Particle zoo in a doped spin chain Correlated states of mesons and magnons Petar Cubela1 2Annabelle Bohrdt3 4Markus Greiner4and Fabian Grusdt1 2 1Department of Physics and Arnold Sommerfeld Center for Theoretical Physics ASC

2025-05-02 0 0 1.89MB 31 页 10玖币
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Particle zoo in a doped spin chain: Correlated states of mesons and magnons
Petar ˇ
Cubela,1, 2 Annabelle Bohrdt,3, 4 Markus Greiner,4and Fabian Grusdt1, 2,
1Department of Physics and Arnold Sommerfeld Center for Theoretical Physics (ASC),
Ludwig-Maximilians-Universit¨at M¨unchen, Theresienstr. 37, M¨unchen D-80333, Germany
2Munich Center for Quantum Science and Technology (MCQST), Schellingstr. 4, D-80799 M¨unchen, Germany
3ITAMP, Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA
4Department of Physics, Harvard University, Cambridge, MA 02138, USA
(Dated: October 6, 2022)
It is a widely accepted view that the interplay of spin- and charge-degrees of freedom in doped
antiferromagnets (AFMs) gives rise to the rich physics of high-temperature superconductors. Nev-
ertheless, it remains unclear how effective low-energy degrees of freedom and the corresponding
field theories emerge from microscopic models, including the tJand Hubbard Hamiltonians. A
promising view comprises that the charge carriers have a rich internal parton structure on inter-
mediate scales, but the interplay of the emergent partons with collective magnon excitations of the
surrounding AFM remains unexplored. Here we study a doped one-dimensional spin chain in a stag-
gered magnetic field and demonstrate that it supports a zoo of various long-lived excitations. These
include magnons; mesonic pairs of spinons and chargons, along with their ro-vibrational excitations;
and tetra-parton bound states of mesons and magnons. We identify these types of quasiparticles in
various spectra using DMRG simulations [1,2]. Moreover, we introduce a strong-coupling theory
describing the polaronic dressing and molecular binding of mesons to collective magnon excitations.
The effective theory can be solved by standard tools developed for polaronic problems, and can be
extended to study similar physics in two-dimensional doped AFMs in the future. Experimentally,
the doped spin-chain in a staggered field can be directly realized in quantum gas microscopes.
I. INTRODUCTION
Field theoretic approaches to quantum spin models in
lattices, such as the Heisenberg antiferromagnet (AFM),
provide very successful descriptions of these paradigmatic
quantum many-body systems [3] and have led to a thor-
ough understanding of their various quantum phase tran-
sitions in different dimensions [4]. Key to their success is
the underlying hypothesis that the coarse-grained fields
on long length-scales feature similar behavior as the mi-
croscopic local magnetic moments underlying the spin
model. More formally, a simple renormalization-group
(RG) procedure yielding the effective low-energy field
theory does not change the particle-content of the an-
alyzed fields. However, in dimensions larger than one
and with mobile dopants included, this approach has
not been able to explain the rich phase diagram of high-
temperature superconductors so far.
In this article, we take a different perspective and ex-
plore emergent structures, at low- to intermediate ener-
gies, in a doped quantum spin chain. The zoo of con-
stituents we find defies a naive field-theoretic descrip-
tion: we identify emergent parton structures of spinons
and chargons, forming mesonic bound states with a rich
spectrum of ro-vibrational internal excitations. More-
over, these mesons interact with collective magnon exci-
tations in the surrounding spin system, which leads to po-
laronic dressing on the one hand and, more exotically, to
long-lived meson-magnon bound states. In a phenomeno-
Corresponding author email: fabian.grusdt@physik.uni-
muenchen.de
logical field-theoretic model, each of these constituents
should be described by a separate quantum field, with
mutual interactions between all of them. Describing how
these new fields emerge at intermediate length- or energy-
scales in a thorough RG procedure is a challenging task,
even for the simple toy model we consider. Hence we fo-
cus on a microscopic description of the individual emer-
gent bound states and analyze their characterizing prop-
erties, such as their dispersion relations, zero-point ener-
gies, and mutual interactions. To this end, we apply the
powerful theoretical tools developed for the description
of Bose polarons [59].
