Abrupt Switching of Crystal Fields during Formation of Molecular

Subscriber access provided by Kaohsiung Medical University

Abrupt Switching of Crystal Fields during Formation of Molecular Contacts Jinjie Chen, Hironari Isshiki, Clemens Baretzky, Timofey Balashov, and Wulf Wulfhekel ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b07927 • Publication Date (Web): 22 Mar 2018 Downloaded from http://pubs.acs.org on March 22, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

Abrupt Switching of Crystal Fields during Formation of Molecular Contacts Jinjie Chen,∗,† Hironari Isshiki,†,‡ Clemens Baretzky,† Timofey Balashov,† and Wulf Wulfhekel† Physikalisches Institut, Karlsruhe Institute of Technology (KIT), Wolfgang-Gaede-Straße 1, 76131 Karlsruhe, Germany E-mail: [email protected]

Abstract Magnetic molecules have the potential to be used as building blocks for bits in quantum computers. The spin states of the magnetic ion in the molecule can be represented by the effective spin Hamiltonian describing the zero field splitting (ZFS) of the magnetic states. We determined the ZFS of mechanically flexible metal-chelate molecules (Co, Ni and Cu as metal ions) adsorbed on Cu2 N/Cu(100) by inelastic tunneling spectroscopy at temperatures down to 30 mK. When moving the tip towards the molecule, the tunneling current abruptly jumps to higher values indicating the sudden deformation of the molecule bridging the tunneling junction. Hand in hand with the formation of the contact, an abrupt change of the ZFS occurs. This work also implies that ZFS expected in mechanical break junctions can drastically deviate from those of adsorbed molecules probed by other techniques. ∗

To whom correspondence should be addressed Physikalisches Institut, Karlsruhe Institute of Technology (KIT), Wolfgang-Gaede-Straße 1, 76131 Karlsruhe, Germany ‡ Current address: ISSP, Wakashiba 226-1, Kashiwa-shi, Chiba-ken, 277-8581, Japan †

1

ACS Paragon Plus Environment

ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Keywords single-molecule magnets, scanning tunneling microscopy, molecular contacts, ligand fields, Kondo effect, quantum bits

Single magnetic ions are of interest due to their magnetic quantum behaviour. 1–10 In contrast to being in free space, ions in molecules are influenced by the electrostatic interaction with the organic ligands. Together with the spin-orbit interaction, this leads to magnetic anisotropy, or more precisely to a splitting of the magnetic multiplet of the 2S + 1 states even in absence of a magnetic field. 11–13 In general, this effect can be expressed as a zero field splitting (ZFS) Hamiltonian, which consists of a sum over Stevens’ operators, which are themselves polynomials of the three components of the effective spin operator. 14 Due to time reversal symmetry, only even powers of the spin operators are allowed. 11 Additionally, some terms in the ZFS may be forbidden due to the point symmetry of the ligand field. 11,15 In the pioneering works of A. Heinrich et al. 1,16 it was shown that magnetic excitations of the ion from its ground state or ground state doublet can be observed with inelastic tunneling spectroscopy. The excitations show up as symmetric steps in the differential tunneling conductance dI/dV marking the energy of the excitation from the lower energy state to an excited state via spin flip scattering of tunneling electrons. 1,17–23 By studying the dependence of the excitations on an external magnetic field, the spin Hamiltonian can be determined. The excitation process by inelastic scattering is a non-coherent process as the ion is either left in the original state or is excited. Thus it is not in a superposition state after inelastic scattering. As in a scanning tunneling microscope (STM), an electric current is needed, the magnetic ion is necessarily in contact with an electron bath, i.e. the substrate. This electron bath can also promote spin scattering with the local ion leading in first or higher orders to the screening of the localized spin by the Kondo effect. 24,25 The presence or absence of a 2


Page 2 of 22

Page 3 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

Kondo effect is intimately linked to the nature of the spin Hamiltonian. 26–31 Similarly, Kondo effect is also observed in magnetic molecules in break junctions. 32–35 Spin excitations studies in metal-organic molecules as a function of the tip-molecule distance have recently been performed. 13,36 A continuous and slight change of the excitation energies was observed and was interpreted by a tip-molecule interaction. In this work, we systematically studied the ZFS and the Kondo effect in mechanically soft M-bis(2,2,6,6-tetramethyl-3, 5-heptanedionato) (M = Co, Ni and Cu) molecules (for short M(thd)2 ) 37 on Cu2 N/Cu(100) surface 38 at low temperatures with STM. Interestingly, we find that the ZFS can be abruptly switched by the sudden formation of a molecular contact and that this modification involves the energetic crossing of eigenstates of the spin system, i.e. a complete change of the magnetic anisotropy of the system. We further discuss the consequences for break junction experiments and applications of the observed effects.

