Theoretical Physics
|
Open Access
Abstract: The nature and origin of electronic nematicity remains a significant challenge in our understanding of the iron-based superconductors. This is particularly evident in the iron chalcogenide, FeSe, where it is currently unclear how the experimentally determined Fermi surface near the M point evolves from having two electron pockets in the tetragonal state, to exhibiting just a single electron pocket in the nematic state. This has posed a major theoretical challenge, which has become known as the missing electron pocket problem of FeSe, and is of central importance if we wish to uncover the secrets behind nematicity and superconductivity in the wider iron-based superconductors. Here, we review the recent experimental work uncovering this nematic Fermi surface of FeSe from both ARPES and STM measurements, as well as current theoretical attempts to explain this missing electron pocket of FeSe, with a particular focus on the emerging importance of incorporating the dxy orbital into theoretical descriptions of the nematic state. Furthermore, we will discuss the consequence this missing electron pocket has on the theoretical understanding of superconductivity in this system and present several remaining open questions and avenues for future research.
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May 2022
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I05-ARPES
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M.
Berben
,
S.
Smit
,
C.
Duffy
,
Y.-T.
Hsu
,
L.
Bawden
,
F.
Heringa
,
F.
Gerritsen
,
S.
Cassanelli
,
X.
Feng
,
S.
Bron
,
E.
Van Heumen
,
Y.
Huang
,
F.
Bertran
,
T. K.
Kim
,
C.
Cacho
,
A.
Carrington
,
M. S.
Golden
,
N. E.
Hussey
Abstract: Once doped away from their parent Mott insulating state, the hole-doped cuprates enter into many varied and exotic phases. The onset temperature of each phase is then plotted versus
p
—the number of doped holes per copper atom—to form a representative phase diagram. Apart from differences in the absolute temperature scales among the various families, the resultant phase diagrams are strikingly similar. In particular, the
p
values corresponding to optimal doping (
p
opt
∼
0.16
) and to the end of the pseudogap phase
(
p
∗
∼
0.19
–
0.20
)
are essentially the same for all cuprate families bar one: the single-layer Bi-based cuprate
Bi
2
+
z
−
y
Pb
y
Sr
2
−
x
−
z
La
x
CuO
6
+
δ
(Bi2201). This anomaly arises partly due to the complex stoichiometry of this material and also to the different
p
values inferred from disparate (e.g., bulk or surface) measurements performed on samples with comparable superconducting transition temperatures
T
c
. Here, by combining measurements of the in-plane resistivity in zero and high magnetic fields with angle-resolved photoemission spectroscopy studies in the superconducting and normal state, we argue that the phase diagram of Bi2201 may in fact be similar to that realized in other families. This study therefore brings Bi2201 into the fold and supports the notion of universality of
p
opt
and
p
∗
in all hole-doped cuprates.
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Apr 2022
|
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I05-ARPES
|
F.
Mazzola
,
C.-M.
Yim
,
V.
Sunko
,
S.
Khim
,
P.
Kushwaha
,
O. J.
Clark
,
L.
Bawden
,
I.
Markovic
,
D.
Chakraborti
,
T. K.
Kim
,
M.
Hoesch
,
A. P.
Mackenzie
,
P.
Wahl
,
P. D. C.
King
Diamond Proposal Number(s):
[12469, 14927, 1626]
Open Access
Abstract: Controlling spin wave excitations in magnetic materials underpins the burgeoning field of magnonics. Yet, little is known about how magnons interact with the conduction electrons of itinerant magnets, or how this interplay can be controlled. Via a surface-sensitive spectroscopic approach, we demonstrate a strong electron–magnon coupling at the Pd-terminated surface of the delafossite oxide PdCoO2, where a polar surface charge mediates a Stoner transition to itinerant surface ferromagnetism. We show how the coupling is enhanced sevenfold with increasing surface disorder, and concomitant charge carrier doping, becoming sufficiently strong to drive the system into a polaronic regime, accompanied by a significant quasiparticle mass enhancement. Our study thus sheds light on electron–magnon interactions in solid-state materials, and the ways in which these can be controlled.
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Feb 2022
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I05-ARPES
|
D. F.
Liu
,
E. K.
Liu
,
Q. N.
Xu
,
J. L.
Shen
,
Y. W.
Li
,
D.
Pei
,
A. J.
Liang
,
P.
Dudin
,
T. K.
Kim
,
C.
Cacho
,
Y. F.
Xu
,
Y.
Sun
,
L. X.
Yang
,
Z. K.
Liu
,
C.