Concretely, we consider doped one-dimensional spin
chains. When featuring SU(2) invariance, these systems
display spin-charge separation [1014]: the collective ex-
citations of the spin-chain are fractionalized spinons,
which co-exist with free chargons. In this limit, non-
trivial bound states of the constituents are absent [12]
and bosonization techniques provide a powerful field-
theoretic description of the doped system in terms of
Luttinger liquids [15]. As we demonstrate, the situa-
tion changes drastically when a staggered magnetic field
is included, breaking the SU(2) symmetry, see Fig. 1a):
Now spinons and chargons are confined [1517], the un-
doped ground state has gapped collective magnon excita-
tions, and spin-charge separation breaks down. Despite
this confinement, the situation is far from trivial: As we
will show, doped holes in this system host a zoo of exci-
tations reflecting their rich internal structure, and their
interaction with gapped magnons can lead to even more
complicated multi-parton bound states, see Fig. 2.
In several regards, our 1D model is motivated by the
physics of mobile holes in a SU(2)-invariant 2D Hubbard
arXiv:2210.02320v1 [cond-mat.str-el] 5 Oct 2022
2
FIG. 1. Particle zoo in a doped spin chain: We consider
a doped mobile hole in a spin-chain subject to a staggered
magnetic field. a) Ignoring transverse spin fluctuations, the
staggered field leads to a confining force between spinons and
chargons connected by a string of overturned spins (top row),
which leads to meson formation. A similar situation is real-
ized in a mixed-dimensional model, where a strong gradient
prevents hole motion along the direction of the gradient (bot-
tom row). b) Transverse spin couplings give rise to Holstein-
Primakoff (HP) magnon fluctuations in the surrounding spin
background. The latter interact with the spinon, which is sur-
rounded by the strongly fluctuating but tightly bound char-
gon cloud. All constituents making up the zoo of excitations
are summarized in c), where we also indicate the background
Ising fields τz
j, affected by the hole motion, around which we
expand in the generalized 1/S approximation employed here.
model. In contrast to the 1D case with SU(2) symmetry,
the ground state of the two-dimensional (2D) Heisen-
berg model has long-range magnetic order and gapless
spin-1 magnon excitations. This effect is mimicked by
the external staggered magnetic field we consider in our
model, which introduces magnetic order and leads to sim-
ilar spin-1 magnon modes, although with a non-vanishing
gap. There is strong evidence that a doped hole in the
2D AFM features a rich internal meson structure [18],
with discrete vibrational [1921] and rotational excita-
tions [22,23]. In our 1D model, we reveal similar struc-
tures and develop an effective strong-coupling descrip-
tion.
Remarkably, the coupling of mesons to collective
magnon excitations remains poorly understood in 1D and
2D, in particular around zero momentum where we show
that the competition of mesons and magnons is most pro-
nounced. In the present article we fill this gap and ap-
ply a powerful theoretical framework, the so-called gen-
eralized 1/S expansion [22], to describe the coupling of
mesons to magnons in a systematic manner. As a result,
we obtain an effective polaron Hamiltonian describing
the dressing, or even binding, of a spinon-chargon meson
with additional magnon excitations. Our work paves the
way for similar studies in doped 2D Mott insulators, and
may lead to a better understanding of the charge carri-
ers and their interactions with magnons in underdoped
copper oxides. In particular we expect that our formal-
ism will be useful for understanding transport measure-
ments involving magnetic polarons, such as the long-time
spreading dynamics of a hole reported in [2427].