Results and discussion Figure 1(a) shows a model of the M(thd)2 molecule. The central metal ion is surrounded by two thd groups, which are only loosely bound by chelate coordinate bonds. The thd groups themselves contain mainly sp3 bonds that in principle allow the rotation around the bond axes giving rise to the flexibility of the molecule. The STM image of the molecule on an atomically resolved Cu2 N/Cu(100) surface reveals the adsorption site of the molecule in the force fields of the surface (see Figure 1(b)). Note that the central Co ion surrounded by two symmetric thd groups displays, however, only a mirror symmetry (white dashed line) with respect to the Cu2 N substrate (C 1v symmetry, for more details see Figure S1 in Supporting Information). As the molecule is rather flexible, we expect that the interaction between the tip and the molecule can lead to changes of the adsorption geometry. 31 To check for these effects, we measured the current I as a function of the tip position z while approaching the tip towards the molecule at the Co ion position (see Figure 1(c)). Compared to the trivial

3


ACS Nano

(a)

(b) C H

M O

M

Cu

(d)

N

1 nm

Cu2N

-100 pm

1.6 -120 pm

1.2

10

-1

10

-2

1.4 tunneling Cu2N

1.0

1

100

-140 pm

Normalized dI/dV

10 Current (nA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 22

(c)

200 100 z-offset (pm)

1.2 1.0

-180 pm -200 pm

0.8

Co(thd)2 center Cu2N

(e)

0

-220 pm

contact 0.8 10 0 10 Bias V (mV)

z-offset = -240 pm

8 10 12 Bias V (mV)

Figure 1: (a) Model and (b) STM image of a M(thd)2 molecule. The white arrow indicates the [110] direction of Cu(100) and the white dashed line the mirror plane of symmetry of the Co ion’s surrounding (U =100 mV, I=40 pA). The black cross indicates the position of the tip for the contact experiments. (c) I(z) plots of Co(thd)2 molecule center (green) and Cu2 N (gray). Negative z-offsets represent approaching the tip from the initial set point (U =20 mV, I=20 pA). (d) Normalized dI/dV spectra of Co(thd)2 with a z-offset of −140 pm (tunneling) and −250 pm (contact) as well as bare Cu2 N for comparison. The spectra in the tunneling regime are vertically offset by 0.3 for clarity. Green solid lines are fits with a Fano function. (e) Normalized dI/dV spectra on Co(thd)2 measured during the approaching of the tip. Subsequent spectra are vertically offset by 0.1. Green dots illustrate the positions of the inelastic excitation (mid of the step). All data was measured at 900 mK.

4


Page 5 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

exponential rise of the current observed on bare Cu2 N (gray line), the tunneling current through the Co(thd)2 molecule displays a prominent jump at an offset of about −160 pm from 0.18 × 10−3 G0 to 1.0 × 10−3 G0 , where G0 =

2e2 h

≈ 7.7 × 10−5 S is the quantum

of conductance, followed by a relatively slow increase upon further approach (green solid line). When retracting the tip again, a hysteresis was observed (green dashed line) and the current only returns to the same values at an offset of about −60 pm. This hysteresis is highly reproducible (see Figure S2 in Supporting Information). It indicates a closing and opening of a molecular contact involving a molecular deformation. 39–43 Similar hysteresis in closing and opening curves are commonly observed in break junctions with molecules in between. 44–46 Accordingly, we label the two states as tunneling and contact regime. Images after contacting show no modification of the molecule or its adsorption site. This means that the molecule is only slightly deformed within the elastic limit during contact returning to its original state after breaking the contact. These observations also exclude chemical modifications due to the process of closing and opening again the contact. To study the magnetic behaviour of the molecule, we recorded dI/dV spectra in the tunneling and contact regime, which significantly differ (see Figure 1(d)). After breaking the contact, again contact formation reveals identical dI/dV spectra (See Figure S3 in Supporting Information), i.e. contact formation and breaking is reversible. The spectra display symmetric steps below 10 mV indicative of an inelastic excitation. As we will show below, they are of magnetic origin. Additionally, a sharp resonance peak at the Fermi energy is observed for both conditions. This resonance can be split by a magnetic field confirming a Kondo resonance, as discussed below. In contrast to the spectra on molecules, data taken on bare Cu2 N show a relatively flat behaviour without clear signs of inelastic excitations or zero bias peaks. Accurately speaking, the Kondo resonance and the inelastic excitation were only clearly observed when contacting the molecule in the center at the location of the metal ion (see Figure S4 in Supporting Information). Clearly, when forming the contact, inelastic excitations move to higher energies. Thus, the ZFS of the magnetic states is modified by

