Dramatic Plasmon Response to the Charge-Density-Wave Gap Development in 1T-TiSe2

2025-05-03 0 0 3.44MB 7 页 10玖币

侵权投诉

Dramatic Plasmon Response to the Charge-Density-Wave Gap Development in

1T-TiSe2

Zijian Lin,1, 2, ∗Cuixiang Wang,1, 2, ∗A. Balassis,3J. P. Echeverry,4A. S. Vasenko,5, 6 V. M. Silkin,7, 8, 9

E. V. Chulkov,7, 8, 5 Youguo Shi,1, 10 Jiandi Zhang,1Jiandong Guo,1, 2, 10, †and Xuetao Zhu1, 2, 10, ‡

1Beijing National Laboratory for Condensed Matter Physics and Institute of Physics,

Chinese Academy of Sciences, Beijing 100190, China

2School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China

3Department of Physics and Engineering Physics, Fordham University,

441 East Fordham Road, Bronx, NY 10458, USA

4Universidad de Ibagu´e, Carrera 22 Calle 67 B, Av. Ambal´a, Ibagu´e-Tolima, Colombia

5HSE University, 101000 Moscow, Russia

6I. E. Tamm Department of Theoretical Physics, P. N. Lebedev Physical Institute,

Russian Academy of Sciences, 119991 Moscow, Russia

7Donostia International Physics Center (DIPC),

20018 San Sebasti´an/Donostia, Basque Country, Spain

8Departamento de Pol´ımeros y Materiales Avanzados: F´ısica,

Qu´ımica y Tecnolog´ıa, Facultad de Ciencias Qu´ımicas,

Universidad del Pa´ıs Vasco UPV/EHU, Apartado 1072,

20080 San Sebasti´an/Donostia, Basque Country, Spain

9IKERBASQUE, Basque Foundation for Science, 48013 Bilbao, Basque Country, Spain

10Songshan Lake Materials Laboratory, Dongguan, Guangdong 523808, China

1T-TiSe2is one of the most studied charge density wave (CDW) systems, not only because of its

peculiar properties related to the CDW transition, but also due to its status as a promising candidate

of exciton insulator signaled by the proposed plasmon softening at the CDW wave vector. Using

high-resolution electron energy loss spectroscopy, we report a systematic study of the temperature-

dependent plasmon behaviors of 1T-TiSe2. We unambiguously resolve the plasmon from phonon

modes, revealing the existence of Landau damping to the plasmon at ﬁnite momentums, which does

not support the plasmon softening picture for exciton condensation. Moreover, we discover that

the plasmon lifetime at zero momentum responds dramatically to the bandgap evolution associated

with the CDW transition. The interband transitions near the Fermi energy in the normal phase is

demonstrated serving as a strong damping channel of plasmons, while such a channel in the CDW

phase is suppressed due to the CDW gap opening, which results in the dramatic tunability of the

plasmon in semimetals or small-gap semiconductors.

In a charge density wave (CDW) material, the CDW

gap development, which is often served as the order pa-

rameter to characterize the CDW transition [1,2], can

strongly inﬂuence the emergent phenomena of the sys-

tem. 1T-TiSe2, a quasi-two-dimensional layered mate-

rial, undergoes a three-dimensional second order CDW

transition at Tc∼200 K with qCDW = (1/2,0,1/2) [3].

The CDW origin of 1T-TiSe2was described by several

diﬀerent mechanisms [3–5]. One of the most-studied sce-

narios is the electron-phonon coupling [4,6–13], which is

the CDW origin in many quasi-two-dimensional systems

[2]. On the other hand, the CDW order in 1T-TiSe2is

proposed to be induced by the formation of the exciton

insulator (EI) [14–21]. And some recent studies claimed

that the EI and the electron-phonon coupling may coop-

eratively induce the CDW transition [22–27].

