Two-dimensional heavy fermion in a monoatomic-layer Kondo lattice YbCu2

概要

Title

Two-dimensional heavy fermion in a monoatomiclayer Kondo lattice YbCu2

Author(s)

Nakamura, Takuto; Sugihara, Hiroki; Chen, Yitong
et al.

Citation

Nature Communications. 2023, 14, p. 7850

Version Type VoR
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rights

https://hdl.handle.net/11094/93528
This article is licensed under a Creative
Commons Attribution 4.0 International License.

Note

Osaka University Knowledge Archive : OUKA
https://ir.library.osaka-u.ac.jp/
Osaka University

Article

https://doi.org/10.1038/s41467-023-43662-9

Two-dimensional heavy fermion in a
monoatomic-layer Kondo lattice YbCu2
Received: 21 June 2023
Accepted: 16 November 2023

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Takuto Nakamura 1,2 , Hiroki Sugihara2, Yitong Chen 2, Ryu Yukawa
Yoshiyuki Ohtsubo 4, Kiyohisa Tanaka 5, Miho Kitamura6,
Hiroshi Kumigashira 7 & Shin-ichi Kimura 1,2,5

3

,

The Kondo effect between localized f-electrons and conductive carriers leads
to exotic physical phenomena. Among them, heavy-fermion (HF) systems, in
which massive effective carriers appear due to the Kondo effect, have fascinated many researchers. Dimensionality is also an important characteristic of
the HF system, especially because it is strongly related to quantum criticality.
However, the realization of the perfect two-dimensional (2D) HF materials is
still a challenging topic. Here, we report the surface electronic structure of the
monoatomic-layer Kondo lattice YbCu2 on a Cu(111) surface observed by
synchrotron-based angle-resolved photoemission spectroscopy. The 2D conducting band and the Yb 4f state, located very close to the Fermi level, are
observed. These bands are hybridized at low-temperature, forming the 2D HF
state, with an evaluated coherence temperature of about 30 K. The effective
mass of the 2D state is enhanced by a factor of 100 by the development of the
HF state. Furthermore, clear evidence of the hybridization gap formation in the
temperature dependence of the Kondo-resonance peak has been observed
below the coherence temperature. Our study provides a new candidate as an
ideal 2D HF material for understanding the Kondo effect at low dimensions.

Heavy fermion (HF) systems in rare-earth (RE) intermetallic compounds originating from hybridization between localized f-electrons
and conduction electrons, namely c-f hybridization, are central
topics in the field of the strongly-correlated electron systems1. At low
temperatures, depending on the strength of the c-f hybridization, the
physical properties change from itinerant f electrons because of the
Kondo effect to a magnetic order originating with magnetic
moments of localized f electrons due to Ruderman–Kittel–
Kasuya–Yosida (RKKY) interactions. The competition between itinerant and localized characters of the f-electrons make a quantum
critical point (QCP), resulting in the emergence of fertile quantum
phenomena such as non-Fermi liquid behavior, and non-BCS HF
superconductivity2,3.

On the other hand, the dimensionality in the system characterizes
the fundamental physical property. In low-dimensional systems, the
enhancement of the electron-electron correlation and/or breaking of
the inversion symmetry leads to novel quantum states such as Rashbatype spin-splitting4, Tomonaga–Luttinger liquid5,6, and unconventional
superconductivity7,8. The combination of the HF state and low
dimensionality modifies the ground state of the system because the
order parameter of these systems is much more sensitive to
dimensionality9,10. The ground state of two-dimensional (2D) HF can be
easily controlled to the vicinity of a quantum critical point, which is the
host to realize unconventional physical properties such as HF superconductivity, by simple external fields such as gate-tuning11,12, and
surface doping13 in addition to traditional external perturbations;

1

Graduate School of Frontier Biosciences, Osaka University, Suita 565-0871, Japan. 2Department of Physics, Graduate School of Science, Osaka University,
Toyonaka 560-0043, Japan. 3Graduate School of Engineering, Osaka University, Suita 565-0871, Japan. 4National Institutes for Quantum Science and
Technology, Sendai 980-8579, Japan. 5Institute for Molecular Science, Okazaki 444-8585, Japan. 6Photon Factory, Institute of Materials Structure Science,
High Energy Accelerator Research Organization (KEK), 1-1 Oho, Tsukuba 305-0801, Japan. 7Institute of Multidisciplinary Research for Advanced Materials
e-mail: nakamura.takuto.fbs@osaka-u.ac.jp; kimura.shin-ichi.fbs@osaka-u.ac.jp
(IMRAM), Tohoku University, Sendai 980-8577, Japan.