5


ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

the formation of the molecular contact. Further, the Kondo peak in contact is much more pronounced and wider than in tunneling and the fitted Kondo temperature TK 47,48 increases from 1.2 ± 0.5 K to 11.4 ± 0.2 K (details about fitting are shown in Supporting Information Table S1). Note that the electronic coupling Γ of the localized spin to the electron bath largely determines the Kondo temperature. An increase of Γ results in first approximation to an exponential increase of TK . 49 This makes the Kondo temperature a very sensitive indicator for the coupling. The Kondo temperature in the tunneling regime is less than half of that of single Co atom on Cu2 N/Cu(100), 29 but much lower than that of single Co atoms on bare metal surfaces, 50 since the interaction between Co ion and conduction electrons from copper substrate is decoupled by Cu2 N. In our case, the Co ion is decoupled from the substrate by both the Cu2 N and the organic ligands. In comparison, after molecular contact, the local spin on Co ion can additionally couple to conduction electrons from tungsten tip, which results in a higher Kondo temperature and in a larger current through the junction. To investigate the details of the deformation induced changes to the magnetic excitations, dI/dV spectra were recorded during approaching the molecule (cf. Figure 1(e)). We here focus on the energy range near the inelastic step. Clearly, the spectra illustrate that changing the z-offset influences the position of the inelastic step. For z-offsets before the jump to contact (indicated by the blue ellipse) we observe a slight tendency of the excitation to shift to lower energies (position of the center of the step in dI/dV is indicated by the green dot). Under these tunneling conditions, the molecule has not yet bridged the contact but the Co ion starts to feel the presence of the tip. When, however, the molecular junction is suddenly formed, both the intensity and energy of the inelastic excitation step are abruptly changed. This illustrates that the ZFS of adsorbed molecules may largely differ from that of contacted molecules in break junctions, and shows that ZFS parameters of molecules in junctions cannot be taken from e.g. measurements of bulk probes. Further approaching the tip leads to a further gradual increase of the excitation energy (indicated by the red ellipse). We ascribe the sudden modification of the ZFS to the jump of the molecule from tunneling

6


Page 6 of 22

Page 7 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

into the contact configuration. The hysteresis in the contacting curve indicates that the two configurations are local minima in the configuration space and the jump corresponds to a quick transition of the molecule between the two states. Similarly, further deformation of the molecule leads to additional changes of the ZFS, but without jumps. Here, the configuration of the molecule is changed continuously with approaching the tip. We stress that particularly the sudden distorts of the molecule during contact formation abruptly modify the ligand field acting on Co ion. Identical experiments on Ni(thd)2 and Cu(thd)2 revealed the same symmetry of the adsorbed molecule and a similar behaviour with a jump to contact upon approaching the tip (see Supporting Information Figure S5). Thus, these are general properties of the organic ligands and are not affected by the central ion confirming the interpretation of the molecular distortion by the presence of the tip. Note that for all molecules studied, the observed features in the tunneling spectra are moved in energy by contact formation, but the qualitative shape of the spectra, e.g. the number of inelastic steps or the existence of a Kondo resonance, is not altered. This excludes charge transfer processes and changes of the spin of the magnetic ion due to contact formation. To study the ZFS in more detail, we recorded dI/dV spectra of all three molecules both in the tunneling and the contact regime as a function of a magnetic field applied normal to the surface plane at a temperature of 30 mK. The results are depicted in Figure 2(a-f). Overall, we notice that Co(thd)2 shows a Kondo resonance at zero bias in both tunneling and contact conditions and one magnetic excitation, while Ni(thd)2 shows no Kondo effect but two excitations. Finally, Cu(thd)2 again displays a Kondo resonance. Further, a weak inelastic step that is independent of magnetic field is present, which is due to a non-magnetic excitation. Since these ions share the same oxidation state of +2, the expected spins S for Co2+ , Ni2+ and Cu2+ are 3/2 (4 states), 1 (3 states) and 1/2 (2 states), respectively. We use these spins of the ions to explain in detail the observed behaviour as the function of the magnetic field.

7


ACS Nano

Co(thd)2

Ni(thd)2

(a)

1.4

4T

Normalized dI/dV contact tunneling

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

3T

1.2

Page 8 of 22

Cu(thd)2

(c)

1.4

(e)

5T 4T

1.2

5T

2.6

4T

2T

2 T 1.0

0T

3T

1.8

2T

1 T 0.8

1.0 −10

0

10

(b)

1.4

0T −4

5T 4T

0

4

(d)

1.4

−2

0

2

(f)

5T

5T

2.6

1.2

4T

3T

1.2

3T

2 T 1.0

1.0

1T

1.0

0 T 0.6

1 T 1.8

2T

0 T 0.8

1T 1.0

0.8 −10

0

10

−4

0

4

Bias V (mV)

0T −2

0

2

Figure 2: (a-f) dI/dV spectra under various out of plane magnetic fields measured on Co(thd)2 , Ni(thd)2 and Cu(thd)2 in both tunneling (z-offset: −140 pm) and contact (zoffset: −250 pm) conditions at 30 mK. Solid lines represent fits with a spin Hamiltonian. Neighboring spectra are vertically offset for clarity. Green dots indicate the positions of the inelastic excitation on each spectrum (mid of the step). Vertical lines through excitation steps of lowest spectra are guides to the eye for the shifts of excitation energies.