Experimentally, the model we consider can be realized
with ultracold atoms in optical lattices, which have re-
cently made significant advances in studying doped quan-
tum magnets [28]. On a mean-field level, our model more-
over maps to a doped mixed-dimensional tJmodel [29],
which can be realized by subjecting a Fermi-Hubbard
system to a strong tilt along one of the lattice directions
[30]. Ultracold atom realizations allow to measure spec-
tra [3134] like the ones we calculate here to identify the
emergent zoo of excitations; moreover they can directly
visualize string patterns [13,35,36] or the dressing cloud
of magnetic polarons in configuration-space [37], making
them ideal platforms to explore the emergent structures
we predict on intermediate length scales.
II. MODEL AND MAIN RESULTS
In this article, we study a simple but rich one-
dimensional model of a doped AFM. Our starting point is
an SU(2)-invariant Heisenberg spin chain. An additional
staggered magnetic field of strength ±hon alternating
sites along the z-direction breaks the SU(2) symmetry,
introduces long-range magnetic correlations, and leads
to collective magnon excitations with a tunable gap con-
trolled by |h|. To describe mobile holes doped into this
model, we use a tJHamiltonian:
ˆ
H=tX
j,σ
ˆ
Pˆc
j+1ˆcj,σ + h.c.ˆ
P
+JX
j
ˆ
Sj+1 ·ˆ
SjhX
j
(1)jˆ
Sz
j.(1)
Since we will only consider a single doped hole in
this article, we dropped the nearest-neighbor interaction
J/4 ˆnj+1 ˆnjtypically included in the tJmodel [38].
A similar model, including phonons, has been studied in
Ref. [39].
A. Lattice gauge Hamiltonian
For later purposes, we find it convenient to write the
Hamiltonian as a sum of two separate parts: (i) a tJz
part which conserves each individual spin in the so-called
squeezed space [12,13,40] obtained by removing holes
from the chain:
ˆ
HtJz=tX
j,σ
ˆ
Pˆc
j+1ˆcj,σ + h.c.ˆ
P
+JzX
j
ˆ
Sz
j+1 ˆ
Sz
jhX
j
(1)jˆ
Sz
j.(2)
To keep our analytical formalism later on general, we
introduced the coupling Jz, which is simply Jz=Jfor
the original model in Eq. (1).
3
Remarkably, the Hamiltonian in Eq. (2) is exactly
equivalent to a Z2lattice gauge theory (LGT), as shown
in Refs. [17,41]. In this mapping, chargons (i.e. spin-
less holes) and spinons (i.e. Ising domain walls) carry
Z2gauge charges and are connected by a Z2electric
string τx
hi,ji. The staggered field ±hleads to a term
hPhi,jiˆτx
hi,jiin the Z2gauge invariant Hamiltonian. The
latter has been shown to cause spinon-chargon confine-
ment for any infinitesimal h6= 0 [16,17]. This Z2LGT
formalism forms the basis for our mesonic description of
a doped hole.
In addition, the full Hamiltonian in Eq. (1) includes
(ii) transverse spin fluctuations,
ˆ
H=ˆ
HtJz+ˆ
HJ,(3)
where we find it most convenient to write
ˆ
HJ=J
2X
jˆ
S+
j+1 ˆ
S
j+ h.c..(4)
Again we introduced the more general coupling strength
Jin this term, although for our original model in Eq. (1)
J=J. Later on, we will include such transverse
spin fluctuations on top of a N´eel ordered ground state
distorted by the hole motion by introducing Holstein-
Primakoff bosons (magnons), see Fig. 1b).
Finally, we note that in the limit h/J→ ∞, the trans-
verse fluctuations ˆ
HJcan always be treated perturba-
tively, independent of the ratios Jz/Jor t/J. To low-
est order, only the tJzpart of the Hamiltonian, Eq. (2),
remains and it follows that the model has an emergent
Z2gauge structure for large values of h.