Felser
,
S. S. P.
Parkin
,
Y. L.
Chen
Open Access
Abstract: The spin–orbit coupling (SOC) lifts the band degeneracy that plays a vital role in the search for different topological states, such as topological insulators (TIs) and topological semimetals (TSMs). In TSMs, the SOC can partially gap a degenerate nodal line, leading to the formation of Dirac/Weyl semimetals (DSMs/WSMs). However, such SOC-induced gap structure along the nodal line in TSMs has not yet been systematically investigated experimentally. Here, we report a direct observation of such gap structure in a magnetic WSM Co3Sn2S2 using high-resolution angle-resolved photoemission spectroscopy. Our results not only reveal the existence and importance of the strong SOC effect in the formation of the WSM phase in Co3Sn2S2, but also provide insights for the understanding of its exotic physical properties.
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Jan 2022
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I05-ARPES
|
Y.
Falke
,
N.
Ehlen
,
G.
Marini
,
A. V.
Fedorov
,
V. Y.
Voroshnin
,
B. V.
Senkovskiy
,
K.
Nikonov
,
M.
Hoesch
,
T. K.
Kim
,
L.
Petaccia
,
G.
Di Santo
,
T.
Szkopek
,
G.
Profeta
,
A.
Gruneis
Diamond Proposal Number(s):
[17064]
Abstract: We investigate electron-phonon coupling (EPC) in the charge density wave (CDW) phase of
V
Se
2
by Raman spectroscopy, angle-resolved photoemission spectroscopy (ARPES), and ab initio calculations. Zone folding induced by the
4
×
4
in-plane CDW phase promotes the appearance of a Raman peak at
∼
170
cm
−
1
. The suppression of ARPES intensity in parts of the Fermi surface is also a result of CDW-induced zone folding and anticrossing of the electron energy bands. The appearance of the new Raman peak is in line with the ARPES observation of a kink feature in the spectral function at the same energy. A self-energy analysis yields an EPC constant of
λ
=
0.3
. Our calculations of the EPC are in excellent agreement and reveal that the kink is caused by several optical phonon branches close in energy. Our paper highlights the CDW phase as a means of inducing EPC pathways to optical phonons that directly affect its Raman spectrum.
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Dec 2021
|
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I05-ARPES
|
Ilya
Belopolski
,
Tyler A.
Cochran
,
Xiaoxiong
Liu
,
Zi-Jia
Cheng
,
Xian P.
Yang
,
Zurab
Guguchia
,
Stepan S.
Tsirkin
,
Jia-Xin
Yin
,
Praveen
Vir
,
Gohil S.
Thakur
,
Songtian S.
Zhang
,
Junyi
Zhang
,
Konstantine
Kaznatcheev
,
Guangming
Cheng
,
Guoqing
Chang
,
Daniel
Multer
,
Nana
Shumiya
,
Maksim
Litskevich
,
Elio
Vescovo
,
Timur K.
Kim
,
Cephise
Cacho
,
Nan
Yao
,
Claudia
Felser
,
Titus
Neupert
,
M. Zahid
Hasan
Diamond Proposal Number(s):
[17924, 19313]
Abstract: The manipulation of topological states in quantum matter is an essential pursuit of fundamental physics and next-generation quantum technology. Here we report the magnetic manipulation of Weyl fermions in the kagome spin-orbit semimetal
Co
3
Sn
2
S
2
, observed by high-resolution photoemission spectroscopy. We demonstrate the exchange collapse of spin-orbit-gapped ferromagnetic Weyl loops into paramagnetic Dirac loops under suppression of the magnetic order. We further observe that topological Fermi arcs disappear in the paramagnetic phase, suggesting the annihilation of exchange-split Weyl points. Our findings indicate that magnetic exchange collapse naturally drives Weyl fermion annihilation, opening new opportunities for engineering topology under correlated order parameters.
|
Dec 2021
|
|
I05-ARPES
|
Diamond Proposal Number(s):
[4906, 5008]
Abstract: The electronic spectral function of BaNi2As2 is investigated using both angle-resolved photoemission spectroscopy (ARPES) and a combined computational scheme of local density approximation and dynamical mean-field theory (LDA + DMFT). In contrast to the well-studied isostructural iron arsenide high-temperature superconductors, BaNi2As2 demonstrates weak correlation effects, although Ni 3d electrons have an even larger on-site interaction than Fe 3d electrons. The LDA + DMFT effective mass enhancement for bands crossing the Fermi level is found to be only about 1.2, which agrees well with ARPES data. This reduction of the correlation manifestation with respect to iron pnictides comes from the increase of 3d-orbital filling when going from Fe to Ni. The electron correlations cause a remarkable reconstruction of the bare BaNi2As2 LDA band structure below −0.8 eV due to a self-energy effect. A simplified toy model to understand the weakness of correlation effects in BaNi2As2 and to describe the LDA + DMFT self-energy shape is discussed. For a more realistic comparison of LDA + DMFT spectral function maps with ARPES data, we take into account several experimental features: the photoemission cross section, the experimental energy and angular resolutions, and the photo-hole lifetime effects. Thus, the LDA + DMFT calculations presented here with experimental features provide a nearly qualitative agreement with ARPES data and assure the observation of an apparent dramatic decrease of the correlation strength compared to the Fe compounds.