8


Page 9 of 22

For Co2+ with a half integer spin, the four magnetic states will be split by the ZFS into two Kramers doublets (see Figure 3(a,b)). We expect an inelastic excitation from the ground state doublet to the excited state doublet at a finite energy and a Kondo effect from elastic scattering of conduction electrons with the two states of the ground state doublet. These two features are clearly visible (see Figure 2(a,b)). By the application of a magnetic field, the ground state doublet responsible for the Kondo effect splits into two states and the Kondo peak disappears. Instead, an inelastic excitation emerges due to spin flip scattering within the Zeeman split doublet. Similarly, the excitation from the lower doublet to the higher doublet is modified by the magnetic field. In both tunneling and contact conditions, the inelastic excitation shifts to higher energies with increasing field (see green dots and vertical black line). To extract the ligand field parameters (see Table 1), we fitted the spectra to a spin Hamiltonian, which can be represented by a sum of Stevens’ operators respecting the C 1v symmetry and the Zeeman energy of the external magnetic field. The Hamiltonian for

(a)

Co2+

tunneling

(b)

Co2+

contact

2+

2+

Ni Ni (e) Cu (c)tunneling (d) contact

2+

Energy

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

-3/2 -1/2 1/2 3/2

-3/2 -1/2 1/2 3/2

-1 0 1

-1 0 1

-1/2

1/2

Figure 3: Sketch of ligand field models of spin states for the three molecules in tunneling and contact geometry. Full dots represent eigenstates. Vertical solid arrows indicate the energies of spin excitations. Horizontal solid arrows indicate Kondo resonances. Dashed circles represent the eigenstates of the B20 -term only. Dashed arrows in (a,b) show B22 -term mixing second neighboring states. Green dots in (c,d) represent the state mainly composed of the Sz = 0 wave function. Dashed arrows in (c,d) illustrate that B22 and B21 -terms split the doublet to two singlets with < Sz >= 0.

9


ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 22

the eigenstates is: 14

Heff = −gµB Bz Sz + 3B20 Sz2 +

B22 2 B1 (S+ + S−2 ) + 2 [Sz (S+ + S− ) + (S+ + S− )Sz ], 2 4

(1)

where g is gyromagnetic factor, µB is the Bohr magneton, Bz is the magnetic field, Sz , S+ and S− are the spin operators (z-component, raising and lowering operator), B20 , B22 and B21 are the axial and transversal magnetic anisotropy and the anisotropy term due to the low symmetry, respectively. Here, the z-axis is normal to the plane and the x-axis is in the mirror plane of the molecule. The tunneling spectra were calculated from the transitions between the eigenstates by spin flip excitation, as discussed in detail in Ref. 51 This model describes both the position and height of the steps in the dI/dV spectra, but does not capture Kondo physics. Thermal broadening and thermal population of the initial states were considered. 52,53 The fit to the spectra describes the spectra well. It naturally fails to describe the Kondo resonance and the overshoots in dI/dV near the steps. For the tunneling case, B21 is compatible with zero, but the positions of the steps only weakly depend on B21 causing a large uncertainty for B21 . Also note that in this case, g is somewhat lower than 2 indicating some orbital contribution to the magnetic moment. B20 is negative indicating an easy axis normal to the plane. When going to contact condition, B20 , B22 , and B21 change dramatically. B21 deviates significantly from zero and artificially fixing B21 to zero in this case leads to unphysical fits. Interestingly the sign of B20 reverses upon contact formation, i.e. the anisotropy is altered to an easy plane system by contact formation. As depicted in Figure 3(a), the four magnetic states of the Co ion with Sz = −3/2, −1/2, 1/2 and 3/2 are mixed by the Stevens’ operators. The B22 -term mixes states with ∆Sz = 2, i.e. Sz = +3/2 with -1/2 and +1/2 with -3/2, leading to a reduction of the expectation values of Sz . Further, the B21 -term mixes neighboring states and the B20 -term is negative, i.e. overall a downward parabola is formed with a ground state doublet of high content of the Sz = ±3/2 states (the eigenstates and energies are listed in Supporting Information Table S2). As the

10


Page 11 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

Table 1: Fitted coefficients of the spin Hamiltonian of Co2+ and Ni2+ in tunneling and contact conditions. Energies are given in meV.

g B20 B22 B21

Co(thd)2 tunneling contact 1.50 ± 0.05 2.06 ± 0.07 −0.71 ± 0.05 0.3 ± 0.2 1.74 ± 0.06 1.5 ± 0.3 −0.003 ± 18 6.6 ± 0.2

Ni(thd)2 tunneling contact 2.09 ± 0.05 2.2 ± 0.2 0.818 ± 0.001 0.64 ± 0.03 1.058 ± 0.004 2.26 ± 0.04 −0.04 ± 0.1 0.73 ± 0.07

spin is half integer, all eigenstates are Kramers doublets. Upon contact formation, the sign of B20 reverses and by this, the energetic order of the two doublets is reversed. The ground state doublet in contact has a higher content of Sz = ±1/2 (see Figure 3(b)). The Kondo effect in both cases is caused by transitions between the two states of the ground state doublet from elastic spin flip scattering by a single electron. This is possible since in both cases, the ground state doublet contains components with Sz = ±1/2 (see Supporting Information Table S2). For Ni2+ with an integer spin, the three magnetic states will be split by the ZFS into three singlets (see Figure 3(c,d)). Note that the magnetic states with Sz = +1 and −1 are coupled by the B22 -term. As the doublet is not of a Kramers kind, this coupling will lead to a splitting of the doublet and formation of two singlets with vanishing expectation value of Sz . Excitations from the ground state singlet are possible to both excited states and we therefore observe two inelastic excitations. As there is only one ground state, no Kondo effect is observed. Note that in tunneling conditions, the lower energy excitation shifts to lower energies and the higher excitation to higher energies with rising field (see green dots in Figure 2(c)). In contact, however, both excitation energies rise with field (cf. Figure 2(d)). We further observe a minor dip at zero bias, which does not depend on magnetic field. Thus, this particular excitation cannot be of magnetic origin and most likely is caused by a vibronic excitation. Similar to the case of Co2+ , contacting the molecule drastically modifies the ZFS. The fits to the spectra reveal again a g-factor close to 2 and a drastic increase of B21 11


ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

upon contact formation. As depicted in Figure 3(c,d), the order of the states changes upon contact formation, similar to the case of Co2+ . While in the tunneling regime, the ground state is mainly composed of the Sz = 0 wave function, it is moved upward between the two other states when the molecule is contacted. In this situation, the application of a magnetic field will lead to an increase of both excitation energies, in agreement with the experimental observation. Finally, we discuss the case of Cu2+ , which should display a spin of 1/2. In this case, one obtains a Kramers doublet, all Stevens’ parameters vanish, and the Hamiltonian only contains the Zeeman energy (see Figure 3(e)). In agreement to that, we observe a Kondo peak at zero bias, which splits into two steps with the magnetic field (see Figure 2(e,f)). In addition to the magnetic excitation, there is a vibronic excitation, which does not move with magnetic field. Thus, our results indicate that the ZFS can be drastically modified by forming or breaking a contact and we relate this to the sudden deformation of the molecule. Care has to be taken, when extrapolating the ZFS obtained from crystals of molecules to break junction experiments. Even opening and closing the junction can fully reverse the sign of the leading anisotropy term and exchange the energetic order of the states.

Outlook These observations indicate to take great care of the microscopic deformation in magnetic molecules especially in break junction experiments but also offers ways to manipulate the quantum state of molecules. For example, an excitation of the break junction with acoustic waves in the GHz range would allow to use the coupling between mechanical and spin degrees of freedom to manipulate the spin state of the molecule. Note that the selection rules for spin excitations driven by the modification of the ZFS and driven by microwave absorption 54 are different opening additional ways to prepare quantum states. Further, the abrupt nature of

12


Page 12 of 22

Page 13 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

contact formation can potentially be used to prepare superposition states. 55–57 If we assume that the molecular spin is in one of the ground states when contacted (i.e. the contact is kept for a time longer than the lifetimes of all excited states), the abrupt modification of the ZFS by breaking the contact will lead to a sudden transformation of the state with the result of a superposition final state according to time dependent perturbation theory. For this to function, the changes in ZFS need to be non-adiabatic. The involved time scales of snapping open of molecular contacts are roughly given by the inverse phonon frequencies, 58 i.e. are of the order of ps. The involved time scales in molecular spins, i.e. the coherence times, are often in the range of 100 ns to 10 µs. 59 To effectively prepare these superposition states, the energetic order of the states needs to be altered by contact opening (or closing). Under these assumptions, the final quantum state after breaking the junction is a superposition state that can be calculated by projecting the initial state with closed contact onto the final states of the open contact.

Experimental methods The M(thd)2 molecules were deposited onto room-temperature Cu2 N/Cu(100) substrates prepared as described elsewhere

60

under a pressure better than 1 × 10−7 mbar by subliming

molecular powder at 30 ◦C from a crucible. The samples were transferred into the STM immediately after preparation without breaking ultra-high vacuum (UHV). All the STM tips were prepared from tungsten wire by chemical etching followed by Ar+ sputtering and flashing in UHV. I(z) curves were measured by ramping the z-offset in a loop from 0 pm to −280 pm and backwards while keeping the sample bias constant, in other words, approaching the tip 280 pm from the initial set point of 20 mV, 20 pA and retracting tip subsequently. A lock-in amplifier was used to obtain the dI/dV spectra. Tunneling spectra were recorded at fixed tip positions by opening the feedback loop. For all dI/dV spectra, the tip was approached towards the molecules with a particular distance from a common initial set

13


ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

point of 20 mV, 20 pA. Data in Figure 1 were measured at 900 mK with a Joule-Thomson STM. 61 A modulation of 0.2 mV RMS amplitude and 487 Hz was used. Data in Figure 2 were taken at 30 mK in a 3 He/4 He dilution refrigerator STM with a modulation of 0.1 mV and 2.21 kHz. dI/dV spectra were individually normalized by dividing by the conductance at large bias. The dI/dV spectra under various magnetic field on a particular molecule were fitted as a whole set. Not only the positions but also the amplitudes of the excitation-steps were fitted. Temperature effect and experimental broadening were included.

Acknowledgement W.W. acknowledges funding from the German Science Foundation (DFG) under the grants INST 121384/30-1 FUGG and WU 349/13-1. J.C. acknowledges funding from the Landesstiftung Baden-W¨ urttemberg and H.I. from the Alexander-von-Humboldt Foundation.

Supporting Information Available Additional data prove the C 1v symmetry, the reproducibility of our results, the localization of Kondo resonances and inelastic spin excitations, the jump of tunneling current on Ni(thd)2 and Cu(thd)2 molecule, determined coefficients of Kondo resonance fits and eigenstates and eigenenergies to spin Hamiltonians. This material is available free of charge via the Internet at http://pubs.acs.org/.