Understanding the the band structure evolution is es-

sential for identifying the origin of the CDW transition

[28,29]. In the normal phase above Tc[Fig. 1(a)], 1T-

TiSe2is a semiconductor with a small indirect gap [28]

(or a semimetal with a small band overlap [9]), where the

bottom of the conduction band is almost tangent to the

Fermi level EF[30,31]. Interestingly, there exists a CDW

ﬂuctuating band above Tc[7,8,14,32,33], which may

provide a precursor of the CDW gap [34]. In the CDW

phase below Tc[Fig. 1(b)], the gap opens by the valence

band gradually shifting toward to higher binding energy,

leaving the conduction band still tangent to EFand ac-

companying with a CDW band folding [8,9,15,16,32].

Due to the coexistence of the metallic nature and CDW

gap in both the normal and CDW phases, the plasmon

provides a good window to visualizing the eﬀect of the

CDW gap development on the electronic properties. Re-

cently, the plasmon softening at qCDW around Tcwas

proposed to serve as the signature of the EI [21]. How-

ever, a recent theoretical study attributed the seemingly

plasmon softening signal to the interband transitions [35].

In this letter, using the momentum-resolved high reso-

lution electron energy loss spectroscopy (HREELS) with

the capability of two-dimensional energy-momentum

mapping [36], we systematically measured the plasmon

behaviors in 1T-TiSe2. Our results unambiguously re-

solve the plasmon from phonon modes and demonstrate

the existence of Landau damping [37–39] to the plas-

mon at ﬁnite momentums within the full temperature

range, revealing that there is no plasmon softening at

arXiv:2210.14635v1 [cond-mat.str-el] 26 Oct 2022

-1

Energy (eV)

(a) Normal

Direct

Gap Fluctuating

Gap Eg

Fluctuating

Band

ΓMLA

(b) CDW

CDW

Gap Eg

Folding

Band

ΓMLA

Folding

Band

FIG. 1. Schematics of the band structure in 1T-TiSe2. (a)

band structure in the normal phase. The red and blue lines

represent the valence and conduction bands, respectively. The

ﬂuctuating band is represented by the gray shadow. The

dashed lines indicate the Fermi level. (b) band structure in

the CDW phase. Black lines represent the folding bands due

to the distortion.

qCDW. Extraordinarily, we discover that the plasmon at

zero momentum responds to the CDW transition dra-

matically, from a broad feature covering the energy of

50 ∼150 meV to a sharp feature with the well-deﬁned

resonant frequency at ∼50 meV in the low-temperature

CDW phase. Such a wide-range tunability is attributed

to the gap opening associated with the CDW transition

that suppresses the interband damping channels of the

plasmon.

Temperature-dependent HREELS results. - The mea-

surements were performed on cleaved 1T-TiSe2single-

crystalline samples in situ in a HREELS system with

reﬂected scattering geometry. The detailed experimen-

tal methods and sample characterizations are described

in the Supplementary Material (SM) [40]. The HREELS

data were collected with the sample temperature varying

from 35 to 300 K along the two high symmetry directions

Γ-M and Γ-K in the surface Brillouin zone (BZ). The in-

cident electron beam energy Eifrom 7 to 110 eV (110 eV

data presented in the main paper, while others in the SM

[40]) is used , with a typical energy resolution of 3 meV.

Figure 2shows the HREELS results of 1T-TiSe2at var-

ious temperatures. The temperature-dependent E-q//

mappings along Γ-M and Γ-K directions are presented in

Figs. 2(a)-(d).

In HREELS, the strong intensity distributions near the

Γ point are dominated by the dipole scattering, and the

relatively weak features away from the Γ point to the

BZ boundary correspond to the impact scattering regime

[41]. Then, it is obvious that there are two kinds of dis-

tinct energy loss features divided by the energy of 45 meV

at 300 K. The loss features higher than 45 meV, labeled

as P, are only located near the Γ point, which are pure

dipole scattering features and regarded as the plasmon

originating from the charge carrier both in the normal

and CDW phases [42,43]. The loss features lower than

45 meV exist throughout the BZ, which are typical im-

pact scattering signals from phonons [41]. To show their

dispersions more clearly, Figs. 2(e)-(h) display the second

diﬀerential images of the original spectra superimposed

to the calculated surface phonon dispersions (red lines)

using an 11-layer slab model (see the details in the SM

[40]). Overall, the calculated phonon dispersions, espe-

cially the optical phonon branches, match well with the

experimental results.