B. Main results: particle zoo in the spin chain
The separation of the Hamiltonian in two components
lends a natural understanding of our results. Our main
goal is to understand the ground and excited states of a
mobile dopant in the spin chain. As described in detail
below, we find that the Z2gauge structure of the tJz
part of the Hamiltonian, or equivalently (in our model)
the string-picture of magnetic polarons [22,42,43], intro-
duces parton constituents, namely spinons and chargons,
which are confined by the linear string tension generated
by the staggered field h. The resulting mesonic spinon-
chargon bound state has a rich internal structure con-
stituted by inversion-even and inversion-odd vibrational
modes of the Z2electric string, or equivalently the string
of overturned Ising spins, connecting the spinon and the
chargon. We probe these states directly in spectra cal-
culated by time-dependent matrix product states (td-
MPS), see Sec. III, and compare to an effective strong-
coupling description that we develop here, see Sec. IV.
The transverse couplings introduced by ˆ
HJlead to
vacuum fluctuations of magnons in the absence of a
doped hole. This effect can be captured by a simple lin-
ear spin-wave expansion around the classical N´eel state,
012345
hin units of t
2
1
0
1
2
G.S. Energy in units of t
Bare Mes
LLP + Gauss
DMRG
h in units of t
Ground state energy in units of t
bare meson
LLP + Gauss
DMRG
FIG. 2. Polaronic bands in the presence of meson-magnon
interactions at low energies: The overall ground state at
momentum k=π/2 is realized by a weakly dressed meson
(solid blue line). Before the broad meson-magnon continuum
is reached at higher energies (wide red band), we predict a
weakly dispersing meson-magnon bound state (dark red line),
corresponding to a tetra-parton configuration. The black lines
indicate the bare meson dispersion (solid) and edges of the
meson-magnon continuum (dashed) in the absence of meson-
magnon interactions, respectively. At higher energies (not
shown) we find ro-vibrational internal meson excitations. Cal-
culations were performed using the strong-coupling general-
ized 1/S approximation introduced in the text; we chose pa-
rameters h= 0.6Jand t= 5J.
which we achieve by a Holstein-Primakoff approximation.
In the vicinity of the meson, the distortion of the N´eel
background caused by the spinon-chargon pair introduces
additional couplings to magnons which give rise to addi-
tional rich physics: On one hand, they lead to polaronic
dressing and weak mass renormalization of the meson
around the dispersion minimum at momentum k=π/2.
This is shown for the lowest-energy mesonic state (solid
blue line) in Fig. 2.
More dramatically, the interactions with magnons can
give rise to meson-magnon bound states. Since the
magnon itself can be viewed as a bound state of two con-
fined spinons, this state constitutes an emergent tetra-
parton composite. As demonstrated in Fig. 2, for suffi-
ciently large values of hour effective model of the meson-
magnon coupling predicts a low-lying meson-magnon
bound state at relatively low energies below the meson-
magnon scattering continuum. This should be contrasted
with the higher excitation energies of ro-vibrational inter-
nal meson modes. We confirm our prediction of meson-
magnon bound states in td-MPS calculations of one-hole
spectra in a sector with total spin Sz= 3/2, see Sec. III.
Finally, meson-magnon interactions can have a pro-
nounced effect on the quasiparticle dispersion of the
dressed meson around momentum k= 0. In this re-
4
gion of momentum space, the bare meson dispersion ap-
proaches the meson-magnon scattering continuum most
closely, as indicated by the dashed and solid black lines
in Fig. 2. Without meson-magnon interactions and for
sufficiently weak fields ht, J, we find that they can
even cross, leading to a decaying bare meson state inside
the meson-magnon scattering continuum. However, in
Sec. V B we analyze our effective meson-magnon Hamil-
tonian and find indications that meson-magnon interac-
tions in the 1D chain are strong enough to avoid such
quasiparticle decay [44]. Namely, the meson and magnon
bands repel and an isolated quasiparticle band of the
mesonic magnetic polaron survives even around k= 0.
This prediction is further supported by td-MPS simula-
tions at small fields where the effect is most pronounced.
Methodologically, we deviate from the standard ap-
proach typically used to describe magnetic polaron for-
mation in an AFM [4547]. As mentioned above, we first
take into account how the mobile hole distorts the N´eel
background with pure Ising interactions. This allows us
to make a direct connection to the Z2LGT and identify
the parton content of the meson. Moreover, we can rela-
tively easily capture the competition between the tunnel-
ing term tand the linear string tension h, to all orders
in t/h. This is achieved within a strong-coupling theory.