References 1. Hirjibehedin, C. F.; Lin, C.-Y.; Otte, A. F.; Ternes, M.; Lutz, C. P.; Jones, B. A.; Heinrich, A. J. Large Magnetic Anisotropy of a Single Atomic Spin Embedded in a Surface Molecular Network. Science 2007, 317, 1199–1203. 2. Bogani, L.; Wernsdorfer, W. Molecular Spintronics Using Single-Molecule Magnets. Nat. Mater. 2008, 7, 179–186. 14


Page 14 of 22

Page 15 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

3. Vincent, R.; Klyatskaya, S.; Ruben, M.; Wernsdorfer, W.; Balestro, F. Electronic Readout of a Single Nuclear Spin Using a Molecular Spin Transistor. Nature 2012, 488, 357. 4. Miyamachi, T.; Schuh, T.; Märkl, T.; Bresch, C.; Balashov, T.; Stöhr, A.; Karlewski, C.; André, S.; Marthaler, M.; Hoffmann, M.; Geilhufe, M.; Ostanin, S.; Hergert, W.; Mertig, I.; Schön, G.; Ernst, A.; Wulfhekel, W. Stabilizing the Magnetic Moment of Single Holmium Atoms by Symmetry. Nature 2013, 503, 242–246. 5. Thiele, S.; Balestro, F.; Ballou, R.; Klyatskaya, S.; Ruben, M.; Wernsdorfer, W. Electrically Driven Nuclear Spin Resonance in Single-Molecule Magnets. Science 2014, 344, 1135–1138. 6. Feltham, H. L.; Brooker, S. Review of Purely 4f and Mixed-Metal nd -4f Single-Molecule Magnets Containing Only One Lanthanide Ion. Coord. Chem. Rev. 2014, 276, 1–33. 7. Dreiser, J. Molecular Lanthanide Single-Ion Magnets: from Bulk to Submonolayers. J. Phys.: Condens. Matter 2015, 27, 183203. 8. Craig, G. A.; Murrie, M. 3d Single-Ion Magnets. Chem. Soc. Rev. 2015, 44, 2135–2147. 9. Frost, J. M.; Harriman, K. L. M.; Murugesu, M. The Rise of 3-d Single-Ion Magnets in Molecular Magnetism: Towards Materials from Molecules? Chem. Sci. 2016, 7, 2470– 2491. 10. Gaita-Arino, A.; Prima-Garcia, H.; Cardona-Serra, S.; Escalera-Moreno, L.; Rosaleny, L. E.; Baldovi, J. J. Coherence and Organisation in Lanthanoid Complexes: from Single Ion Magnets to Spin Qubits. Inorg. Chem. Front. 2016, 3, 568–577. 11. Bleaney, B.; Stevens, K. W. H. Paramagnetic Resonance. Rep. Prog. Phys. 1953, 16, 108–159.

15


ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

12. Boˇca, R. Zero-Field Splitting in Metal Complexes. Coord. Chem. Rev. 2004, 248, 757– 815. 13. Heinrich, B. W.; Braun, L.; Pascual, J. I.; Franke, K. J. Tuning the Magnetic Anisotropy of Single Molecules. Nano Lett. 2015, 15, 4024–4028. 14. Stevens, K. W. H. Matrix Elements and Operator Equivalents Connected with the Magnetic Properties of Rare Earth Ions. Proc. Phys. Soc., London, Sect. A 1952, 65, 209–215. 15. Misra, S. K.; Poole, C. P.; Farach, H. A. A Review of Spin Hamiltonian Forms for Various Point-Group Site Symmetries. Appl. Magn. Reson. 1996, 11, 29–46. 16. Heinrich, A. J.; Gupta, J. A.; Lutz, C. P.; Eigler, D. M. Single-Atom Spin-Flip Spectroscopy. Science 2004, 306, 466–469. 17. Hirjibehedin, C. F.; Lutz, C. P.; Heinrich, A. J. Spin Coupling in Engineered Atomic Structures. Science 2006, 312, 1021–1024. 18. Chen, X.; Fu, Y.-S.; Ji, S.-H.; Zhang, T.; Cheng, P.; Ma, X.-C.; Zou, X.-L.; Duan, W.H.; Jia, J.-F.; Xue, Q.-K. Probing Superexchange Interaction in Molecular Magnets by Spin-Flip Spectroscopy and Microscopy. Phys. Rev. Lett. 2008, 101, 197208. 19. Fransson, J. Spin Inelastic Electron Tunneling Spectroscopy on Local Spin Adsorbed on Surface. Nano Lett. 2009, 9, 2414–2417. 20. Tsukahara, N.; Noto, K.-i.; Ohara, M.; Shiraki, S.; Takagi, N.; Takata, Y.; Miyawaki, J.; Taguchi, M.; Chainani, A.; Shin, S.; Kawai, M. Adsorption-Induced Switching of Magnetic Anisotropy in a Single Iron(II) Phthalocyanine Molecule on an Oxidized Cu(110) Surface. Phys. Rev. Lett. 2009, 102, 167203. 21. Kahle, S.; Deng, Z.; Malinowski, N.; Tonnoir, C.; Forment-Aliaga, A.; Thontasen, N.; Rinke, G.; Le, D.; Turkowski, V.; Rahman, T. S.; Rauschenbach, S.; Ternes, M.; Kern, K.