Nature Communications | (2023)14:7850

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Article

https://doi.org/10.1038/s41467-023-43662-9

temperature, pressure, and magnetic field. Fabricating artificial lowdimensional strongly correlated electron systems and quantizing a
three-dimensional HF state by quantum confinement14 are suitable
methods to investigate the novel electronic phase. In the Ce-based
artificial superlattice, the suppression of antiferromagnetic (AFM)
ordering as well as the increase of the effective electron mass with
decreasing of the thickness of the Ce-layer15 and the emergence of the
strong-coupling superconductivity16 have been reported. To understand the fundamental properties of 2D HF systems, it is necessary to
clarify the electronic band structure and the formation mechanism of
the HF. However, the details have remained unclear due to the lack of
promising materials and the extremely low transition temperatures of
less than a few kelvin to HF even in known materials14,15,17,18.
The growth of the well-ordered atomically thin film on a single
crystal substrate is a suitable technique to access such 2D electron
systems. So far, various 2D Kondo-lattice has been fabricated on the
substrates; multilayer CePt5 thin-film on Pt(111)19, CePb3 on Si(111)20,
Graphene on SmB621, and a checkerboard pattern of organic molecules
on Au(111)22. However, Yb-based 2D HF material, in which the Yb ion is
the most fundamental element to realize HF23,24 and has a symmetrical
electronic-hole configuration to the Ce one, has not been reported. In
particular, the RE-based monoatomic layer Kondo-lattice showing HF
state has never been reported.
In this study, we report the HF electronic structure of a novel Ybbased monoatomic layer Kondo lattice; synchrotron-based angleresolved photoemission spectroscopy (ARPES) on monoatomic
layered YbCu2 on Cu(111). The surface atomic structure of YbCu2 on
Cu(111) is shown in Fig. 1a. The Yb atoms surrounded by Cu atoms are
arranged in a triangular lattice. In a similar surface alloy RE NM2/
NM(111) (NM = noble metal), various physical properties appear such
as ferromagnetic ordering25–27 and Weyl nodal-line fermion28 depending on the containing RE element, but there is no report on the
appearance of HF character so far. Figure 1c, d shows the LEED patterns

a

of the Cu(111) substrate and the Yb-adsorbed Cu(111) surface at 70 K,
respectively. In addition to the primitive (1 × 1) spots originating
pffiffiffi pffiffiffi from
the Cu(111) substrate indicated by yellow arrows, the ( 3 × 3)R30∘
structure with the Moiré patterns, originating from the small lattice
mismatch between Cu(111) and the topmost surface alloy layer, was
observed, indicating the successful fabrication of the monoatomic
YbCu2 layer, one possible model of the Yb-Cu surface alloy system, on
the Cu(111) substrate. Note that the overall trend of the LEED patterns
is consistent with those of other RE NM2/NM(111) systems25–30.

Results and discussion
The itinerant or localized character of Yb 4f electrons is strongly
reflected in the valency of the Yb ions. Figure 2a shows Yb 3d core-level
spectrum of YbCu2/Cu(111) at 15 K. The photoelectron peaks at the
binding energies of 1528 and 1538 eV originate from the Yb2+ and
Yb3+3d final states, respectively, after photoexcitation. From the
intensity ratio between the Yb2+ and Yb3+ peaks after subtracting the
background indicated by the dotted line in the figure, the mean
valence of Yb ions was evaluated as 2.41 ± 0.01. To confirm the consistency of the coexistence of Yb2+ and Yb3+ observed in the Yb 3d corelevel spectra to the electronic state near the Fermi level (EF), angleintegrated valence-band photoelectron spectra of the Cu(111) clean
substrate and the YbCu2/Cu(111) surface are shown in Fig. 2b. The Cu
3d states at the binding energy of 3 eV are dominant in the Cu(111)
substrate. In the YbCu2/Cu(111) spectrum, there are two narrow peaks
originating from the Yb2+4f spin-orbit pair near EF, and broad peaks of
Yb3+4f final states and Cu 3d states at the binding energy of 3–13 eV.
These results strongly suggest that the Yb ions in monoatomic layer
YbCu2 are mixed valence. Note that in the YbAu2/Au(111), which has a
similar atomic structure to YbCu2/Cu(111), Yb ions are almost
divalent29. The reason for the difference in the Yb valence between
YbCu2 and YbAu2 would be due to the in-plane lattice compression,
which can be explained by the analogy from the bulk Yb-based

c

YbCu2

Cu(111)

Cu
Cu

z
y

x

b

d

(√3×√3)R30°

YbCu₂/Cu(111)

Cu
Yb

Cu
YbCu2
ky

y
z

x

Fig. 1 | Monoatomic-layer YbCu2 on Cu(111) substrate. a A surface atomic
structure of YbCu2/Cu(111). b Top view of monoatomic-layer YbCu2. The dashed
line indicates the unit cell of YbCu2. c LEED pattern of Cu(111)-(1 × 1) substrate.
pffiffiffi pffiffiffi
d Same as (b) but for YbCu2/Cu(111)-( 3 × 3)R30∘. Both LEED patterns were taken
pffiffiffi pffiffiffi
at the temperature of 70 K. The primitive (1 × 1) and ( 3 × 3)R30∘ are indicated by

Nature Communications | (2023)14:7850

kx

Ep = 117 eV

orange and blue arrows, respectively. The distortions of the LEED image are due to
the flat microchannel plate used for the LEED measurement. The satellite spots
around the integer spots represent the moiré superstructure originating from a
small lattice mismatch of YbCu2 and Cu(111).

2

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