Next we introduce generalized Holstein-Primakoff bosons
(loosely speaking, magnons) by expanding around the al-
ready distorted eel state (we refer to this approach as
the generalized 1/S approximation [22]). This yields ad-
ditional couplings of the meson to the magnons; impor-
tantly, the strength of these couplings is only of order
J, and a fraction of tfor some further corrections we
identify. Hence, perturbative or simple variational ap-
proaches are sufficient to capture the additional meson-
magnon interactions. This should be contrasted with
the traditional 1/S approximation [4547] where the hole
hopping titself leads to magnon creation: as a result, the
effective Hamiltonian is strongly coupled when t>hand
direct analytical insights are harder to obtain.
C. Possible experimental realizations
Experimentally, the model in Eq. (1) we study can be
realized in different ultracold atom setups. We propose
to use ultracold fermionic Lithium or Potassium atoms
which have very successfully explored the SU(2)-invariant
2D Fermi-Hubbard model [28]. The main obstacle in
these systems is to implement the staggered magnetic
field, which requires local addressability on the scale of
an optical wavelength, see e.g. [48], and sizable magnetic
moments in order to distinguish different spin states, in a
regime close to an atomic Feshbach resonance to realize
super-exchange couplings.
A first option is to use Potassium atoms in a quantum
gas microscope [49] which have a sizable magnetic mo-
ment [50], allowing for a local modulation of the magnetic
field. A second option is to work in a mixed-dimensional
setting where tunneling is strongly suppressed by strong
gradients along all but one lattice direction [29]. More-
over, we assume that nearest-neighbor AFM Ising cou-
plings between all spins are present, which dominate over
the weak super-exchange couplings along the gradient di-
rections. This can be realized in an optical lattice by
adding Rydberg dressing [51]. When doping only the
central chain with one hole and keeping all neighboring
chains at half filling, the surrounding spin chains can gen-
erate an effective staggered field term ±hif they are suffi-
ciently cold. Here we assumed, in a mean-field spirit, that
the wavefunctions of the different chains approximately
factorize. Similarly, in mixed-dimensional settings with
SU (2) invariant spin-exchange interactions [29,30] we
expect a ground state with broken SU (2) symmetry in
qualitatively very similar physics.
Finally, we note that the 1D model in Eq. (1) can be
equally realized with bosons as long as one ensures to
have AFM Heisenberg couplings between the spins [52,
53]. The statistics of the dopants is irrelevant, as can be
shown by a Jordan-Wigner transformation. Hence the
model in Eq. (1) can also be simulated in qubit arrays
or digital quantum computers [54], without the need to
incorporate fermionic statistics.
III. NUMERICAL DMRG SPECTRA
In this section we present our numerical results, largely
based on td-MPS simulations [55], which support our
main findings about the structure and interactions of
doped holes in the 1D spin chain with a staggered field.
We already compare our numerical results to predictions
by the semi-analytical strong-coupling meson-magnon
theory introduced in the subsequent sections. This the-
ory provides a unified understanding of all our key nu-
merical observations.
Detailed descriptions of the numerical td-MPS simu-
lations we performed can be found in Refs. [21,23]; our
algorithm builds upon the earlier works [5658]. To en-
sure proper convergence of the MPS calculations, we per-
formed the same convergence checks, in time and bond-
dimension, as described in [21,23].