16


Page 16 of 22

Page 17 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

The Quantum Magnetism of Individual Manganese-12-Acetate Molecular Magnets Anchored at Surfaces. Nano Lett. 2012, 12, 518–521. 22. Choi, T.; Badal, M.; Loth, S.; Yoo, J.-W.; Lutz, C. P.; Heinrich, A. J.; Epstein, A. J.; Stroud, D. G.; Gupta, J. A. Magnetism in Single Metalloorganic Complexes Formed by Atom Manipulation. Nano Lett. 2014, 14, 1196–1201. 23. Ormaza, M.; Bachellier, N.; Faraggi, M. N.; Verlhac, B.; Abufager, P.; Ohresser, P.; Joly, L.; Romeo, M.; Scheurer, F.; Bocquet, M.-L.; Lorente, N.; Limot, L. Efficient Spin-Flip Excitation of a Nickelocene Molecule. Nano Lett. 2017, 17, 1877–1882. 24. Kondo, J. Resistance Minimum in Dilute Magnetic Alloys. Prog. Theor. Phys. 1964, 32, 37–49. 25. Anderson, P. W. Localized Magnetic States in Metals. Phys. Rev. 1961, 124, 41–53. 26. Li, J.; Schneider, W.-D.; Berndt, R.; Delley, B. Kondo Scattering Observed at a Single Magnetic Impurity. Phys. Rev. Lett. 1998, 80, 2893–2896. 27. Wahl, P.; Diekhöner, L.; Wittich, G.; Vitali, L.; Schneider, M. A.; Kern, K. Kondo Effect of Molecular Complexes at Surfaces: Ligand Control of the Local Spin Coupling. Phys. Rev. Lett. 2005, 95, 166601. 28. Romeike, C.; Wegewijs, M. R.; Hofstetter, W.; Schoeller, H. Quantum-TunnelingInduced Kondo Effect in Single Molecular Magnets. Phys. Rev. Lett. 2006, 96, 196601. 29. Otte, A. F.; Ternes, M.; von Bergmann, K.; Loth, S.; Brune, H.; Lutz, C. P.; Hirjibehedin, C. F.; Heinrich, A. J. The Role of Magnetic Anisotropy in the Kondo Effect. Nat. Phys. 2008, 4, 847–850. 30. González, G.; Leuenberger, M. N.; Mucciolo, E. R. Kondo Effect in Single-Molecule Magnet Transistors. Phys. Rev. B 2008, 78, 054445.

17


ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

31. Hiraoka, R.; Minamitani, E.; Arafune, R.; Tsukahara, N.; Watanabe, S.; Kawai, M.; Takagi, N. Single-Molecule Quantum Dot as a Kondo Simulator. Nat. Commun. 2017, 8, 16012. 32. Park, J.; Pasupathy, A. N.; Goldsmith, J. I.; Chang, C.; Yaish, Y.; Petta, J. R.; Rinkoski, M.; Sethna, J. P.; Abruna, H. D.; McEuen, P. L.; Ralph, D. C. Coulomb Blockade and the Kondo Effect in Single-Atom Transistors. Nature 2002, 417, 722–725. 33. Liang, W.; Shores, M. P.; Bockrath, M.; Long, J. R.; Park, H. Kondo Resonance in a Single-Molecule Transistor. Nature 2002, 417, 725–729. 34. Scott, G. D.; Natelson, D. Kondo Resonances in Molecular Devices. ACS Nano 2010, 4, 3560–3579. 35. Frisenda, R.; Gaudenzi, R.; Franco, C.; Mas-Torrent, M.; Rovira, C.; Veciana, J.; Alcon, I.; Bromley, S. T.; Burzur, E.; van der Zant, H. S. J. Kondo Effect in a Neutral and Stable All Organic Radical Single Molecule Break Junction. Nano Lett. 2015, 15, 3109–3114. 36. Heinrich, B.; Braun, L.; Pascual, J.; Franke, K. Protection of Excited Spin States by a Superconducting Energy Gap. Nat. Phys. 2013, 9, 765–768. 37. Sans-Lenain, S.; Gleizes, A. Structural Features of Homo- and Heteroleptic Complexes of Copper(II) with 2,2,6,6-Tetramethyl-3,5-Heptanedione and 3-Chloro-2,4-Pentanedione. Inorg. Chim. Acta 1993, 211, 67–75. 38. Leibsle, F.; Dhesi, S.; Barrett, S.; Robinson, A. STM Observations of Cu(100)-c(2×2)N Surfaces: Evidence for Attractive Interactions and an Incommensurate c(2×2) Structure. Surf. Sci. 1994, 317, 309–320. 39. Joachim, C.; Gimzewski, J. K.; Schlittler, R. R.; Chavy, C. Electronic Transparence of a Single C60 Molecule. Phys. Rev. Lett. 1995, 74, 2102–2105. 18


Page 18 of 22

Page 19 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

40. Takács, A. F.; Witt, F.; Schmaus, S.; Balashov, T.; Bowen, M.; Beaurepaire, E.; Wulfhekel, W. Electron Transport Through Single Phthalocyanine Molecules Studied Using Scanning Tunneling Microscopy. Phys. Rev. B 2008, 78, 233404. 41. Temirov, R.; Lassise, A.; Anders, F. B.; Tautz, F. S. Kondo Effect by Controlled Cleavage of a Single-Molecule Contact. Nanotechnology 2008, 19, 065401. 42. Schmaus, S.; Bagrets, A.; Nahas, Y.; Yamada, T. K.; Bork, A.; Bowen, M.; Beaurepaire, E.; Evers, F.; Wulfhekel, W. Giant Magnetoresistance Through a Single Molecule. Nat. Nanotechnol. 2011, 6, 185–189. 43. Karan, S.; Jacob, D.; Karolak, M.; Hamann, C.; Wang, Y.; Weismann, A.; Lichtenstein, A. I.; Berndt, R. Shifting the Voltage Drop in Electron Transport Through a Single Molecule. Phys. Rev. Lett. 2015, 115, 016802. 44. Xu, B.; Xiao, X.; Tao, N. J. Measurements of Single-Molecule Electromechanical Properties. J. Am. Chem. Soc. 2003, 125, 16164–16165. 45. Kim, Y.; Song, H.; Strigl, F.; Pernau, H.-F.; Lee, T.; Scheer, E. Conductance and Vibrational States of Single-Molecule Junctions Controlled by Mechanical Stretching and Material Variation. Phys. Rev. Lett. 2011, 106, 196804. 46. Tsutsui, M.; Morikawa, T.; He, Y.; Arima, A.; Taniguchi, M. High Thermopower of Mechanically Stretched Single-Molecule Junctions. Sci. Rep. 2015, 5, 11519. 47. Fano, U. Effects of Configuration Interaction on Intensities and Phase Shifts. Phys. Rev. 1961, 124, 1866–1878. 48. Madhavan, V.; Chen, W.; Jamneala, T.; Crommie, M. F.; Wingreen, N. S. Tunneling into a Single Magnetic Atom: Spectroscopic Evidence of the Kondo Resonance. Science 1998, 280, 567–569.

19


ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

49. Haldane, F. D. M. Scaling Theory of the Asymmetric Anderson Model. Phys. Rev. Lett. 1978, 40, 416–419. 50. Wahl, P.; Diekhöner, L.; Schneider, M. A.; Vitali, L.; Wittich, G.; Kern, K. Kondo Temperature of Magnetic Impurities at Surfaces. Phys. Rev. Lett. 2004, 93, 176603. 51. Fernández-Rossier, J. Theory of Single-Spin Inelastic Tunneling Spectroscopy. Phys. Rev. Lett. 2009, 102, 256802. 52. Lambe, J.; Jaklevic, R. C. Molecular Vibration Spectra by Inelastic Electron Tunneling. Phys. Rev. 1968, 165, 821–832. 53. Ternes, M. Spin Excitations and Correlations in Scanning Tunneling Spectroscopy. New J. Phys. 2015, 17, 063016. 54. del Barco, E.; Kent, A. D.; Yang, E. C.; Hendrickson, D. N. Quantum Superposition of High Spin States in the Single Molecule Magnet Ni4 . Phys. Rev. Lett. 2004, 93, 157202. 55. Schrödinger, E. Die gegenwärtige Situation in der Quantenmechanik. Naturwissenschaften 1935, 23, 807–812. 56. Monroe, C.; Meekhof, D. M.; King, B. E.; Wineland, D. J. A “Schrödinger Cat” Superposition State of an Atom. Science 1996, 272, 1131–1136. 57. Wineland, D. J. Nobel Lecture: Superposition, Entanglement, and Raising Schrödinger’s Cat. Rev. Mod. Phys. 2013, 85, 1103–1114. 58. Paulsson, M.; Krag, C.; Frederiksen, T.; Brandbyge, M. Conductance of Alkanedithiol Single-Molecule Junctions: A Molecular Dynamics Study. Nano Lett. 2009, 9, 117–121. 59. Aromi, G.; Aguila, D.; Gamez, P.; Luis, F.; Roubeau, O. Design of Magnetic Coordination Complexes for Quantum Computing. Chem. Soc. Rev. 2012, 41, 537–546.

20


Page 20 of 22

Page 21 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

60. Miyamachi, T.; Gruber, M.; Davesne, V.; Bowen, M.; Boukari, S.; Joly, L.; Scheurer, F.; Rogez, G.; Yamada, T. K.; Ohresser, P.; Beaurepaire, E.; Wulfhekel, W. Robust Spin Crossover and Memristance Across a Single Molecule. Nat. Commun. 2012, 3, 938. 61. Zhang, L.; Miyamachi, T.; Tomani, T.; Dehm, R.; Wulfhekel, W. A Compact SubKelvin Ultrahigh Vacuum Scanning Tunneling Microscope with High Energy Resolution and High Stability. Rev. Sci. Instrum. 2011, 82, 103702.

21


ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Graphical TOC Entry

22


Page 22 of 22

Abrupt Switching of Crystal Fields during Formation of Molecular

Recommend Documents