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Dec 2021
|
|
|
|
D. F.
Liu
,
Q. N.
Xu
,
E. K.
Liu
,
J. L.
Shen
,
C. C.
Le
,
Y. W.
Li
,
D.
Pei
,
A. J.
Liang
,
P.
Dudin
,
T. K.
Kim
,
C.
Cacho
,
Y. F.
Xu
,
Y.
Sun
,
L. X.
Yang
,
Z. K.
Liu
,
C.
Felser
,
S. S. P.
Parkin
,
Y. L.
Chen
Abstract: Topological Weyl semimetals (TWSs) are exotic crystals possessing emergent relativistic Weyl fermions connected by unique surface Fermi arcs (SFAs) in their electronic structures. To realize the TWS state, certain symmetries (such as the inversion or time reversal symmetry) must be broken, leading to a topological phase transition (TPT). Despite the great importance in understanding the formation of TWSs and their unusual properties, direct observation of such a TPT has been challenging. Here, using a recently discovered magnetic TWS
Co
3
Sn
2
S
2
, we were able to systematically study its TPT with detailed temperature dependence of the electronic structures by angle-resolved photoemission spectroscopy. The TPT with drastic band structure evolution was clearly observed across the Curie temperature
(
T
C
=
177
K
)
, including the disappearance of the characteristic SFAs and the recombination of the spin-split bands that leads to the annihilation of Weyl points with opposite chirality. These results not only reveal important insights on the interplay between the magnetism and band topology in TWSs, but also provide a method to control their exotic physical properties.
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Nov 2021
|
|
I05-ARPES
|
D. F.
Liu
,
L. Y.
Wei
,
C. C.
Le
,
H. Y.
Wang
,
X.
Zhang
,
N.
Kumar
,
C.
Shekhar
,
N. B. M.
Schröter
,
Y. W.
Li
,
D.
Pei
,
L. X.
Xu
,
P.
Dudin
,
T. K.
Kim
,
C.
Cacho
,
J.
Fujii
,
I.
Vobornik
,
M. X.
Wang
,
L. X.
Yang
,
Z. K.
Liu
,
Y. F.
Guo
,
J. P.
Hu
,
C.
Felser
,
S. S. P.
Parkin
,
Y. L.
Chen
Diamond Proposal Number(s):
[18005]
Open Access
Abstract: Dirac semimetals are classified into different phases based on the types of Dirac fermions. Tuning the transition among different types of Dirac fermions in one system remains a challenge. Recently, KMgBi was predicted to be located at a critical state in which various types of Dirac fermions can be induced owing to the existence of a flatband. Here, we carried out systematic studies on the electronic structure of KMgBi single crystals by combining angle-resolve photoemission spectroscopy and scanning tunneling microscopy/spectroscopy. The flatband was clearly observed near the Fermi level. We also revealed a small bandgap of ∼20 meV between the flatband and the conduction band. These results demonstrate the critical states of KMgBi that transition among various types of Dirac fermions can be tuned in one system.
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Jun 2021
|
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I05-ARPES
|
C. E.
Matt
,
O.
Ivashko
,
M.
Horio
,
J.
Choi
,
Q.
Wang
,
D.
Sutter
,
N.
Dennler
,
M. H.
Fischer
,
S.
Katrych
,
L.
Forro
,
J.
Ma
,
B.
Fu
,
B. Q.
Lv
,
M. V.
Zimmermann
,
T. K.
Kim
,
N. C.
Plumb
,
N.
Xu
,
M.
Shi
,
Johan
Chang
Diamond Proposal Number(s):
[16104]
Open Access
Abstract: The interplay between structural and electronic phases in iron-based superconductors is a central theme in the search for the superconducting pairing mechanism. While electronic nematicity is competing with superconductivity, the effect of purely structural orthorhombic order is unexplored. Here, using x-ray diffraction and angle-resolved photoemission spectroscopy, we reveal a structural orthorhombic phase in the electron-doped iron-pnictide superconductor
Pr
4
Fe
2
As
2
Te
0.88
O
4
(
T
c
=
25
K), which is distinct from orthorhombicity in the nematic phase in underdoped pnictides. Despite the high electron doping we find an exceptionally high orthorhombic onset temperature (
T
ort
∼
250
K), no signatures of phase competition with superconductivity, and absence of electronic nematic order as the driving mechanism for orthorhombicity. Combined, our results establish a high-temperature phase in the phase diagram of iron-pnictide superconductors and impose strong constraints for the modeling of their superconducting pairing mechanism.
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Jun 2021
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