A. Ground state: Dressed hole
In Fig. 3we start by showing the standard one-hole
angle-resolved photoemission spectrum (ARPES), de-
fined by
S(k, ω) = 1
πImZ
0
dtei(ωE0)t
LX
j
eikj Gj(t),(5)
with the Green’s function
Gj(t) = X
σhΨ0|ˆc
j,σeiˆ
Htˆc0|Ψ0i(6)
5
0.0 0.2 0.4 0.6 0.8 1.0
k/π
8
7
6
5
4
3
2
1
0
ω/J
S(kx, ky= 0.0π, ω)
0.2
0.4
0.6
0.8
1.0
1.2
FIG. 3. The standard one-hole ARPES spectrum reveals
a pronounced quasiparticle peak at the lowest energy. The
dispersion minimum is located at k=π/2, as predicted by
our semi-analytical theory (solid blue line). Here we consider
h= 1.0Jand t= 5J; the color scale is in a.u..
where |Ψ0iis the ground state with energy E0of ˆ
Hwith
zero holes.
In the spectrum, we observe a pronounced quasiparti-
cle peak at low energy which corresponds to the magnetic
polaron. The comparison with our semi-analytical the-
ory shows that it is located around the expected energy,
and shows the same dispersion relation with a minimum
at k=π/2. At higher energies the spectrum is relatively
featureless for the considered value of h/J = 1.0 in Fig. 3.
As we show next, additional features becomes visible for
larger values of h.
B. Ro-vibrational excitations: Mesonic states
Now we calculate a rotational variant of the ARPES
spectrum, where spinon-chargon excitations with odd
(ξ=1) and even (ξ= +1) inversion symmetry can
be detected. It is defined as in Eq. (5) but using the
rotational Green’s function [23]
Grot
j,ξ (t) = X
σhΨ0|ˆc
j,σ ˆ
X
j,ξeiˆ
Htˆ
X0ˆc0|Ψ0i(7)
where ˆ
Xj,ξ =Pσˆc
j,σ(ˆcj+1+ξˆcj1 ) creates an addi-
tional excitation of the spinon-chargon pair.
In Fig. 4we show our results for h= 4J. In both parity
sectors ξ=±1 we observe pronounced vibrational peaks,
which correspond to vibrational modes of the spinon-
chargon string. The absence of even (odd) peaks in the
odd (even) spectrum indicates that the parity ξis a good
emergent quantum number at all momenta, not only at
k= 0, π/2 where the system is strictly inversion sym-
metric. This is a direct indication for the existence of an
internal meson structure [23].
0.0 0.2 0.4 0.6 0.8 1.0
k/π
10
5
0
5
10
15
20
25
ω/J
S(kx, ky= 0.0π, ω)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.0 0.2 0.4 0.6 0.8 1.0
k/π
10
5
0
5
10
15
20
25
ω/J
S(kx, ky= 0.0π, ω)
0.0
0.2
0.4
0.6
0.8
FIG. 4. The rotational one-hole ARPES spectrum reveals a
series of long-lived vibrational excitations with even (ξ= +1,
top) and odd (ξ=1, bottom) parity. We compare the td-
MPS spectra with bare meson resonances calculated from our
strong-coupling theory (gray solid lines: ξ= +1 even; gray
dashed lines: ξ=1 odd). Here we consider h= 4.0Jand
t= 5J; the color scale is in a.u..
In Fig. 4we also compare the peak positions observed
in td-MPS with predictions by our strong-coupling the-
ory. The observed peaks in our full numerical spectra
are in excellent agreement with our semi-analytical pre-
dictions. In the latter, for simplicity, we neglected cor-
rections from magnon-dressing which are weak at large
values of h. Nevertheless, note that significant charge
fluctuations are present since we consider t>hin the
figure.
C. Meson-magnon bound states
Next we show that even more complex excitations can
arise when the mesonic hole interacts with its spin envi-
ronment. Specifically, the meson can form a stable bound
state with a magnon excitation. To demonstrate the ex-
摘要:

Particlezooinadopedspinchain:CorrelatedstatesofmesonsandmagnonsPetarCubela,1,2AnnabelleBohrdt,3,4MarkusGreiner,4andFabianGrusdt1,2,1DepartmentofPhysicsandArnoldSommerfeldCenterforTheoreticalPhysics(ASC),Ludwig-Maximilians-UniversitatMunchen,Theresienstr.37,MunchenD-80333,Germany2MunichCenterfor...

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