The evolution of the plasmon with temperature is il-

lustrated by a stacking plot of the loss features at the Γ

point [Fig. 2(i)]. As the temperature decreases from 300

K to Tc, the energy of the plasmon gets closer to the op-

tical phonon. When the temperature further decreases

below Tc, the energy of the plasmon slightly increases

and the linewidth of the plasmon changes dramatically.

These phenomena will be discussed later in more detail.

Compared to the previous HREELS study in Ref. [21],

our results show similar temperature-dependent plasmon

behavior at the Γ point, but surprisingly, we did not ob-

serve the plasmon softening at qCDW. In our energy-

momentum mappings at any temperature, the plasmon

only exists near the Γ point and does not disperse to the

low-energy range at the BZ boundary. In detail, Fig. 2(j)

shows dispersion behaviors of the plasmon and phonons

in the qspace along the Γ-M direction [44]. The plas-

mon decays from a sharp peak at q= 0.01 r.l.u. to a

weaker peak at q= 0.03 r.l.u. As qincreases, the plas-

mon is not a well-deﬁned peak anymore at q= 0.10 r.l.u.

and disappears (indistinguishable from noise) beyond q

= 0.14 r.l.u. The reported energy loss signal over the

entire BZ at 17 K in Ref. [21], interpreted as a disper-

sionless plasmon, could be from diﬀusion scattering [41].

In our results, the delicate optical phonon dispersions

[those highlighted by the dashed-line rectangles in Fig.

2], which were not observed in Ref. [21], can be clearly

resolved from the plasmon. The energy range of these

optical phonon branches are close to the claimed soften-

ing plasmon energy, so the softening plasmon dispersion

around Tc[21] seems from the envelope of phonon sig-

nals. Details of the comparison are described in the SM

[40].

Landau damping of the plasmon. - The observed

plasmon damping behaviors can be well understood by

the calculations of the loss functions, which were car-

ried in the framework of time-dependent density func-

tional theory (see details in the SM [40]). For simplicity,

the calculated band structure in the CDW phase does

not include the eﬀects of band folding and renormaliza-

tion. The calculated single particle excitation (SPE) re-

gions for the normal and CDW band structures are in-

dicated by white dashed line and shadow areas centered

at thick white dashed lines in Fig. 3, and plasmon dis-

persions are represented by color mappings of the calcu-

lated loss functions. At the lowest q, plasmons are almost

delta functions. However, plasmons broaden and weaken

rapidly approaching SPE borders. Beyond the calculated

qc= 0.024 r.l.u. [45], plasmons are not well-deﬁned col-

lective excitations anymore due to the strong decay to

SPEs. This explains the experimentally observed fast

文档加载中……请稍候！
如果长时间未打开，您也可以点击刷新试试。

下载文档到电脑，查找使用更方便

10 玖币 0人已下载

立即下载

摘要：

DramaticPlasmonResponsetotheCharge-Density-WaveGapDevelopmentin1T-TiSe2ZijianLin,1,2,CuixiangWang,1,2,A.Balassis,3J.P.Echeverry,4A.S.Vasenko,5,6V.M.Silkin,7,8,9E.V.Chulkov,7,8,5YouguoShi,1,10JiandiZhang,1JiandongGuo,1,2,10,yandXuetaoZhu1,2,10,z1BeijingNationalLaboratoryforCondensedMatterPhysicsand...

展开>> 收起<<

Dramatic Plasmon Response to the Charge-Density-Wave Gap Development in 1T-TiSe2.pdf

共7页,预览2页

还剩页未读，继续阅读

声明：本站为文档C2C交易模式，即用户上传的文档直接被用户下载，本站只是中间服务平台，本站所有文档下载所得的收益归上传人(含作者)所有。玖贝云文库仅提供信息存储空间，仅对用户上传内容的表现方式做保护处理，对上载内容本身不做任何修改或编辑。若文档所含内容侵犯了您的版权或隐私，请立即通知玖贝云文库，我们立即给予删除！

Dramatic Plasmon Response to the Charge-Density-Wave Gap Development in 1T-TiSe2

相关推荐

开通VIP享超值会员特权

作者详情

相关内容

热门标签

举报选择: