] reported

high chemical activity of oxygen in thin BSCCO ﬂakes.

More detailed studies [7] revealed that water molecules

can also quickly deteriorate the surface of BSCCO ﬂakes.

In addition, oxygen dopants in cuprates are mobile above

200 K [8, 9], destroying high-quality superconductivity re-

quiring ordered distribution of oxygen defects [10, 11].

In comparison with the bulk crystalline order, the ro-

bustness of the spatially-correlated superlattice orders

in BSCCO down to a few unit cells is remarkable [12].

Thus, cryogenic temperatures and a well-controlled en-

vironment are necessary to prevent detrimental disorder

eﬀects in the spatially correlated super-lattices and to

freeze oxygen defects in their functional original posi-

tions for realizing the high quality cuprates-based vdW

heterostructures.

The possibility of making twisted HTSC-based het-

erostructures has attracted substantial interest because

of the d-wave pairing symmetry [13]. Previously twisted

BSCCO junctions using the bulk crystals or ﬂakes were

realized through an annealing process at high tempera-

ture in oxygen atmosphere [14, 16], which can reconstruct

the interfacial structure [15]. The obtained structures did

not demonstrate any angular dependence of the Joseph-

son current [14, 16]. Nevertheless, the non-monotonic an-

arXiv:2210.02124v2 [cond-mat.supr-con] 6 Mar 2023

gular behavior of the critical current has been reported

on the cross-whisker HTSCs junctions, with the critical

current being much reduced as compared to the critical

current of the bulk intrinsic junctions [17]. On the other

hand, the angular dependence of the critical current in

cuprate in-plane grain boundary junctions is indeed well

described by the d-wave pairing symmetry because of

the large in-plane coherence length as compared to that

in the out-of-plane junctions [18, 19], which reduces the

detrimental disorder eﬀects. However, the transmission

electron microscopy has shown that the grain-boundary

Josephson junctions (GBJJs) are generally composed of

facets in the range of 10–100 nm and demonstrate a

strong dependence of their properties upon the partic-

ular HTSC, the substrate, the conditions of the ﬁlm de-

position, and of the presence of defects [20]. Facets could

take place in all three dimensions as a consequence of the

adopted fabrication techniques for bicrystals, biepitaxial

growth, or step edges [21]. The GBJJs have been well

studied, and their electronic properties were found to be

controlled by the misorientation between two grains [22].

Because of that, the facets create additional complexity

and diﬃculties in controlling the JJs’ properties.

An angular dependence of the critical current of an

out-of-plane Josephson junction resulting in its change

over two orders of magnitude has been demonstrated

in thin BSCCO twisted heterostructures prepared by

the cryogenic stacking technique while preserving the

coherence of the crystalline and oxygen order at the

interface [23]. An additional important evidence of the

signiﬁcant reduction of the Josephson critical current

when changing the twist angle has been reported in [24].

It was found that the corresponding critical currents are,

on average, lower than the critical current of an intrinsic

Josephson junction for the BSCCO [23], given that the

fabrication did not occur under cryogenic conditions.

The general improvement of the control over the BSCCO

properties in the low-dimensional limit stimulated the

theoretical activities. The emergence of the topological

states in the twisted vdW heterostructures of HTSC

BSCCO layers with the d-wave superconducting order

parameter was suggested [25–30]. The twist angle close

to 45°was found to result in a time-reversal symmetry

(TRS) broken chiral superconducting dx2

−y2±idxy

phase, which was also reported at the intermediate

twist angles and was attributed to the unconventional

sign structure of the d-wave order parameter [26, 31].

Strong support for this theoretical proposal came from

the experimental detection of some new interfacial su-

perconductivity [23], manifesting as a dominant second

harmonic of the Josephson current close to 45°angle.

However, the TRS breaking in the high temperature

superconducting phase can be suppressed by strong

disorder at the interface [31], hence careful studies of

detrimental disorder eﬀects on the interfaces and novel

methods that rely on cheaper and/or innovative process

of fabrication are required.

II. FABRICATION

We fabricate six 0°Josephson junctions and one 43.2°-

twisted Josephson junction based on the optimally doped

BSCCO ﬂakes using a cryogenic dry transfer technique

in a pure argon atmosphere. This technique consists of

cleaving a sequential pair of fresh surfaces of the BSCCO

from the pre-exfoliated single crystal and stacking the

two resulting ﬂakes on top of each other. This procedure

of the junction fabrication is sketched in Fig. 1 and can be

described as follows. First, the BSCCO crystals are me-

chanically exfoliated using a scotch tape on the SiO2/Si

substrates, previously treated with oxygen plasma and

baked overnight to get rid of water molecules. Next, uti-

lizing the optical contrast, we identify the BSCCO ﬂake

with a thickness in the range between 80 and 100 nm and

cool the sample stack down to −90 °C, to preserve the

crystalline structure and the superconducting state of the

interface while building the junction. After that, we cover

the BSCCO ﬂake with an edge of a polydimethylsiloxane

(PDMS) stamp placed on a glass slide mounted on a mi-

cromanipulator and let the assembly thermalize. At the

temperature, being a little bit above the glass transition

of our PDMS (Tg=−120 °C), the stamp becomes very

adhesive. By quickly detaching the stamp from the sub-

strate, we cleave the crystal along a ﬂat plane between

the BiO planes, obtaining two thinner ﬂakes. The ﬂake

standing on the PDMS stamp is aligned and placed back

onto the bottom ﬂake on the substrate within 40 seconds.

The time between cleaving and stacking makes a huge

impact on the junction quality. We ﬁnd out that both

ﬂakes should be thicker than 30 nm, otherwise they are

not rigid enough to create a ﬂat interface without modu-

lating the surface. Finally, the stack is slowly heated up

to −30 °C and the top ﬂake is released from the bottom

one as the stamp is no longer adhesive. In Figure 1, the

bottom color bar illustrates representative temperatures

(eg. melting temperature TM) of the commonly used

polymers for exfoliation, such as polycarbonate(PC) and

polypropylene carbonate(PPC)[32]. The control over the

adhesion of the PDMS stamp allows us to fabricate the

junctions by taking advantage of a solvent-free and dry

transfer at low temperatures. In this way, the encapsu-

lated sample survives after being exposed to an ambient

condition for at least three hours.

For three out of the seven junctions (two at 0°, and one

twisted at 43.2°), we opt to additionally protect the inter-

face above the BSCCO heterostructures, especially from

water molecules by placing an encapsulating hexagonal

boron nitride (hBN) ﬂake on top of the stacked ﬂakes

immediately afterward. The bottom surface of the lower

ﬂake in the heterostructure is attached closely to the sub-

strate and, therefore, is not exposed to the ambient atmo-

sphere, which is critical for the degradation of BSCCO.

Electrical contacts are then deposited in two steps using

a chemical-free stencil mask technique [3] in an evapo-

ration chamber directly connected to the glovebox clus-

ter. First, gold electrodes are evaporated right on the

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Encapsulatinghigh-temperaturesuperconductingtwistedvanderWaalsheterostructuresblocksdetrimentaleectsofdisorderYejinLee,1,2MickeyMartini,1,2TommasoConfalone,1,3SanazShokri,1,2ChristianN.Saggau,1DanielWolf,1GendaGu,4KenjiWatanabe,5TakashiTaniguchi,6DomenicoMontemurro,7ValeriiM.Vinokur,8KorneliusNiels...

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Encapsulating high-temperature superconducting twisted van der Waals heterostructures blocks detrimental eects of disorder Yejin Lee1 2Mickey Martini1 2Tommaso Confalone1 3Sanaz Shokri1 2

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